COMPUTER NETWORKING

Introduction

A network is a group of computers and other devices, such as printers and modems, connected to each other. This enables the computers to effectively share data and resources.

The concept of sharing resources over a network is called networking. The computers in a network can share data, messages, graphics, printers, fax machines, modems, and other hardware and software resources.

Network elements

• Servers

Servers are the computers that provide the shared resources to network users. There is usually only one server in a small network, but bigger networks may have more than one server.

• Clients

Clients are the computers that can access the shared network resources provided by a server.

• Media

The computers in a network are connected to each other by hardware components, such as cables. These components are called the media.

• Shared data

A server provides shared resources and data over a network. The files that are provided by the server over the network are called shared data. This shared data can be a document, a worksheet or a folder.

• Resources

Files, printers or other items that can be used by network users are known as resources. These resources can be either hardware or software resources.

Benefits of the computer Network

In most organizations, a network offers benefits, such as data sharing and online communication that are not provided by a stand-alone environment (An environment in which there is several computers that are not connected to each other is called a stand-alone environment.). In this topic, you will identify the benefits of a network.

• Data sharing

The data can be easily sharable in a network so other user can access it at any time. For example, a user working in a team wants to share some data with the other team members. In a stand-alone environment, to share the data you can verbally exchanging information or writing memos or putting the data on a floppy and copying it to the machines of other members. If it network the data can be easily made sharable in a network so that the other team members can access.

A centralized data storage system enables multiple users to access data in different locations.

• Resource sharing

Resource sharing is also an important benefit of a computer network. For example, if there are four people in a family, each having their own computer, they will require four modems (for the Internet connection) and four printers, if they want to use the resources at the same time. A computer network, on the other hand, provides a cheaper alternative by the provision of resource sharing.

In this way, all the four computers can be interconnected, using a network, and just one modem and printer can efficiently provide the services to all four members. The facility of shared folders can also be availed by family members.

• Centralized Software Management

One of the greatest benefits of installing a network is the fact that all of the software can be loaded on one computer (the file server). This eliminates that need to spend time and energy installing updates and tracking files on independent computers throughout the building.

• Communication medium

A computer network can provide a powerful communication medium among people spread widely at different physical locations. It acts as a powerful communication medium when it comes to sharing information and resources.

• Data Security and Management

In a business environment, a network allows the administrators to much better manage the company's critical data. Instead of having this data spread over dozens or even hundreds of small computers in a haphazard fashion as their users create it, data can be centralized on shared servers. This makes it easy for everyone to find the data, makes it possible for the administrators to ensure that the data is regularly backed up, and also allows for the implementation of security measures to control who can read or change various pieces of critical information.

• Speed

Sharing and transferring files within Networks are very fast. Networks provide a very rapid method for sharing and transferring files. The sharing of data with the other users in a stand-alone environment requires physical transfer of data. This can be done by copying the data on a floppy disk or taking a printout. This requires more time and is inconvenient as compared to sharing the data over a network.

Disadvantages of the computer Network

• Expensive to Install

Although a network will generally save money over time, the initial costs of installation can be prohibitive. Cables, network cards, and software are expensive, and the installation may require the services of a technician.

• Data Security

If a network is implemented properly, it is possible to greatly improve the security of important data. In contrast, a poorly secured network puts critical data at risk, exposing it to the potential problems associated with hackers, unauthorized access.

• Cables May Break

One broken cable can stop the entire network.

• File Server May Fail

Although a file server is no more susceptible to failure than any other computer, when the files server "goes down," the entire network may come to a halt. When this happens, the entire hospital may lose access to necessary programs and files.

• Virus Infections

If any computer system in a network gets affected by computer virus , there is a possible threat of other systems getting affected too. Viruses get spread on a network easily because of the interconnectivity of workstations. Such spread can be dangerous if the computers have important database which can get corrupted by the virus.

To overcome Network Disadvantages

• Security issues and Virus Infections issues

The increase in businesses connecting their systems and using the Internet has its drawbacks. When all computers are networked, one user's problems may affect everyone. You should consider the greater potential for data loss, security breaches and viruses when creating a network.

It is important to ensure data security through regular backing up the files, password routines, and system logs. It is important to remove access from employees who leave, otherwise they may still be able to access customer records.

As your data will be stored in one location on the server, physical security is very important. You should improve the quality of your system security to protect your business from potential virus attacks and hacking. This should include anti virus software and a firewall or software barrier.

If your staffs need to access the network while off-site, consider a virtual private network. This creates a secure link and protects information sent and received.

• File Server Fail down issues

For this we can have backup fail server. When the files server "goes down," then we can use the backup fail server. Having backup server we can overcome this problem.

