Compiled by :

S. AgarwalS. AgarwalLecturer & Systems Incharge

St. Xavier’s Computer Centre

St. Xavier’s College, Kolkata.

IntroductionIntroduction

CommunicationCommunication - means to an exchange of information between two or more parties and it can be exchange in a variety of way, that might be in the form of word, letters, messages drawing, body movement or any other symbols that represents the idea which make to other understandable.

Data communicationData communication : entails electronically exchanging data or information. It is the movement of computer information from one point to another by means of electrical or optical transmission system. This system often is called data communication data communication networks. networks.

The key technology of the information age is computer communicationscomputer communications.  •The value of high-speed data communication network is that it brings the message sender and receiver closer together in time. 

•It facilitates more efficient use of computers and improves the day to day control of business by providing faster information flow.

A computer network is a system in which a set of computers and/or other communication facilities are connected to each other. It is a collection of computers which are in some way connected such that they can exchange data between themselves and other computers on the network. It might involve physical wire, infrared or radio frequency communication, and can be usually used to share files, devices and connections to other computers. In short, a computer network allows your computers to talk to each other.

Reasons for networking:

To allow users to communicate with To allow users to communicate with each other.each other.

•Pass data between users without the use of floppy disks — Files can be copied and accessed across the network, eliminating the time wasted and inconvenience caused by using floppy disks to transfer files. There is also less restriction on the size of file that can be transferred over the network.

To share hardware.To share hardware.

•Share expensive peripherals, such as printers — All of the computers can access the same printer, modems or CD-ROM tower.

To share software and information.To share software and information.•Centralize key computer programs, such as finance and accounting programs, word processing, time and billing — It is often important that all users have access to the same program (and not copies of it) so that they can work on it simultaneously. Networking allows offices to have such a central program that all users can access. •Automate unattended backup of valuable — It is always essential to keep backups of any important files. You can automate this procedure by having a computer program that backs up the files for you. Without a network, you would have to manually copy files, which is time consuming.

Basic Components of a Network

Server or host computer: A server is a machine that provides clients with service. Servers share a specific resource with other computers. Examples of the servers are the database server, that provides the large database and the communication server that provides connection to another network, to commercial database, or to a powerful processor. In larger LANs, the server is dedicated to being a server. In a peer-to-peer LAN, the server may be both a server and a client computer. The server stores data or software that can be accessed by the client. There are file, database, network, access, modem, facsimile, printer, and geteway server. Servers are usually microcomputers (often more powerful than the other microcomputers on the network), but they may be minicomputers or mainframes.

Client: A client is a computer such as a PC or a workstation attached to the network, which is used to access shared network resources. Client computer uses the resources shared by server computer. The client is the input/output hardware device at the other end of communication circuit. It typically provides users with access to the network and data and software on the server. There are three major categories of clients: terminals, minicomputers/workstation, and special purpose terminals.

Circuit: The circuit is the pathway through which the message passes. Circuits can be twisted pair cable, coaxial cable, fiber optic cable, microwave transmission, and so forth. There are many devices in the circuit that perform special functions, such as hubs, switches, routers and gateways.

•Cables & Connectors – Cables carry electronic signals from one computer to another & Connectors attach cables to network interface cards.

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.

Unshielded Twisted Pair (UTP) CableTwisted pair cabling comes in two varieties: shielded and unshielded.

The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot.

Unshielded Twisted Pair ConnectorThe standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector. A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.

Shielded Twisted Pair (STP) CableA disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky.

Coaxial CableCoaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable.

Coaxial Cable ConnectorsThe most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector.

Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.

Fiber Optic CableFiber Optic CableFiber optic cabling consists of a center glass core surrounded by several layers of protective materials It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify.

Wireless LANsWireless LANs

Not all networks are connected with cabling; some networks are wireless. These use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.

Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.

Wireless LANs have several disadvantages. They are very expensive, provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.

Bits per secondBits per second

In data communications, bits per second (abbreviated bps and, by some, bit/sec) is a common measure of data speed for computer modems and transmission carriers. As the term implies, the speed in bps is equal to the number of bits transmitted or received each second.

Larger units are sometimes used to denote high data speeds. One kilobit per second (abbreviated Kbps) is equal to 1,000 bps. One megabit per second (Mbps) is equal to 1,000,000 bps or 1,000 Kbps.

Protocol - Protocol can be defined as an agreed-upon format for transmitting data between two devices. The protocol determines the following:

•The type of error checking to be used •Data compression method, if any •How the sending device will indicate that it has finished sending a message •How the receiving device will indicate that it has received a message

There are many standard protocols, among them: AppleTalk, Ethernet, NetBEUI, and TCP/IP

EthernetThe Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). 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 effect the speed of transmission on the network.

The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps.

LocalTalk / AppletalkLocalTalk 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.

Token RingThe Token Ring protocol was developed by IBM in the mid-1980s. The access method used involves token-passing. In Token Ring, the computers are connected so that the signal travels around the network from one computer to another in a logical ring. A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If a computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the data is captured by the receiving computer. 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. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased.

FDDIFiber Distributed Data Interface (FDDI) is a network protocol that is used primarily to interconnect two or more local area networks, often over large distances. The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete ring. A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbps.