Peer-to-Peer Networks In a peer-to-peer network, there are no dedicated servers, and there is no hierarchy among the computers. All the computers are equal and therefore are known as peers. Each computer functions as both a client and a server. And there is no administrator responsible for the entire network. The user at each computer determines what data on that computer is shared on the network. Below figure shows a peer-to-peer network in which each computer functions as both a client and a server. Computers in a peer-to-peer network are called peers. In a peer-to-peer network, all computers are considered equal; they all have the same abilities to use the resources available on the network. Each computer can function both as a client and a server. Computers are not dedicated to function as servers. They use the network to share resources among the independent peers. The computer whose applications are required by the other networked computers functions as a server. The other computers function as clients. Therefore, a dedicated administrator is not assigned for network management. A peer-to-peer network is a small group of people using a network. Peer-to-peer networks members usually perform similar tasks, which necessitates the sharing of resources. The peer-to-peer networks support 10 computers. The users in a peer-to-peer network are located in the same geographical area. Operating systems, such as Microsoft Windows 98 or Microsoft Windows XP, can be used to set up a peer-to-peer network. Additional software is not required because peer-to-peer networking is built into the systems. Another important point of peer-to-peer networks is that the users of each computer plan and control the security of their resources. The users determine the resources on their computers, which can be shared on the network. The shared network resources, such as disk space, printers or faxes, can be used by anyone who has access to the network. This is possible only if the shared network resources are not password protected. Peer-to-peer networks have weak and intrusive security because a central server is not used to administer and secure the network. In addition, some users may not implement security. A peer-to-peer network does not support a central login process. This implies that a user who logs on to one peer can access any shared network resource, which is not controlled by a specific password. Peer-to-peer networks are relatively simple. Because each computer functions as a client and a server, there is no need for a powerful central server or for the other components required for a high-capacity network. Peer-to-peer networks can be less expensive than server-based networks. Peer-to-peer networks are simple and inexpensive to install and maintain. The cost of implementing peer-to-peer networks is low because a central server is not used to administer the network. In addition, the components for a high-capacity network are not required in a peer-to-peer network. In a peer-to-peer network, the users handle administration. This means that all the users need to be trained in how to share files, folders, and printers. In a peer-to-peer network, suddenly shutting down your computer can cause one of your colleagues to be unable to print. Peer-to-peer networks are appropriate for environments where all the users are located in the same geographical area and the network security is not an important factor. In addition, these networks are useful when the network expansion is limited. Advantages of a peer-to-peer network: Less initial expense - No need for a dedicated server. Setup - An operating system (such as Windows XP) already in place may only need to be reconfigured for peer-to-peer operations. Disadvantages of a peer-to-peer network: Decentralized - No Centralized server. Security - Does not provide the security available on a peer-to-peer network. Server Based Networks A dedicated server is one that functions only as a server and is not used as a client or workstation. Server based networks (see below Figure) have become the standard models for networking. In a server-based network, clients rely on the services that the server provides, such as file storing and printing. Client computers are generally less powerful than server computers. A server-based network using network operating system is that the networks are organized into domains. A domain is a collection of networks and clients that share security information. Domain security and logon permissions are controlled by special servers called domain controllers. Users cannot access the resources of servers in a domain until a domain controller has authenticated them. In server-based networks, a network administrator centrally manages the resource security. The administrator defines and manages user access to network resources. Another beneficial of server-based networks is central file storage. Server-based networks provide easy backup of critical data. Data backup is another useful characteristic of server based networks. Server based networks can support a larger number of users than peer-to-peer networks. To support a large number of users, server-based networks use monitoring and network management tools. Servers must perform varied and complex tasks. (See below figure)   Security is often the primary reason for choosing a server-based approach to networking. In a server-based environment, one administrator who sets the policy and applies it to every user on the network can manage security. (see below figure)   Advantages of a client/server network Centralized - Resources and data security are controlled through the server. Security - More security then Peer-to-peer network. Flexibility - New technology can be easily integrated into system. Interoperability - All components (client /server) work together. Accessibility - Server can be accessed remotely and across multiple platforms. Disadvantages of a client/server network Expense - Requires initial investment in dedicated server. Maintenance - Large networks will require a staff to ensure efficient operation. Dependence - When server goes down, operations will cease across the network. Wired Local Area Network (LAN) It is the simplest type of network in which computers are connected to each other by cables.   Each of the computers on the LAN is also called a node . A LAN is characterized by three primary attributes: Topology The topology is the pattern used to connect the computers together. With a bus topology, a network cable connects each computer to the next one, forming a chain. With a star topology, each of the computers is connected to a central nexus called a hub/Switch. A ring topology is essentially a bus network with the two ends joined together. (You will see more about network topologies in Chapter 6) Medium Four basic types of media are used in local-area networks; coaxial cable, twisted-pair wires, fiber-optic cable, And wireless. Each medium has its advantages and disadvantages relative to cost, speed, and expandability. Coaxial cables provide high speed and low error rates. Twisted-pair wires are cheaper than coaxial cables, can sustain the speeds common to most personal computers, and are easy to install. Fiber-optic cable is the medium of choice for high-speed local-area networks. Wireless local-area networks have the advantage of expandability. (You will see more about network medium in Chapter 5.2) Protocols The topology and the medium used on a particular network are specified by the protocol. (You will see more about protocol in Chapter 8). LAN computer networks that usually cover a limited range, say, within the boundary of a building. A LAN computer network is two or more computers that communicate with each other through some medium. The primary usage of local-area networks (LANs) is the sharing of hardware, software, or information, such as data files, multimedia files, or electronic mail. Resource sharing provided by local-area networks improves efficiency and reduces overhead. There are a number of ways in which nodes can communicate over a network. The simplest is to establish a dedicated link between the transmitting and receiving stations. This technique is known as circuit switching. A better way of communicating is to use a technique known as packet switching, in which a dedicated path is not reserved between the source and the destination. Data are wrapped up in a packet and launched into the network. In this way, a node only has exclusive access to the medium while it is sending a packet. During its inactive period, other nodes can transmit. A typical packet is divided into preamble, address, control, data, and error-check fields. The computers in a LAN are connected by using cables. This method cannot be used to connect computers that are in different locations, for example, in buildings across a town or city. Therefore, a LAN is not suitable for large businesses with offices in several locations. Wireless Local Area Network The term wireless networking refers to technology that enables two or more computers to communicate using standard network protocols, but without network cabling. Peer-to-peer wireless network consists of a number of computers each equipped with a wireless networking interface card. Each computer can communicate directly with all of the other wireless enabled computers. They can share files and printers this way, but may not be able to access wired LAN resources, unless one of the computers acts as a bridge to the wired LAN using special software.   A wireless network can also use an access point, or base station. In this type of network the access point acts like a hub, providing connectivity for the wireless computers. It can connect the wireless LAN to a wired LAN, allowing wireless computer access to LAN resources, such as file servers or existing Internet Connectivity. There are two types of access points: Hardware Access Points (HAP) Hardware access points offer complete support of most wireless features, but check your requirements carefully. Software Access Points Software Access Points which run on a computer equipped with a wireless network interface card as used in peer-to-peer wireless network. The software routers that can be used as a basic Software Access Point, and include features not commonly found in hardware solutions. Connected wireless LAN to wired LAN To do this you will need some sort of bridge between the wireless and wired network. This can be accomplished either with a hardware access point or a software access point. Hardware access points are available with various types of network interfaces, such as Ethernet or Token Ring, but typically require extra hardware to be purchased if you're networking requirements change. If networking requirements go beyond just interconnecting a wired network to a small wireless network, a software access point may be the best solution. A software access point does not limit the type or number of network interfaces you use. It may also allow considerable flexibility in providing access to different network types, such as different types of Ethernet, Wireless and Token Ring networks. Such connections are only limited by the number of slots or interfaces in the computer used for this task. Further to this the software access point may include significant additional features such as shared Internet access, web caching or content filtering, providing significant benefits to users and administrators. Wireless networking offers a cost-effective solution to users with difficult physical installations such as campuses, hospitals or businesses with more than one location in immediate proximity but separated by public thoroughfare. This type of installation requires two access points. Each access point acts as a bridge or router connecting its own LAN to the wireless connection. The wireless connection allows the two access points to communicate with each other, and therefore interconnect the two LAN's.   Wireless Network Range Each access point has a finite range within which a wireless connection can be maintained between the client computer and the access point. The actual distance varies depending upon the environment; manufacturers typically state both indoor and outdoor ranges to give a reasonable indication of reliable performance. Also it should be noted that when operating at the limits of range the performance may drop, as the quality of connection deteriorates and the system compensates. Typical indoor ranges are 150-300 feet, but can be shorter if the building construction interferes with radio transmissions. Longer ranges are possible, but performance will degrade with distance. Outdoor ranges are quoted up to 1000 feet, but again this depends upon the environment. There are ways to extend the basic operating range of Wireless communications, by using more than a single access point or using a wireless relay /extension point. Multiple access points can be connected to a wired LAN, or sometimes even to a second wireless LAN if the access point supports this. In most cases, separate access points are interconnected via a wired LAN, providing wireless connectivity in specific areas such as offices or rooms, but connected to a main wired LAN for access to network resources, such as file servers. If a single area is too large to be covered by a single access point, then multiple access points or extension points can be used. Note that an "extension point" is not defined in the wireless standard, but have been developed by some manufacturers. When using multiple access points, each access point wireless area should overlap its neighbors. This provides a seamless area for users to move around in using a feature called "roaming." See Roaming for further information. Some manufacturers produce extension points, which act as wireless relays, extending the range of a single access point. Multiple extension points can be strung together to provide wireless access to far away locations from the central access point. Roaming A wireless computer can "roam" from one access point to another, with the software and hardware maintaining a steady network connection by monitoring the signal strength from in-range access points and locking on to the one with the best quality. Usually this is completely transparent to the user; they are not aware that a different access point is being used from area to area. Some access point configurations require security authentication when swapping access points, usually in the form of a password dialog box. Access points are required to have overlapping wireless areas to achieve this as can be seen in the following diagram. A user can move from Area 1 to Area 2 transparently. The Wireless networking hardware automatically swaps to the Access Point with the best signal. Sharing an internet connection in wireless network To share an Internet connection across a LAN you need two things: an Internet sharing hardware device or software program a LAN If your LAN is wireless. You need hardware or software access point and a wireless LAN. Any computer equipped with a wireless network card running suitable Internet sharing software can be used as a software access point. A number of vendors offer hardware access points. A hardware access point may provide Internet Sharing capabilities to Wired LAN computers, but does not usually provide much flexibility beyond very simple configurations.   If an existing wired LAN already has an Internet connection, then the hardware access points simply connect to LAN and allow wireless computers to access the existing Internet connection in the same way as wired LAN computers. Wireless Network security Wireless communications obviously provide potential security issues, as an intruder does not need physical access to the traditional wired network in order to gain access to data communications. However, 802.11 wireless communications cannot be received much less decoded by simple scanners, short wave receivers etc. This has led to the common misconception that wireless communications cannot be eavesdropped at all. However, eavesdropping is possible using specialist equipment. To protect against any potential security issues, 802.11 wireless communications have a function called WEP (Wired Equivalent Privacy), a form of encryption which provides privacy comparable to that of a traditional wired network. If the wireless network has information that should be secure then WEP should be used, ensuring the data is protected at traditional wired network levels. IEEE 802.11 Wireless networking hardware requires the use of underlying technology that deals with radio frequencies as well as data transmission. The most widely used standard is 802.11 produced by the Institute of Electrical and Electronic Engineers (IEEE). This is a standard defining all aspects of Radio Frequency Wireless networking. Also it should be noted that traditional Virtual Private Networking (VPN) techniques will work over wireless networks in the same way as traditional wired networks. Wide Area Network (WAN) A wide area network (WAN) is a telecommunications network, usually used for connecting computers, that spans a wide geographical area. WANs can by used to connect cities, states, or even countries. An example of a WAN connection would be a company with two offices in distant cities, each with its own LAN and connected by a leased telephone line. This type of WAN is illustrated in below figure. Each end of the leased line is connected to a router and the routers are connected to individual LANs. Any computer on either of the LANs can communicate with any one of the other computers at the other end of the WAN link or with a computer on its own LAN.   WANs are often used by larger corporations or organizations to facilitate the exchange of data and in a wide variety of industries, corporations with facilities at multiple locations have embraced WANs. Increasingly, however, even small businesses are utilizing WANs as a way of increasing their communications capabilities. Although WANs serve a purpose similar to that of local area networks (LANs), WANs are structured and operated quite differently. The user of a WAN usually does not own the communications lines that connect the remote computer systems; instead, the user subscribes to a service through a telecommunications provider. Unlike LANs, WANs typically do not link individual computers, but rather are used to link LANs. WANs also transmit data at slower speeds than LANs. WANs have existed for decades, but new technologies, services, and applications have developed over the years to dramatically increase their efficacy for business. WANs were originally developed for digital leased-line services carrying only voice, rather than data. As such, they connected the private branch exchanges (PBXs) of remote offices of the same company. WANs are still used for voice services, but today they are used more frequently for data and image transmission (such as video conferencing). These added applications have spurred significant growth in WAN usage, primarily because of the surge in LAN connections to the wider networks. WANs are either point-to-point, involving a direct connection between two sites, or operate across packet-switched networks, in which data is transmitted in packets over shared circuits. Point-to-point WAN service may involve either analog dial-up lines, in which a modem is used to connect the computer to the telephone line, or dedicated leased digital telephone lines, also known as "private lines." Analog lines, which may be either part of a public-switched telephone network or leased lines, are suitable for batch data transmissions, such as congruent order entry and point-of-sale transactions. Dedicated digital phone lines permit uninterrupted, secure data transmission at fixed costs. Point-to-point WAN service providers include both local telephone companies and long distance carriers. Packet-switched network services are typically chosen by organizations which have low volumes of data or numerous sites, for which multiple dedicated lines would be too expensive. Depending on the service, WANs can be used for almost any data sharing purpose for which LANs can be used. Slower transmission speeds, however, may make some applications less practical for WANs. The most basic uses of WANs are for electronic mail and file transfer, but WANs can also permit users at remote sites to access and enter data on a central site's database, such as instantaneously updating accounting records. New types of network-based software that facilitate productivity and production tracking, such as groupware and work-flow automation software, can also be used over WANs. Using groupware, workers at dispersed locations can more easily collaborate on projects. WANs also give remote offices access to a central office's other data communications services, including the Internet. Wireless Fidelity Wi-Fi A typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at least 1 Mbit/s. Based on the settings (e.g. the SSID), the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide which of the two APs to make a connection to. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than the other. Since Wi-Fi transmits in the air, it has the same properties as a non-switched ethernet network. Wi-Fi Devices : Wireless Access Point (WAP) A wireless access point (AP) connects a group of wireless stations to an adjacent wired local area network (LAN). An access point is similar to anethernet hub or switch, but instead of relaying LAN data only to other LAN stations, an access point can relay wireless data to all other compatible wireless devices as well as to a single (usually) connected LAN device, in most cases an ethernet hub or switch, allowing wireless devices to communicate with any other device on the LAN. Wireless Routers A wireless router integrates a wireless access point with an IP router and an ethernet switch . The integrated switch connects the integrated access point and the integrated ethernet router internally, and allows for external wired ethernet LAN devices to be connected as well as a (usually) single WANdevice such as cable modem or DSL modem . A wireless router advantageously allows all three devices (mainly the access point and router) to be configured through one central configuration utility, usually through an integrated web server . Wireless Ethernet Bridge A wireless Ethernet bridge connects a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer . Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes. Advantages of Wi-Fi Allows LANs to be deployed without cabling, typically reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs. Wi-Fi silicon pricing continues to come down, making Wi-Fi a very economical networking option and driving inclusion of Wi-Fi in an ever-widening array of devices. Wi-Fi products are widely available in the market. Different brands of access points and client network interfaces are interoperable at a basic level of service. Products designated as Wi-Fi CERTIFIED by the Wi-Fi Alliance are interoperable and include WPA2 security. Wi-Fi networks support roaming, in which a mobile client station such as a laptop computer can move from one access point to another as the user moves around a building or area. Wi-Fi networks support roaming, in which a mobile client station such as a laptop computer can move from one access point to another as the user moves around a building or area. Wi-Fi is a global set of standards. Unlike cellular carriers, the same Wi-Fi client works in different countries around the world. Disadvantages of Wi-Fi Wi-Fi can be interrupted by other devices, notably 2.4 GHz cordless phones and microwave ovens. Power consumption is fairly high compared to some other standards, making battery life and heat a concern. The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be breakable even when correctly configured. Wi-Fi Access Points typically default to an open ( encryption -free) mode. Novice users benefit from a zero configuration device that works out of the box but might not intend to provide open wireless access to their LAN. Wi-Fi networks have limited range. A typical Wi-Fi home router using 802.11b or802.11g with a stock antenna might have a range of 45 m (150 ft) indoors and 90 m (300 ft) outdoors. Range also varies with frequency band, as Wi-Fi is no exception to the physics of radio wave propagation. Wi-Fi in the 2.4 GHz frequency block has better range than Wi-Fi in the 5 GHz frequency block, and less range than the oldest Wi-Fi (and pre-Wi-Fi) 900 MHz block. Outdoor range with improved antennas can be several kilometres or more with line-of-sight. Wi-Fi networks can be monitored and used to read and copy data (including personal information) transmitted over the network when no encryption such asVPN is used. Ethernet Network architecture combines standards, topologies and protocols to produce a working network. Currently, the most popular network architecture is Ethernet. A network that follows the Ethernet architecture standard is known as an Ethernet network. The first Ethernet network was introduced in 1975 by Robert Metcalfe and David Boffs at Xerox Palo Alto Research Center . This network was designed as a 2.94 Mbps system that could connect over 100 computers on a one kilometer cable. Xerox, Intel and Digital extended the original specification to 10 Mbps. This design formed the basis for the IEEE 802.3 specification defined by the IEEE 802 committee. The IEEE 802.3 specification defines Ethernet standards including cabling and topology alternatives. The term Ethernet refers to the family of local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol. This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally affect the speed of transmission on the network. It is one of the most widely implemented LAN standards. Three data rates are currently defined for operation over optical fiber and twisted-pair cables: 10 Mbps-10Base-T Ethernet 100 Mbps-Fast Ethernet 1000 Mbps-Gigabit Ethernet 10Base-T Ethernet 10Base-T provides Manchester-encoded 10-Mbps bit-serial communication over two unshielded twisted-pair cables. Although the standard was designed to support transmission over common telephone cable, the more typical link configuration is to use two pair of a four-pair Category 3 or 5 cable, terminated at each NIC with an 8-pin RJ-45 connector (the MDI), as shown in Figure 2.1.1. Because each active pair is configured as a simplex link where transmission is in one direction only, the 10Base-T physical layers can support either half-duplex or full-duplex operation.   Each 10Base5 cable segment can have a maximum of 100 computers. The maximum permissible distance between a computer and a transceiver is 50 meters (164 feet). The maximum total length of joined segments in a 10Base5 network is 2,500 meters (8,200 feet). Fast Ethernet To allow for an increased speed of transmission, the Ethernet protocol has developed a new standard that supports 100 Mbps. This is commonly called Fast Ethernet. Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet uses the CSMA/CD access method. As 10BaseT, Fast Ethernet also supports star and bus topology. Gigabit Ethernet The most recent development in the Ethernet standard is a protocol that has a transmission speed of 1 Gbps. Gigabit Ethernet is primarily used for backbones on a network at this time. In the future, it will probably be used for workstation and server connections also. It can be used with both fiber optic cabling and copper. 10-Gigabit Ethernet is under development and will likely be published as the IEEE 802.3ae supplement Ethernet has survived as the major LAN technology (it is currently used for approximately 85 percent of the world's LAN-connected PCs and workstations) because its protocol has the following characteristics: Is easy to understand, implement, manage, and maintain Allows low-cost network implementations Provides extensive topological flexibility for network installation Guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer Ethernet LANs consist of network nodes and interconnecting media. The network nodes fall into two major classes: Data terminal equipment (DTE) Devices that are either the source or the destination of data frames. DTEs are typically devices such as PCs, workstations, file servers, or print servers that, as a group, are all often referred to as end stations. Data communication equipment (DCE) Intermediate network devices that receive and forward frames across the network. DCEs may be either standalone devices such as repeaters, network switches, and routers, or communications interface units such as interface cards and modems. The current Ethernet media options include two general types of copper cable: unshielded twisted-pair (UTP) and shielded twisted-pair (STP), plus several types of optical fiber cable. Traditionally, Ethernet uses the linear bus topology for data transmission. Some Ethernet networks also use the star bus topology. Bus Topology In a bus topology, all devices on the network connect to one main cable (trunk cable). It is easy to install and configure, and inexpensive. Amplification or regeneration does not require special equipment. If a device wants to send information must first determine if the bus is being used by any other device. If no other device is attempting to transmit, the device sends the data. If the trunk cable fails, all devices are affected. (You can see more about Bus topology in Chapter 6)   Star Topology In a star topology, each device is connected by separate cable to a central device (hub or switch). If a cable fails it affects only the one device connected to the failed cable unlike the bus. Star networks are easily expanded, easier to troubleshoot and support many types of cables. Passive or active hub or switch is used to connect more than two devices together in a star topology. (You can see more about Star topology in Chapter 6)      Ethernet networks can use thinnet coaxial, thicknet coaxial or unshielded twisted pair (UTP) cables. Ethernet uses baseband transmission to transmit encoded signals over a cable.   Ethernet is popular in the scientific and academic communities because it supports multiple protocols. Examples of the protocols supported by Ethernet are: TCP/IP IPX NetBEUI DLC Ethernet breaks data down into frames. A frame is a package of information transmitted as a single unit. The length of an Ethernet frame can be between 64 and 1,518 bytes, but the Ethernet frame itself uses at least 18 bytes for control information. Therefore, the data in an Ethernet frame can be between 46 and 1,500 bytes. Every Ethernet frame contains control information and follows the same basic organization. For example, the Ethernet frame used for TCP/IP contains preamble, detonation, type, CRC in addition to the data.   Preamble - Makes the start of the frame Detonation - Specify the origin and destination addresses. Type - Identifies the network layer Protocol CRC - Is the error checking field that determines if the frame arrived without being corrupted. Ethernet performance can be improved by dividing a crowded segment into two less-populated segments that are joined with a bridge or a router. This reduces traffic on each segment. Since there are fewer computers attempting to transmit on the segment, access time improves.   Ethernet is the most popular network architecture in use today and can be used with most network operating systems. Ethernet Features Topology: Bus or Star Signal type: Baseband Protocol: TCP/IP, NetBEUL, DLC Access method: IEEE 802.3 Transfer speed : 10 Mbps, 100 Mbps, 1000 Mbps Cable type: Thicknet, Thinnet, UTP Data packet format: Ethernet Frames Operating Systems: Can be used with most Network operating systems Token Ring The Token Ring protocol was developed by IBM in the mid-1980s. The related IEEE 802.5 specification is almost identical to and completely compatible with IBM's Token Ring network. In fact, the IEEE 802.5 specification was modeled after IBM Token Ring, and it continues to shadow IBM's Token Ring development. The term Token Ringgenerally is used to refer to both IBM's Token Ring network and IEEE 802.5 networks. Token Ring and IEEE 802.5 networks are basically compatible, although the specifications differ in minor ways. IBM's Token Ring network specifies a star, with all end stations attached to a device called a multistation access unit (MSAU). In contrast, IEEE 802.5 does not specify a topology, although virtually all IEEE 802.5 implementations are based on a star. Other differences exist, including media type (IEEE 802.5 does not specify a media type, although IBM Token Ring networks use twisted-pair wire) and routing information field size. Below figure summarizes IBM Token Ring network and IEEE 802.5 specifications.   Token Ring Operation The token moves between the computers inside a logical ring(see figure 2.2.8). The actual physical cable ring is through the hub. Users are a part of the ring, but they connect to the ring through the hub.   Token Ring and IEEE 802.5 are two principal examples of token-passing networks (FDDI is the other). Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit. If a node receiving the token has no information to send, it passes the token to the next end station. Each station can hold the token for a maximum period of time. If a station possessing the token does have information to transmit, it seizes the token, alters 1 bit of the token (which turns the token into a start-of-frame sequence), appends the information that it wants to transmit, and sends this information to the next station on the ring. While the information frame is circling the ring, no token is on the network (unless the ring supports early token release), which means that other stations wanting to transmit must wait. Therefore, collisions cannot occur in Token Ring networks. If early token release is supported, a new token can be released when frame transmission is complete. The information frame circulates the ring until it reaches the intended destination station, which copies the information for further processing. The information frame continues to circle the ring and is finally removed when it reaches the sending station. The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination. Unlike CSMA/CD networks (such as Ethernet), token-passing networks are deterministic, which means that it is possible to calculate the maximum time that will pass before any end station will be capable of transmitting. This feature and several reliability features, which are discussed in the section "Fault-Management Mechanisms," later in this chapter, make Token Ring networks ideal for applications in which delay must be predictable and robust network operation is important. Factory automation environments are examples of such applications. Token Ring standard specifies a ring topology, the IBM implementation of token ring uses the star-wired ring topology with all the computers on the network connected to a central hub.The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbps or 16 Mbps. Token Ring networks use STP and UTP cabling, such as IBM Types 1, 2 and 3 cables, to connect the network adapter cards of the computers to the central hub and to interconnect other hubs. Data in a Token Ring network is transmitted in the form of frames with start and end delimiters. The basic format of a Token Ring data frame is showed in below figure with the descriptions of each field in the data frame. Start and end end delimiters - Start and end of the frame. Access control - Specifies frame priority and whether it is a token of a data frame Frame control - Media Access Control information or end station information Destination and source address - Origin and destination address Data - Data begin sect Frame check sequence - CRC error - checking information Frame status - Indicates if the frame was recognized, copied or if the destination address was available Token Ring is a stable architecture with the capacity to handle high-bandwidth applications, such as desktop videoconferencing and multimedia. The right choice of equipment is the key to high-performance, scaleable Token Ring networks. Hardware components of a Token Ring network Hub or Switch In a Token Ring network, the hub contains the actual ring in the network. The hub of a Token Ring network is also referred to as Multistation Access Unit (MAU), MultiStation Access Unit (MSAU) or Smart Multistation Access Unit (SMAU). The hub is attached to individual clients and servers by cables. The hub of a Token Ring network is a passive hub and acts as a connection point. The internal ring converts to an external ring at each connection point when a computer is connected. When all the ports on a hub have computers connected to them, the Token Ring network can be enlarged by adding another hub. However, each hub must be added in such a way that it becomes a part of the logical ring.   STP or UTP cables STP or UTP cables are required to connect the computers on a Token Ring network to a hub. Token Rings use IBM Type 1, 2 and 3 cabling. Most networks use the IBM Cabling System Type 3 UTP cabling. The screen displays the cable distances relevant to a Token Ring network. Media filters Media filters are used in the computers that use Type 3 telephone twisted pair cabling to convert cable connectors and reduce line noise. Media filters connect the token ring adapter card to a standard RJ-11/RJ-45 telephone jack. Repeaters Repeaters enable you to increase Token Ring cable distances. A repeater actively regenerates and retimes the Token Ring signal to extend the distances between the hubs on the network. Network adapter cards Network adapter cards enable a device to be connected to the Token Ring network. Token Ring adapter cards are available in both 4 Mbps and 16 Mbps models. The 16 Mbps cards accommodate an increased frame length that requires fewer transmissions for the same amount of data. Token Ring cards need to be implemented carefully because a Token Ring network runs only at one of two possible speeds, 4 Mbps or 16 Mbps. If 16 Mbps cards are used in a 4 Mbps network, they revert to the 4 Mbps mode. However, a 16 Mbps network will not accept the 4 Mbps cards because they cannot speed up to 16 Mbps. Fiber optic cable Due to the mix of data streaming, high speeds and unidirectional data traveling, Token Ring networks are well suited to fiber optic cable. Fiber optic cables are used in Token Ring networks to increase the range of the network up to 10 times that possible with copper cabling. However, fiber optic cables are more expensive than copper cables. ArcNET Attached Resource Computer Network (ArcNet) was developed by Datapoint Corporation in 1977. It is a simple, inexpensive and flexible network architecture designed for workgroup-sized networks. ArcNet technology is described by the ANSI standard 878.1 and predates the IEEE Project 802 standards. ArcNet should not be confused with the IEEE Token Bus standard, IEEE 802.4. However, ArcNet does loosely comply to this token passing specification. ArcNet technology is described by the ANSI standard 878.1 and predates the IEEE Project 802 standards. ArcNet should not be confused with the IEEE Token Bus standard, IEEE 802.4. However, ArcNet does loosely comply to this token passing specification. The token moves from one computer to another based on node addresses instead of the physical location of computers. This means that ArcNet passes the token to the next address regardless of whether the address is on a workstation in the same room or in a separate building. Each computer in an ArcNet network is connected by a cable to a hub, which can be an active, a passive or a smart hub. The standard cabling used for ArcNet is 93 ohm RG-62 A/U coaxial cable. ArcNet also supports twisted pair and fiber optic cables. The use of star topology and cable filtering make ArcNet networks reliable. In a distributed star design, ArcNet uses passive and active hubs to control and route data tokens from one workstation to the next. Since token passing is done at a fixed rate and collisions do not occur, ArcNet is very stable. LocalTalk LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 Kbps. FDDI The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper. It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI), has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire. This chapter focuses mainly on FDDI specifications and operations, but it also provides a high-level overview of CDDI. FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. As will be discussed in detail later in this chapter, the primary purpose of the dual rings is to provide superior reliability and robustness. Figure shows the counter-rotating primary and secondary FDDI rings. FDDI uses optical fiber as the primary transmission medium, but it also can run over copper cabling. As mentioned earlier, FDDI over copper is referred to as Copper-Distributed Data Interface (CDDI). Optical fiber has several advantages over copper media. In particular, security, reliability, and performance all are enhanced with optical fiber media because fiber does not emit electrical signals. A physical medium that does emit electrical signals (copper) can be tapped and therefore would permit unauthorized access to the data that is transiting the medium. In addition, fiber is immune to electrical interference from radio frequency interference (RFI) and electromagnetic interference (EMI). Fiber historically has supported much higher bandwidth (throughput potential) than copper, although recent technological advances have made copper capable of transmitting at 100 Mbps. Finally, FDDI allows 2 km between stations using multimode fiber, and even longer distances using a single mode FDDI defines two types of optical fiber: single-mode and multimode. A mode is a ray of light that enters the fiber at a particular angle. Multimode fiber uses LED as the light-generating device, while single-mode fiber generally uses lasers. Below figure depicts single-mode fiber using a laser light source and multimode fiber using a light emitting diode (LED) light source. FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling. The FDDI frame format is similar to the format of a Token Ring frame. This is one of the areas in which FDDI borrows heavily from earlier LAN technologies, such as Token Ring. FDDI frames can be as large as 4,500 bytes. Figure shows the frame format of an FDDI data frame and token. Preamble - Gives a unique sequence that prepares each station for an upcoming frame. Start delimiter - Indicates the beginning of a frame. Frame control - Indicates the size of the address. Destination address - Origin and destination address Source address - Identifies the single station that sent the frame. Frame check sequence (FCS) - CRC error - checking End delimiter - End of the frame. Frame status - Identifies whether the frame was recognized and copied by a receiving station. Ethernet Network Interface Card (NIC) A network card (network adapter, network interface card, NIC, Ethernet adapter etc.) is a piece of computer hardware designed to allow computers to communicate over a computer network. It is an OSI model layer 2 item. Every network card in the world has a unique 48-bit serial number called a MAC address, which is written to ROM carried on the card. Every computer on a network must have a card with a unique MAC address. The IEEE is responsible for assigning MAC addresses to the vendors of network interface cards, which means that two cards sharing the same MAC address is impossible. Whereas network cards used to be expansion cards to plug into a computer bus, most new computers have a network interface built into the motherboard, so a separate network card is not required unless multiple interfaces are needed or some other type of network is used. The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as Ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP. There are four techniques used for transfer of data, the NIC may use one or more of these techniques. Polling is where the microprocessor examines the status of the peripheral under program control. Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system's address bus. Interrupt-driven I/O is where the peripheral alerts the microprocessor that its ready to transfer data. DMA is where the intelligent peripheral assumes control of the system bus to access memory directly. A network card typically has a twisted pair and BNC sockets where the network cable is connected, and a few LEDs to inform the user of whether the network is active, and whether or not there is data being transmitted on it. The Network Cards are typically available in 10/100/1000 Mbit/s. This means they can support a transfer rate of 10 or 100 or 1000 Mbit/s. Network Cables Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network. The following sections discuss the types of cables used in networks. Twisted Pair A thin-diameter wire commonly used for telephone and network cabling. The wires are twisted around each other to minimize interference from other twisted pairs in the cable. Twisted pairs have less bandwidth than coaxial cable or optical fiber.   UTP (Unshielded Twisted Pair) & STP (Shielded Twisted Pair) Twisted pair cables are available unshielded (UTP) and shielded (STP), with UTP being the most common. STP is used in noisy and static field interference environments (factories) where the shield around each of the wire pairs, plus an overall shield, protects against excessive electromagnetic interference. A variation of STP, known as ScTP for "Screened Twisted Pair" or FTP for "Foil Twisted Pair," uses only the overall shield and provides more protection than UTP, but not as much as STP. Unshielded Twisted Pair (UTP)   UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. UTP cables are found in many ethernet networks and telephone systems. Shielded Twisted Pair (STP)   STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences). e.g. the 150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications and used with Token Ring networks. Screened Shielded Twisted Pair (S/STP)   S/STP cabling is STP cabling with metal shielding also covering the group of shielded copper pairs. This type of cabling offers still improved protection from interference from external sources. Screened Unshielded Twisted Pair (S/UTP)   S/UTP, also known as Fully shielded (or Foiled) Twisted Pair (FTP) and Screened Fully shielded Twisted Pair (S/FTP), is a screened UTP cable. Twisted pair cabling is standardized into various categories by numbers, which indicate signal integrity attributes. Category 5 cable is commonly used for Ethernet with 10BASE-T or 100BASE-TX. Coaxial cables Coaxial cables are widely used for transmitting voice, video and data over LANs. To select the appropriate type of coaxial cable for our network, it is useful to learn about the benefits and limitations of coaxial cable types. There are two types of cable available based on the thickness of core I. Thinnet II. Thiknet Thinnet Thinnet is a thin and flexible coaxial cable, which has a thickness of about one-fourth of an inch. Since this type of coaxial cable is flexible, it is easy to work with thinnet cables. They can be used for any kind of installation. Another benefit of using thinnet coaxial cable is the low cable cost. Thinnet cable is inexpensive as compared to thicknet cable. An earlier 10 Mbps Ethernet standard that used a thin coaxial cable. Network nodes were attached to the cable via T-type BNC connectors in the adapter cards. Also called "thin Ethernet," "ThinWire," "ThinNet" and "Cheapernet," Thinnet cable had a distance limit of 607 feet. Thinnet (cheapernet or 10BASE2) is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors. The (10BASE2) 10 comes from the maximum transmission speed of 10 Mbit/s. The BASE stands for baseband signaling, and the 2 represents rounded up shorthand for the maximum segment length of 185 meters (606.955 feet). Thiknet The other type of coaxial cable, the thicknet cable, is a thicker cable. It is about half an inch in diameter. The copper core is thicker than a thinnet core. Since the thicknet cable has a thicker copper core, it can carry a signal out of than a thinnet cable. It can carry a signal for 500 meters (1,640 feet). Another benefit of the thicknet cable is that it can be used as a backbone to connect several smaller thinnet-based networks. This is possible because the thicknet cable is capable of supporting data transfer over long distances. Fiber Optic Cable A thin glass strand designed for light transmission. A single hair-thin fiber is capable of transmitting trillions of bits per second. In addition to their huge transmission capacity, optical fibers offer many advantages over electricity and copper wire. Light pulses are not affected by random radiation in the environment, and their error rate is significantly lower. Fibers allow longer distances to be spanned before the signal has to be regenerated by expensive "repeaters." Fibers are more secure, because taps in the line can be detected, and lastly, fiber installation is streamlined due to their dramatically lower weight and smaller size compared to copper cables.   There are two primary types of fiber. For intercity cabling and highest speed, singlemode fiber with a core diameter of less than 10 microns is used. Multimode fiber is very common for short distances and has a core diameter from 50 to 100 microns. The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components. The light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use. The fiber absorption is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission. A local minimum of absorption is found near 850 nm, a wavelength for which low cost transmitters and receivers can be designed, and this wavelength is often used for short distance applications. Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction. For modern glass optical fiber, the maximum transmission distance is limited not by attenuation but by dispersion, or spreading of optical pulses as they travel along the fiber. Dispersion in optical fibers is caused by a variety of factors. Intermodal dispersion, caused by the different axial speeds of different transverse modes, limits the performance of multi-mode fiber. Because single-mode fiber supports only one transverse mode, intermodal dispersion is eliminated. For single-mode fiber performance is limited by chromatic dispersion, which occurs because the index of the glass varies slightly depending on the wavelength of the light, and light from real optical transmitters has nonzero spectral width. Polarization mode dispersion, which can limit the performance of single-mode systems, occurs because although the single-mode fiber can sustain only one transverse mode, it can carry this mode with two different polarizations, and slight imperfections or distortions in a fiber can alter the propagation velocities for the two polarizations. Dispersion limits the bandwidth of the fiber because the spreading optical pulse limits the rate that pulses can follow one another on the fiber and still be distinguishable at the receiver. Because the effect of dispersion increases with the length of the fiber, a fiber transmission system is often characterized by its bandwidth-distance product, often expressed in units of MHzÃ-km. This value is a product of bandwidth and distance because there is a tradeoff between the bandwidth of the signal and the distance it can be carried. For example, a common multimode fiber with bandwidth-distance product of 500 MHzÃ-km could carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km. In single-mode fiber systems, both the fiber characteristics and the spectral width of the transmitter contribute to determining the bandwidth-distance product of the system. Typical single-mode systems can sustain transmission distances of 80 to 140 km (50 to 87 miles) between regenerations of the signal. By using an extremely narrow-spectrum laser source, data rates of up to 40 gigabits per second are achieved in real-world applications. Ethernet Switches An Ethernet Switch is a LAN interconnection device which operates at the data link layer (layer 2) of the OSI reference model. A switch is fundamentally similar to a bridge , but usually supports a larger number of connected LAN segments and has a richer management capability. A network switch is a computer networking device that connects network segments. It uses the logic of a Network bridge but allows a physical and logical star topology. It is often used to replace network hubs. A switch is also often referred to as an intelligent hub or switching hub. As a frame comes into a switch, the switch saves the originating MAC address and the originating port in the switch's MAC address table. The switch then selectively transmits the frame from specific ports based on the frame's destination MAC address and previous entries in the MAC address table. If the destination MAC address is unknown a broadcast address or (for simpler switches) a multicast address the switch simply transmits the frame out of all of the connected interfaces except the incoming port. If the destination MAC address is known, the frame is forwarded only to the corresponding port in the MAC address table. If the destination port is the same as the originating port, the frame is filtered out and not forwarded. Routers A router is an Intermediate System (IS) which operates at the network layer of the OSI reference model. Routers may be used to connect two or more IP networks , or an IP network to an internet connection. A router consists of a computer with at least two network interface cards supporting theIP protocol . The router receives packets from each interface via a network interface and forwards the received packets to an appropriate output network interface. Received packets have all link layer protocol headers removed, and transmitted packets have a new link protocol header added prior to transmission. The router uses the information held in the network layer header (i.e. IP header) to decide whether to forward each received packet, and which network interface to use to send the packet. Most packets are forwareded based on the packet's IP destination address , along with routing information held within the router in a routing table. Before a packet is forwarded, the processor checks the Maximum Transfer Unit (MTU) of the specified interface. Packets larger than the interface's MTU must be fragmented by the router into two or more smaller packets. If a packet is received which has the Don't Fragment (DF) bit set in the packet header , the packet is not fragmented, but instead discarded. In this case, an ICMP error message is returned to the sender (i.e. to the original packet's IP source address) informing it of the interface's MTU size. This forms the basis for Path MTU discovery (PMTU) . The routing and filter tables resemble similar tables in link layer bridges and switches. Except, that instead of specifying link hardware addresses ( MAC addresses ), the router table sepcify network ( IP addresses ). The routing table lists known IP destination addresses with the appropraite network interface to be used to reach that destiantion. A default entry may be specified to be used for all addresses not explicitly defined in the table. A filter table may also be used to ensure that unwanted packets are discarded. The filter may be used to deny access to particular protocols or to prevent unauthorised access from remote computers by discarding packets to specified destination addresses. A router forwards packets from one IP network to another IP network. Like other systems, it determines the IP network from the logical AND of an IP address with the associated subnetwork address mask. One execption to this rule is when a router receives an IP packet to a network broadcast address. In this case, the router discards the packet. Forwarding broadcast packet can lead to severe storms of packets, and if uncontrolled could lead to network overload. Routers are often used to connect together networks which use different types of links (for instance an HDLC link connecting a WAN to a local Ethernet LAN ). The optimum (and maximum) packet lengths (i.e. the maximum transmission unit (MTU) ) is different for different types of network. A router may therefore uses IP to provide segmentation of packets into a suitable size for transmission on a network. Patch panels A panel of network ports contained together, usually within a telecommunications closet, which connects incoming and outgoing lines of a LAN or other communication, electronic or electrical system. In a LAN , the patch panel connects the network's computers to each other and to the outside lines that enable the LAN to connect to the Internet or another WAN. Connections are made with patch cords. The patch panel allows circuits to be arranged and rearranged by plugging and unplugging the patch cords. Switch rack   A rack which can accommodate the Ethernet switch and patch panel, and usually this located on the common place for all the computers. Wall pallets   The wall outlet will be like a plug point which connects the internal and external wiring, and the wall outlet is mostly used for secure the internal cables. Firewall A firewall protects networked computers from intentional hostile intrusion that could compromise confidentiality or result in data corruption or denial of service. It may be a hardware device or a software program running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to. Hardware Firewall : Hardware firewall providing protection to a Local Network Firewall Software : Computer running firewall software to provide protection A firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet. The earliest firewalls were simply routers. The term firewall comes from the fact that by segmenting a network into different physical subnetworks, they limited the damage that could spread from one subnet to another just like firedoors or firewalls. A firewall examines all traffic routed between the two networks to see if it meets certain criteria. If it does, it is routed between the networks, otherwise it is stopped. A firewall filters both inbound and outbound traffic. It can also manage public access to private networked resources such as host applications. It can be used to log all attempts to enter the private network and trigger alarms when hostile or unauthorized entry is attempted. Firewalls can filter packets based on their source and destination addresses and port numbers. This is known as address filtering. Firewalls can also filter specific types of network traffic. This is also known as protocol filtering because the decision to forward or reject traffic is dependant upon the protocol used, for example HTTP, ftp or telnet. Firewalls can also filter traffic by packet attribute or state. A firewall cannot prevent individual users with modems from dialling into or out of the network, bypassing the firewall altogether. Employee misconduct or carelessness cannot be controlled by firewalls. Policies involving the use and misuse of passwords and user accounts must be strictly enforced. These are management issues that should be raised during the planning of any security policy but that cannot be solved with firewalls alone. Bus topology A bus topology network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. The bus topology is often referred to as a "linear bus" because the computers are connected in a straight line. This is the simplest and most common method of networking computers. Below figure shows a typical bus topology. It consists of a single cable called a trunk (also called a backbone or segment) that connects all of the computers in the network in a single line.   Computers on a bus topology network communicate by addressing data to a particular computer and sending out that data on the cable as electronic signals. Network data in the form of electronic signals is sent to all the computers on the network. Only the computer whose address matches the address encoded in the original signal accepts the information. All other computers reject the data. Figure shows a message being sent from 0020af151d8b to 02608c133456.   Only one computer at a time can send messages. Because only one computer at a time can send data on a bus network, the number of computers attached to the bus will affect network performance. The more computers there are on a bus, the more computers will be waiting to put data on the bus and, consequently, the slower the network will be. There is no standard way to measure the impact of a given number of computers on the speed of any given network. The effect on performance is not related solely to the number of computers. The following is a list of factors that in addition to the number of networked computers will affect the performance of a network: Hardware capabilities of computers on the network Total number of queued commands waiting to be executed Types of applications (client-server or file system sharing, for example) being run on the network Types of cable used on the network Distances between computers on the network Computers on a bus either transmit data to other computers on the network or listen for data from other computers on the network. They are not responsible for moving data from one computer to the next. Consequently, if one computer fails, it does not affect the rest of the network. Because the data, or electronic signal, is sent to the entire network, it travels from one end of the cable to the other. If the signal is allowed to continue uninterrupted, it will keep bouncing back and forth along the cable and prevent other computers from sending signals. Therefore, the signal must be stopped after it has had a chance to reach the proper destination address. To stop the signal from bouncing, a component called a terminator is placed at each end of the cable to absorb free signals. Absorbing the signal clears the cable so that other computers can send data. Both ends of each cable segment on the network must be plugged into something. For example, a cable end can be plugged into a computer or a connector to extend the cable length. Any open cable ends not plugged into something must be terminated to prevent signal bounce. Figure 2.2.3 shows a properly terminated bus topology network.   Advantages and Disadvantages of a Bus Network Advantages Easy to implement and extend Well suited for temporary networks (quick setup) Typically the cheapest topology to implement Faster than a ring network. If any node on the bus network fails, the bus its self is not effected. Requires less cable than a Star network. Disadvantages Difficult to administer/troubleshoot Limited cable length and number of stations A cable break can disable the entire network Maintenance costs may be higher in the long run Performance degrades as additional computers are added or on heavy traffic Low security (all computers on the bus can see all data transmissions) One virus in the network will affect all of them (but not as badly as a star or ring network) Proper termination is required.(loop must be in closed path) Star topology Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch , hub or computer which acts as a router to transmit messages. In the star topology, cable segments from each computer are connected to a centralized component called a hub . Figure 2.2.6 shows four computers and a hub connected in a star topology. Signals are transmitted from the sending computer through the hub to all computers on the network. This topology originated in the early days of computing when computers were connected to a centralized mainframe computer.   The star network offers the advantage of centralized resources and management. However, because each computer is connected to a central point, this topology requires a great deal of cable in a large network installation. Also, if the central point fails, the entire network goes down. If one computer or the cable that connects it to the hub fails on a star network, only the failed computer will not be able to send or receive network data. The rest of the network continues to function normally. Another characteristic of the star topology is that it is easy to modify. Computers can be added or removed from the network without disturbing the functioning of the network. The star topology supports the expansion of networks. This is done by placing another hub where a computer can be located. This enables several more computers or hubs to be connected to the main hub. A disadvantage of the star topology is that the cost of cabling is higher in a network based on this topology because all the network cables need to be extended to one central point. Advantages and Disadvantages of a Ring Network Advantages Easy to implement and extend, even in large networks Well suited for temporary networks (quick setup) The failure of a non-central node will not have major effects on the functionality of the network. Reliable market proven system No problems with collisions of Data since each station have its own cable to the server/hub. Security can be implemented in the hub/switch. The centre of a star i.e. the hub or switch is best place to find network faults. It is easy to modify and add new computers to a star network without disturbing the rest of the network. Disadvantages Depending on the transmission media, length limitations may be imposed from the central location used Failure of the central node can disable the entire network Limited cable length and number of stations Maintenance costs may be higher in the long run  Mesh topology A mesh topology network offers superior redundancy and reliability. In a mesh topology, each computer is connected to every other computer by separate cabling. This configuration provides redundant paths throughout the network so that if one cable fails, another will take over the traffic. While ease of troubleshooting and increased reliability is definite pluses, these networks are expensive to install because they use a lot of cabling. Often, a mesh topology will be used in conjunction with other topologies to form a hybrid topology.   Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed. This concept is applicable to wireless networks, wired networks, and software interaction. A mesh network is a networking technique which allows inexpensive peer network nodes to supply back haul services to other nodes in the same network. It effectively extends a network by sharing access to higher cost network infrastructure. Advantages and Disadvantages of a mesh network Advantages Provides redundant paths between devices. The network can be expanded without disruption to current users. Disadvantages Requires more cable than the other LAN topologies. Complicated implementation.  Selecting a Topology There are many factors to consider when deciding which topology best suits the needs of an organization. Below table provides some guidelines for selecting a topology. Topology Advantages Disadvantages Bus Use of cable is economical. Media is inexpensive and easy to work with. System is simple and reliable. Bus is easy to extend. Network can slow down in heavy traffic. Problems are difficult to isolate. Cable break can affect many users. Ring System provides equal access for all computers. Performance is even despite many users. Failure of one computer can impact the rest of the network. Problems are hard to isolate. Network reconfiguration disrupts operation. Star Modifying system and adding new computers is easy. Centralized monitoring and management are possible. Failure of one computer does not affect the rest of the network. If the centralized point fails, the network fails. Mesh System provides increased redundancy and reliability as well as ease of troubleshooting. System is expensive to install because it uses a lot of cabling.  OSI Layer OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunicationnetwork. Its purpose is to guide product implementers so that their products will consistently work with other products. The reference model defines seven layers of functions that take place at each end of a communication. Although OSI is not always strictly adhered to in terms of keeping related functions together in a well-defined layer, many if not most products involved in telecommunication make an attempt to describe them in relation to the OSI model. It is also valuable as a single reference view of communication that furnishes everyone a common ground for education and discussion. Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of interfaces. Instead, the committee decided to establish a common reference model for which others could develop detailed interfaces that in turn could become standards. OSI was officially adopted as an international standard by the International Organization of Standards ( ISO ). Currently, it is Recommendation X.200 of the ITU-TS. The main idea in OSI is that the process of communication between two end points in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Each communicating user or program is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user or program. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system , applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer. OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are: Layer 7: The application layer Layer 6: The presentation layer Layer 5: The session layer Layer 4: The transport layer Layer 3: The network layer Layer 2: The data-link layer Layer 1: The physical layer Layer 7: The application layer The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer . The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal , and job transfer and manipulation protocols. Examples: FTP DNS SNMP SMTP gateways Web browser Network File System (NFS) Telnet and Remote Login (rlogin) X.400 FTAM Database software Print Server Software Layer 6: The presentation layer The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests from the application layer and issues service requests to the session layer . The presentation layer concerns itself not only with the format and representation of actual user data, but also with data structure used by programs. Therefore, the presentation layer negotiates data transfer syntax for the application layer. The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end- usersystems. Note: An example of a presentation service would be the conversion of anEBCDIC -coded text file to an ASCII -coded file. The idea of the application layer should be able to point at the data to be moved, and the Presentation layer will deal with the rest. Encryption is typically done at this level too, though it can be done at the application , session , transport , or network layer ; each having its own advantages and disadvantages. Another example is representing structure, which is normally standardised at this level, often by using XML . As well as simple pieces of data, like strings, more complicated things are standardised in this layer. Two common examples are 'objects' in object-oriented programming , and the exact way that streaming video is transmited. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HTTP , generally regarded as an application layer protocol, has presentation layer aspects such as the ability to identify character encodings for proper conversion, which is then done in the application layer. Examples: AFP, AppleShare File Protocol LPP, Lightweight Presentation Protocol NCP, NetWare Core Protocol NDR, Network Data Representation XDR, eXternal Data Representation X.25 PAD, Packet Assembler/Disassembler Protocol  Layer 5: The session layer The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer . The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either full duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. The Session layer is typically completely unused, but it does have a few places where it is useful. The idea is to allow information on different streams, perhaps originating from different sources, to be properly combined. In particular, it deals with synchronization issues, and ensuring nobody ever sees inconsistent versions of data, and similar things. One application which is fairly intuitively clear is web conferencing . Here, we want to make sure that the streams of audio and video match up - or in other words, that we do not have lipsync problems. We may also want to do "floor control" - ensuring that the person displayed on screen and whose words are relayed is the one selected by the speaker, or by some other criteria. Another big application is in live TV programs, where streams of audio and video need to be seamlessly merged from one to the other so that we do not have half a second of blank airtime, or half a second when we transmit two pictures simultaneously. Examples: ADSP, AppleTalk Data Stream Protocol ASP, AppleTalk Session Protocol NetBIOS, Network Basic Input Output System PAP, Printer Access Protocol PPTP, Point-to-Point Tunnelling Protocol RPC, Remote Procedure Call Protocol RTP, Real-time Transport Protocol RTCP, Real-time Transport Control Protocol SMPP, Short Message Peer-to-Peer SCP, Secure Copy Protocol SSH, Secure Shell  Layer 4: The transport layer In computing and telecommunications , the transport layer is layer four of the seven layer OSI model. It responds to service requests from the session layer and issues service requests to the network layer. The transport layer provides transparent transfer of data between hosts. It is usually responsible for end-to-end error recovery and flow control, and ensuring complete data transfer. In the Internet protocol suite this function is most commonly achieved by the connection oriented Transmission Control Protocol (TCP). The datagram -type transport,User Datagram Protocol (UDP), provides neither error recovery, nor flow control, leaving these to the application . The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer usually turns the unreliable and very basic service provided by the Network layer into a more powerful one. There is a long list of services that can be optionally provided at this level. None of them are compulsory, because not all applications want all the services available. Some can be wasted overhead, or even counterproductive in some cases: Connection-Oriented This is normally easier to deal with than connection-less models, so where the Network layer only provides a connection-less service, often a connection-oriented service is built on top of that in the Transport layer. Same Order Delivery The Network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature, so the Transport layer provides it. The simplest way of doing this is to give each packet a number, and allow the receiver to reorder the packets. Reliable Data The underlying network may well be noisy, and the data received may not always be the same as the data sent. The Transport layer can fix this: typically by providing a checksum of the data which detects if there has been a glitch of some kind. Of course, error free is impossible, but it is possible to substantially reduce the numbers of undetected errors. This layer may also retransmit packets which have gone missing en route. Flow Control The amount of memory on a computer is limited, and without flow control a larger computer might flood a computer with so much information that it can't hold it all before dealing with it. Nowadays, this is not a big issue, as memory is cheap while bandwidth is comparatively expensive, but in earlier times it was more important. Flow control allows the receiver to say "Whoa!" before it is overwhelmed. Sometimes this is already provided by the network, but where it is not, the Transport layer may add it on. Byte Orientation Rather than dealing with things on a packet-by-packet basis, the Transport layer may add the ability to view communication just as a stream of bytes. This is nicer to deal with than random packet sizes, however, it rarely matches the communication model which will normally be a sequence of messages of user defined sizes. Ports Ports are essentially ways to address multiple entities in the same location. For example, the first line of a postal address is a kind of port, and distinguishes between different occupants of the same house. Computer applications will each listen for information on their own ports, which is why you can use more than one network-based application at the same time. On the Internet there are a variety of Transport services, but the two most common are TCP and UDP. TCP is the more complicated, providing a connection and byte oriented stream which is almost error free, with flow control, multiple ports, and same order delivery. UDP is a very simple 'datagram' service, which provides limited error reduction and multiple ports. TCP stands for Transmission Control Protocol, while UDP stands for User Datagram Protocol. Other options are the Datagram Congestion Control Protocol(DCCP) and Stream Control Transmission Protocol (SCTP). Some things, such as connection orientation can be implemented at either Transport or Network layer. The idea is that the Network layer implements whatever set of options is easiest: for some underlying networks it is easiest to implement connectionless communication, while for others it is easiest to implement connection oriented communication. The Transport layer uses this simplest set of options to implement whatever combinations of options are actually desired. Examples: AEP, AppleTalk Echo Protocol ATP, AppleTalk Transaction Protocol DCCP, Datagram Congestion Control Protocol FCP, Fiber Channel Protocol FCIP, Fiber Channel over TCP/IP TCP, Transmission Control Protocol Layer 3: The network layer The network layer is level three of the seven level OSI model. It responds to service requests from the transport layer and issues service requests to the data link layer. The network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching , routing , and controlling the congestion of data packets. In essence, the network layer is responsible for end to end (source to destination) packet delivery, whereas the data link layer is responsible for node to node (hop to hop) packet delivery. The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the transport layer. The Network layer performs network routing, flow control , network segmentation /desegmentation, anderror control functions.The network layer deals with transmitting information all the way from its source to its destination - transmitting from anywhere, to anywhere. Here are some things that the network layer needs to address: i. Network connection-oriented or connectionless The OSI Network Layer protocol can be either connection-oriented, or connectionless. The TCP/IP Internet Layer (equivalent to OSI's Network Layer) supports only the connectionless Internet Protocol (IP). ii. Global Addresses Everybody in the network needs to have a unique address which determines who they are. This address will normally be hierarchical, so you can be "Fred Murphy" to Dubliners, or "Fred Murphy, Dublin " to people in Ireland , or "Fred Murphy, Dublin , Ireland " to people anywhere in the world. On the internet, these addresses are known as IP Numbers. iii. Forward a message This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol ( IPv4 ) doesn't really allow for this, though it has been hacked somewhat since its inception. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make this type of application much smoother. Examples IP/IPv6, Internet Protocol IPSec, Internet Protocol Security IPX, Internetwork Packet Exchange X.25, Packet Level Protocol DDP, Datagram Delivery Protocol Layer 2: The data-link layer The data link layer is layer two of the seven-layer OSI model. It responds to service requests from the network layer and issues service requests to the physical layer. This is the layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment . The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical layer. Examples of data link protocols are Ethernet for local area networks and PPP , HDLC andADCCP for point-to-point connections. The data link is all about getting information from one place to a selection of other places. At this layer one does not need to be able to go everywhere, just able to go somewhere else. So in social contact, one needs to know at least one other person, but not necessarily know Fred, Bob, or James. The data link provides data transfer across the physical link. That transfer might or might not be reliable; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even have any form of checksum to check for transmission errors. In those cases, higher-level protocols must provide flow control, error checking, and acknowledgments and retransmission. In some networks, such as IEEE 802 local area networks, the data link layer is split into MAC and LLC sublayers; this means that the IEEE 802.2 LLC protocol can be used with all of the IEEE 802 MAC layers, such as Ethernet, Token Ring , IEEE 802.11 , etc., as well as with some non-802 MAC layers such as FDDI. Other data link layer protocols, such as HDLC, are specified to include both sublayers, although some other protocols, such as Cisco HDLC , use HDLC's low-level framing as a MAC layer in combination with a different LLC layer. Logical Link Control Sublayer The uppermost sublayer is Logical Link Control (LLC). This sublayermultiplexes protocols running atop the data link layer, and optionally provides flow control, acknowledgment, and error recovery. Media Access Control Sublayer The sublayer below it is Media Access Control (MAC). Sometimes this refers to the sublayer that determines who is allowed to access the media at any one time (usually CSMA/CD ). Other times it refers to a frame structure with MAC addresses inside, specially speaking about ethernet over switches. Examples: ARCnet ATM Controller Area Network (CAN) Ethernet Fiber Distributed Data Interface (FDDI) Frame Relay IEEE 802.2 (provides LLC functions to IEEE 802 MAC layers) IEEE 802.11 wireless LAN LocalTalk Multiprotocol Label Switching (MPLS) Point-to-Point Protocol (PPP) Token ring And most forms of serial communication. Layer 1: The physical layer The physical layer is level one in the seven level OSI model of computer networking. It performs services requested by the data link layer. This level refers to networkhardware , physical cabling or a wireless electromagnetic connection . It also deals with electrical specifications, collision control and other low-level functions. The physical layer is the most basic network layer, providing only the means of transmitting raw bits. The shapes of the electrical connectors , which frequencies to broadcast on, and similar low-level things are specified here. An analogy of this layer in a physical mail network would be a specification for various kinds of paper and ink. The major functions and services performed by the physical layer are: Establishment and termination of a connection to a communications medium. Participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control. Conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel . Examples: EIA standards: RS-232, RS-422, RS-423, RS-449, RS-485 ITU Recommendations: see ITU-T DSL ISDN T1 and other T-carrier links, and E1 and other E-carrier links 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T,1000BASE-SX and other varieties of Ethernet TCP/IP Transmission Control Protocol/Internet Protocol (TCP/IP) is an industry standard protocol stack that is used for communication between Windows based computers. TCP/IP is designed for communication across large-scale networks. The tasks involved in using TCP/IP in the communication process are distributed between protocols that are organized into four distinct layers of the TCP/IP stack. Each protocol in the TCP/IP stack has a distinct role in the communication process. During the communication process, many applications may be in communication at the same time. TCP/IP has the ability to differentiate one application from another. TCP/IP identifies an application on one computer and then moves the data from that application to an application on another computer. TCP/IP Activities The TCP/IP communication process is initiated using an application on the source computer that prepares the data to be transmitted in a format that an application on the destination computer can read. This is similar to writing a letter in a language that the recipient can understand. Then the data is associated with the destination application and computer, much like how you address a letter to a recipient and household. The address of the destination computer is then added to the data, just as the address of the recipient is specified on the letter. After these activities are performed, the data and additional information, including a request for confirmation of its delivery, are sent over the network to the destination. The network medium used for transmitting the data is independent of the above activities, just as the means of transport that transfers the letter from one post office to another is independent of the letters content or address. TCP/IP Protocol: Transmission Control Protocol (TCP) User Datagram Protocol (UDP) Internet Protocol (IP) Internet Control Message Protocol (ICMP) Internet Group Management Protocol (IGMP) Address Resolution Protocol (ARP) TCP/IP Utilities Transmission Control Protocol (TCP) Transmission Control Protocol (TCP) is a required TCP/IP standard protocol that provides a reliable, connection-oriented data delivery service between only two computers. Such a communication is known as a unicast. In connection oriented communication, the connection must be established before data can be transmitted between the two computers. After the connection is established, data is transmitted over this single connection only. Connection-oriented communication is also referred to as reliable communication because it guarantees the delivery of the data at the destination. On the source computer, TCP organizes the data to be transmitted into packets. On the destination computer, TCP reorganizes the packets to recreate the original data.   User Datagram Protocol (UDP) User Datagram Protocol (UDP) is a transport layer protocol that identifies the destination application in network communications. UDP provides a connectionless packet delivery service that offers fast but unreliable, best-effort delivery of the data. UDP does not require an acknowledgment for the data received and does not attempt to retransmit data that is lost or corrupted. This means that less data is sent, but neither the arrival of packets nor the correct sequencing of delivered packets is acknowledged or guaranteed. UDP is used by applications that transmit data to multiple computers by using broadcast or multicast transmissions. It is also used for transmitting small amounts of data or data that is not of high importance. Example uses of UDP include multicasting streaming media, such as during a live videoconference, and broadcasting a list of computer names, which are maintained for local communication. To use UDP, the source application must supply its UDP port number as well as that of the destination application. It is important to note that UDP ports are distinct and separate from TCP ports, even though some of them use the same numbers. Internet Protocol (IP) Internet Protocol (IP) helps to identify the location of the destination computer in a network communication. IP is a connectionless, unreliable protocol that is primarily responsible for addressing packets and routing them between networked computers. Although IP always attempts to deliver a packet, a packet may be lost, corrupted, delivered out of sequence, duplicated, or delayed. However, IP does not attempt to recover from these types of errors by requesting retransmission of the data. Acknowledging the delivery of packets and recovering lost packets is the responsibility of a higher-layer protocol, such as TCP, or of the application itself. Activities Performed by IP You can visualize IP as the mailroom of the TCP/IP stack, where packet sorting and delivery take place. The packets are passed down to IP by UDP or TCP from the transport layer or passed up from the network interface layer. The primary function of IP is to route the packets until they reach their destination. Each packet includes the source IP address of the sender and the destination IP address of the intended recipient. These IP addresses in a packet remain the same throughout the packets journey across a network. If IP identifies a destination address as an address from the same segment, it transmits the packet directly to that computer. If the destination IP address is not on the same segment, IP must use a router to send the information. IP is also responsible for ensuring that a packet does not remain on the network forever by limiting the number of networks across which the packet can travel. This is done by assigning a Time to Live (TTL) number to every packet. A TTL specifies the maximum length of time that the packet can travel on the network before being discarded. Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP) provides troubleshooting facilities and error reporting for undeliverable packets. With ICMP, computers and routers that use IP communication can report errors and exchange limited control and status information. For example, if IP is unable to deliver a packet to a destination computer, ICMP sends a Destination Unreachable message to the source computer. Although the IP protocol is used to move data across routers, ICMP reports errors and control messages on behalf of IP. ICMP does not attempt to make IP a reliable protocol, because ICMP messages are unacknowledged and therefore unreliable. It only attempts to report errors and provide feedback on specific conditions. Although this may not seem effective, it is much more efficient than using bandwidth to acknowledge each ICMP message. Internet Group Management Protocol (IGMP) Internet Group Management Protocol (IGMP) is a protocol that manages the membership lists for IP multicasting in a TCP/IP network. IP multicasting is a process by which a message is transmitted to a select group of recipients, known as a multicast group. IGMP maintains the list of members who subscribe to each multicast group. All of the members of a multicast group listen for IP traffic directed to a specific multicast IP address and receive the packets sent to that IP address. However, because multicasting involves multiple computers, the packets are sent using the unreliable UDP protocol, which does not guarantee the delivery of the packets to the multicast group. When multiple computers need to access information, such as streaming media, an IP address reserved for multicasting is used. Routers that are configured to process multicast IP addresses pick up this information and forward it to all subscribers of the multicast group associated with the multicast IP address. Address Resolution Protocol (ARP) Located in the Internet layer of the TCP/IP suite, Address Resolution Protocol (ARP) performs address resolution for outgoing packets. Address resolution is the process by which IP addresses are mapped to MAC addresses. The network adapters use the MAC address to determine if a packet is meant for that computer. Without the MAC address, the network adapters do not know if they are to pass the data to a higher layer for further processing. As the outgoing packets in the IP layer are being readied for transmission on the network, the source and destination MAC addresses must be added. ARP Cache ARP stores a table containing IP addresses and their corresponding MAC addresses. The area of memory where this table is stored is referred to as the ARP cache. The ARP cache for any computer contains the mappings for only computers and routers that reside on the same segment. Physical Address Resolution ARP compares every outbound packets destination IP address with the ARP cache to determine the MAC address to which the packet will be sent. If there is a matching entry, the MAC address is retrieved from the cache. If not, ARP broadcasts a request for the computer owning the IP address in question to reply with its MAC address. Next, the computer with the corresponding IP address adds the initial computers MAC address to its cache and then replies with its own MAC address. When an ARP reply is received, the ARP cache is updated with the new information and the packet can then be sent. If the packet is going to another segment, ARP resolves the MAC address for the router responsible for that segment, rather than resolving the address for the final destination computer. The router is then responsible for either finding the MAC address of the destination or forwarding the packet to another router.  Network Wiring There are several types of wiring schemas available. So, we have to consider more appropriate schema for our network. EIA/TIA 568A Standard and 568A and 568B Wiring Schemes are most popular wiring standard CAT 5e wiring. 568A Standard: The purpose of EIA/TIA 568A was to create a multiproduct, multivendor, standard for connectivity. Prior to the adoption of this standard, many "proprietary" cabling systems existed. This was very bad for the consumer. Among other things, the standard set the minimum requirements for category-5E cable and hardware. The 568 "standard" is not to be confused with 568A or 568B wiring schemes, which are themselves, part of the "568A standard". 568A & 568B Wiring Schemes: When we refer to a jack or a patch panel's wiring connection, we refer to either the 568A, or 568B wiring scheme, which dictates the pin assignments to the pairs of cat 5E cable. It is very important to note that there is no difference, whatsoever, between the two wiring schemes, in connectivity or performance when connected form one modular device to another (jack to Patch panel, RJ-45 to RJ-45, etc.), so long as they (the two devices) are wired for the same scheme (A or B). The only time when one scheme has an advantage over the other, is when one end of a segment is connected to a modular device, and the other end to a punch block. In which case, the 568A has the advantage of having a more natural progression of pairs at the punch block side. The Category 5 E standard is now officially part of the 568A standard. Category 5e Cable Unshielded Twisted Pair   Category 5 (CAT5) cabling is good, solid cable for 100-Mbps LANs. The Category 5 standard has been around since 1991, so it's well established. You'll find existing Category 5 installations everywhere. If you still have a lot of 10-Mbps equipment, CAT5 cabling will serve your needs. It also handles 100-Mbps Fast Ethernet transmissions very well. Category 5e: The improved Category 5 Category 5e (CAT5e), also known as Enhanced Category 5, was ratified in 1999. It's an incremental improvement designed to enable cabling to support full-duplex Fast Ethernet operation and Gigabit Ethernet. The main differences between Category 5 and Category 5e can be found in the specifications. The performance requirements have been raised slightly in the new standard (see the Buyer's Guide below). Like CAT5, CAT5e is a 100-MHz standard, but it has the capacity to handle bandwidth superior to that of CAT5. With these improvements, you can expect problem-free, full-duplex, 4-pair Ethernet transmissions over your CAT5e UTP.  Network Wiring There are several types of wiring schemas available. So, we have to consider more appropriate schema for our network. EIA/TIA 568A Standard and 568A and 568B Wiring Schemes are most popular wiring standard CAT 5e wiring. 568A Standard: The purpose of EIA/TIA 568A was to create a multiproduct, multivendor, standard for connectivity. Prior to the adoption of this standard, many "proprietary" cabling systems existed. This was very bad for the consumer. Among other things, the standard set the minimum requirements for category-5E cable and hardware. The 568 "standard" is not to be confused with 568A or 568B wiring schemes, which are themselves, part of the "568A standard". 568A & 568B Wiring Schemes: When we refer to a jack or a patch panel's wiring connection, we refer to either the 568A, or 568B wiring scheme, which dictates the pin assignments to the pairs of cat 5E cable. It is very important to note that there is no difference, whatsoever, between the two wiring schemes, in connectivity or performance when connected form one modular device to another (jack to Patch panel, RJ-45 to RJ-45, etc.), so long as they (the two devices) are wired for the same scheme (A or B). The only time when one scheme has an advantage over the other, is when one end of a segment is connected to a modular device, and the other end to a punch block. In which case, the 568A has the advantage of having a more natural progression of pairs at the punch block side. The Category 5 E standard is now officially part of the 568A standard. Category 5e Cable Unshielded Twisted Pair   Category 5 (CAT5) cabling is good, solid cable for 100-Mbps LANs. The Category 5 standard has been around since 1991, so it's well established. You'll find existing Category 5 installations everywhere. If you still have a lot of 10-Mbps equipment, CAT5 cabling will serve your needs. It also handles 100-Mbps Fast Ethernet transmissions very well. Category 5e: The improved Category 5 Category 5e (CAT5e), also known as Enhanced Category 5, was ratified in 1999. It's an incremental improvement designed to enable cabling to support full-duplex Fast Ethernet operation and Gigabit Ethernet. The main differences between Category 5 and Category 5e can be found in the specifications. The performance requirements have been raised slightly in the new standard (see the Buyer's Guide below). Like CAT5, CAT5e is a 100-MHz standard, but it has the capacity to handle bandwidth superior to that of CAT5. With these improvements, you can expect problem-free, full-duplex, 4-pair Ethernet transmissions over your CAT5e UTP. COLOR-CODE STANDARDS Here are the diagrams:   Note that the TX (transmitter) pins are connected to corresponding RX (receiver) pins, plus to plus and minus to minus.  And that you must use a crossover cable to connect units with identical interfaces.  If you use a straight-through cable, one of the two units must, in effect, perform the cross-over function. Two wire color-code standards apply: EIA/TIA 568A and EIA/TIA 568B. The codes are commonly depicted with RJ-45 jacks as follows (see below figure from the front of the jacks): Network Cabling Tools Modular Plug Tool   We need a modular crimping tool during the cabling of the twisted pair and RJ 45 connectors. It's a tool in a shape of pliers which will securely crimp RJ-45 connectors.  Even though the crimper has cutters, which we use to cut cables and individual wires, and perhaps stripes the outer jacket. UTP Stripping Tool   This tool works neat and produces good finishing; as well this tool is used to cut cables and individual wires. Diagonal Cutters   This tool is used for easier to cut the cable off at the reel and to fine tune the cable ends during the process of assembling.  Windows Server 2003 Operating Systems Windows Server 2003 operating systems take the best of Windows 2000 Server technology and make it easier to deploy, manage, and use. The result: a highly productive infrastructure that helps make your network a strategic asset for your organization. As of March 28, 2005 , all Windows Server 2003 operating systems ship with Windows Server Service Pack 1 (SP1). Windows Server 2003 SP1 provides enhanced security, increased reliability, and a simplified administration to help enterprise customers across all industries. Windows Server 2003 includes all the functionality customers need today from a Windows Server operating system to do more with less, such as security, reliability, availability, and scalability. In addition, Microsoft has improved and extended the Windows server operating systems to incorporate the benefits of Microsoft .NET for connecting information, people, systems, and devices. Windows Server 2003 is a multipurpose operating system capable of handling a diverse set of server roles, depending on your needs, in either a centralized or distributed fashion. Some of these server roles include: •  File and print server. •  Web server and Web application services. •  Mail server. •  Terminal server. •  Remote access and virtual private network (VPN) server. •  Streaming media server. Installing Server 2003 Standard Edition To successfully install Windows server 2003 standard edition, it is very important that you assess the system, document it, plan the installation, and then methodically follow through with your plan. Insert the Server 2003 Standard Edition CD ROM and click the applicable Install Windows button. To install windows from across a network, launch the winnt32.exe program on the network drive containing the windows setup files,and then proceed within setup normally. Choose new Installation (Advanced) from the Installation Type box and then click Next to perform a clean install of Windows. following window will be appears   Read the licensee agreement, choose the I accept This Agreement option, press F8 to continue   On the next screen you are asked to choose the disk and partition on which you want to install windows appears   After you finish modifying the partition, select the partition on which you want to install windows and press enter. If you choose to create new partition, you are prompted to format the partition using either the NTFS or FAT file system, using either a quick format or full format, Choose the NTFS Quick format if you know the disk has not bad sectors or history of file corruption, otherwise choose NTFS full Format and press enter Setup confirms the choice of partition and gives you the option of converting the partition to NTFS if it's not an NTFS partition already. If you want to reformat the hard drive, choose the NTFS Quick format option if you know the disk has no bad sector or history on file corruption, otherwise choose NTFS full format and press enter   Press ENTER to format   Formatting   Setup checks the hard disk for error and then copies the appropriate files into the newly created Windows folder. After Setup finishes copying files, it prompts you to remove any floppy or CD -ROM and then it restart the system and starts the windows Setup Wizard   Windows setup wizard   The setup Wizard detects and configures the devices installed on the computer/If Setup can not properly detected a device, it display a Device Configuration dialog box for manual configuration of the device.   After the hardware is detected, you are prompted to configure the regional settings, if you haven't already. These settings affect such factors as keyboard layout and how dates and currency values are displayed. Configure this option as appropriate and then click next   Type the name of the person the computer is to be registered under as well as the organization   Type the product key if you haven't already, and then click next   Choose the licensing mode in the next window, either Per Server or Per Seat. If you choose Per Server, Specify how many CALs(Certificate Authority License) you purchased   Type the name of the computer in the computer Name text Box. The computer name can contain the numbers zero to nine, uppercase and lowercase letters, and hyphen character. The must be DNS compatibility with per-Windows XP clients. Review the date, and time zone information, make any necessary correction, and then click next to configure your network settings   Select custom setting . And then click next   Choose the typical Settings option to install the following commonly used networks protocols and services. Clients for Microsoft Networks, File And Printer Sharing fro Microsoft Networks, and Transmission Control Protocol/ Internet Protocol configured to use DHCP   Highlight the TCP/IP and click properties. In the following window.   In the following window enter the required under the 'General' tab and click ok in the following window.   In the Workgroup or Domain window enter the name of our workgroup or domain. Setup will finish copying Setup will then finish the installation and reboot windows server 2003. After Setup restarts your computer, you see the standard windows logon screen.  File Server Many people mistake file servers for a high-end storage system, but in reality, file servers do not need to possess great power or super fast computer specifications. A form of disk storage that hosts files within a network File servers do not need to be high-end but must have enough disk space to incorporate a large amount of data. File servers generally offer some form of system security to limit access to files to specific users or groups. In a confidential organizations like the Hospital has lot of details of many numbers of patients. These are hierarchical views of the computing environment which treat users, directories, computers, applications and files as distinct but related entities on the network and grant access based on user or group credentials.In many cases, the directory service spans many file servers, potentially hundreds for large organizations. In the past, and in smaller organizations, authentication can take place directly to the server itself. Installing File Server Select the File Server and click next   This wizard will guide us to give the limit and warning disk space for new user. click next   Here this wizard ask us to keep index service to the folders or not select yes and click next   This wizard gives the summary of the service click next.   Then the share folder wizard will be appears. click next   Select a folder path to implement the file server and click next   In the Name, Description, and settings dialog box, type a share name and a description of the Folder's contents   Click change to set weather and how the contents of the folder are available for offline use. Set the permission for the shared and click finish click Close   Printer servers A computer in a network that controls one or more printers is known as the print server. It is either part of the network operating system or an add-on utility that stores the print-image output from users' machines and feeds it to the printer one job at a time. The computer and its printers are known as a "Print Server" or a file server with "print services." Printer servers are computers that manage the communication between printers and the clients' computers that want to print to the printers. Printers can be connected to the print server by network connection, with parallel or serial port connection, or via a Universal Serial Bus or IEEE 1394 (Fire wire or iLink) connection. Although USB printers and old parallel printers are popular among consumers and can be connected to the Windows server 2003 print server, easier to locate, and reduce the amount of processing power used on the print server. Installing Network Printer Click configure your server wizard Click Printer server and Click Next then the following wizard will appears   Click on Windows 2000 and windows XP clients only, to install printer and the drive to Windows XP and windows 2000 clients.   Summary of selection wizard Connect the printer to the network and configure the printer with the proper TCP/IP settings for the network. And click Next Choose Add printer from the file menu to start the printer wizard. Then the following wizard will appears   Click next   Select the Local Printer Attached to My computer option, and clear the Automatically Detect and install My plug and Play Printer check box as it is. Click next   Select Use the following port and click next   From this wizard it ask to select the printer driver software make sure your printer driver software and click next   This wizard ask us to name the printer, here we can keep exiting printer name or we can change the printer name, What we type her that only going to display in printer wizard   Here select share the printer and click next   This wizard will ask us to give a location and the description about the location; there for give location and description where we can remember easily   Click finish, to finish the network printer   The installed printer will be shown with a hand in bottom of the icon.   Printer Permission Levels Printers are resources and thus can be protected like any other resource using the security features of windows server2003. Printers have owners and access control lists, which specify the permissions each user or group has. The Creator of the printer is an automatically made the owner of the printer, and only users with sufficient permissions can take the ownership of the printer from creator. The permission can divide into three types. They are: Print Manage Documents Manage Printer Changing printer Permission 1. Open the printer and Fax Folder located on the Start menu. 2. Right click the printer you want to modify, and then choose properties from the shortcut menu.   3. Click the security tab in the printer's properties dialog box.   4. To change the permission for a group, select the group from the list, and then select the check boxes in the permission section of the dialog box to choose which permission to give to each group. Changing Spool settings Print spooling, or storing a print job on disk before printing, affects how clients perceive printing performance and the actual printing speed. To change the spool settings for a printer, right click the printer you want to modify, select properties from the shortcut menu, and then use the advanced tab to modify the spool settings. Spool print Documents so program finished printing faster spools the print documents to the printer server, freeing the client to perform other tasks more quickly. To reduce the time it takes to print a document, select Start Printing Immediately. To ensure that the entire document is available to the printer when printing begins, select Star printing problems, and also helps high priority documents print before low priority documents.   Hold Mismatched Documents: Holds documents in the queue that did not match the current printer settings. Other documents in the print queue are unaffected by the held documents. Print Spooled Documetns First: Prints the highest priority document that is already spooled first, ahead of higher priority documents that are still spooling. This step speeds overall printer through out put by keeping the printer from waiting for documents in the departments. Keep Printed Documents: Keeps a copy of print job in the printer queue. In this circumstance, the user can resubmit the document directly from the queue rather than printing from his application second time. Implementing VPN VPN is an external link and simply an extension of private network. Virtual Private Networks (VPN) technology uses private encrypted tunneling, allowing networking of remote workers, branch office locations, and central corporate sites without dedicated physical circuits. VPN offers companies a safe, inexpensive means of using the Internet to access their internal networks. Installing VPN Installing VPN, go to configure your server wizard and select Remote Access / VPN server as in figure 10.5.1 Figure 10.5.1: Server Role Figure 10.5.1 wizard helps to connect Remote Access /VPN enable remote clients to connect the network through either dial up connection or VPN security connection, click next will show the summarize selection wizard of the VPN server access   Figure 10.5.2: Summary of Selection Click next to change our selection and continue installation, then the following new wizard will appear. This Routing and Remote Access Server Setup Wizard will help you to connect your clients' remote, click next   Figure 10.5.3: Welcome Figure 10.5.4 help to configuration the services select custom configuration and click next then the following Figure 10.5.5 will display   Figure 10.5.4: Configuration From Figure 10.5.5 select VPN access and LAN routing and click next   Figure 10.5.5: Custom Configuration Complete the Routing and Remote Access Server Setup Wizard will be display and click finish.   Figure 10.5.6: Complete After few minutes the wizard will give the message that Remote Access and VPN access were successfully set up this server as remote access, click finish