ATMAsynchronous Transfer Mode (ATM) is a network protocol that transmits data at a speed of 155 Mbps and higher. ATM works by transmitting all data in small packets of a fixed size; whereas, other protocols transfer variable length packets. ATM supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable.

ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local area networks.

Protocol Cable Speed Topology

Ethernet Twisted Pair, Coaxial, Fiber 10 Mbps Linear Bus, Star, Tree

Fast Ethernet Twisted Pair, Fiber 100 Mbps Star

LocalTalk Twisted Pair .23 Mbps Linear Bus or Star

Token Ring Twisted Pair 4 Mbps - 16 Mbps Star-Wired Ring

FDDI Fiber 100 Mbps Dual ring

ATM Twisted Pair, Fiber 155-2488 Mbps Linear Bus, Star, Tree

Network Architectures

Network architecture guides a network builder to make a plan or structure by which the component parts are related. Network architecture is a framework of rules to ease the operation, maintenance, and growth of a communication network by isolating the user and the application programs from the details of the network.

The work done by any application program can be separated in to four general functions. The first one is data storage, almost all application programs requires data to be stored and retrieved. The section function is data access logic, when data is stored than it need to be accessed. The third one is application and the fourth function is presentation logic. These four functions are the basic building block of any application.

There are three fundamental network architecture: 1. Host Based Architecture 2. Clint Based Architecture 3. Clint-Server Architecture

Host-based architecture: Host-based architecture performs virtually all the network. It was the very first data communication network architecture and the host computer performs all four function of application program. This is the simple architecture and works well.

In host based network all data are flow from the one central host. Client computers enable users to send and receive the message from the host computer. Client computers send the message to the host computer for processing and accepted the instruction from the host on what to display. In host based network all work must be done by the host computer because of these host computers become overloaded and can not process quickly for the all user's demand. Response time become slower and network managers are required to spend more money to upgrade the host computers.

Client based architecture: The clients are the microcomputer on a LAN and host computers are servers on the same network.

In client based architecture all process must be done by the client computer or the application software on the client computer is responsible for the all four application program functions; the server simply stores the data. As host based architecture, this architecture also have overload problem, since all data are process from the client so if more users are in active in the same network than logically the network circuit can not perform well and be overloaded. The fundamental problem of this architecture is all the data from the server must travel to the clients for processing which makes slow the entire network.

Client server architecture: In client-server computing, several servers may work together over the network to support the business application. Client-server architecture makes a balance between client computer and host computer.

In client server architecture, they split the application program function in to two groups, client handles the presentation logic, and server handle the data access logic and data storage while application logic may be on both or can reside either on client or on the server. Because of its nature, the network is not overloaded with entire files being transferred back and forth through the network circuit for processing at each remote terminal. For example; if the insurance company wants to request the list of all the customer and their insurance police, the client would accept the request than format the request to be understood by the server and transmit it to the server. On the other side, upon receiving the request form the client, the server search the database for all the requested and than transmit only the matching request to the client.

Network SoftwareNetwork Software - computer applications used to operate the network and its many components.

What is a Network Operating System?Unlike operating systems, such as DOS and Windows, that are designed for single users to control one computer, network operating systems (NOS) coordinate the activities of multiple computers across a network. The network operating system acts as a director to keep the network running smoothly.

The two major types of network operating systems are: Peer-to-Peer & Client/Server

Peer-to-PeerPeer-to-peer network operating systems allow users to share resources and files located on their computers and to access shared resources found on other computers. However, they do not have a file server or a centralized management source. 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. Peer-to-peer networks are designed primarily for small to medium local area networks. AppleShare and Windows for Workgroups are examples of programs that can function as peer-to-peer network operating systems.

Advantages of a peer-to-peer network:Less initial expense - No need for a dedicated server.

Setup - An operating system (such as Windows) already in place may only need to be reconfigured for peer-to-peer operations.

Disadvantages of a peer-to-peer network:Decentralized - No central repository for files and applications. Security - Does not provide the security available on a client/server network.

Client/ServerClient/server network operating systems allow the network to centralize functions and applications in one or more dedicated file servers. The file servers become the heart of the system, providing access to resources and providing security.

Individual workstations (clients) have access to the resources available on the file servers. The network operating system provides the mechanism to integrate all the components of the network and allow multiple users to simultaneously share the same resources irrespective of physical location. Novell Netware and Windows NT/2000 Server are examples of client/server network operating systems.

Advantages of a client/server network:         Centralized - Resources and data security are controlled through the server.          Scalability - Any or all elements can be replaced individually as needs increase.          Flexibility - New technology can be easily integrated into system.          Interoperability - All components (client/network/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.

LAN TOPOLOGIES :

What is a Topology?

The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations.

Main Types of Physical Topologies

Linear BusA linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals) are connected to the linear cable. Ethernet and LocalTalk networks use a linear bus topology.

Advantages of a Linear Bus Topology         Easy to connect a computer or peripheral to a linear bus.          Requires less cable length than a star topology.

Disadvantages of a Linear Bus Topology         Entire network shuts down if there is a break in the main cable.          Terminators are required at both ends of the backbone cable.          Difficult to identify the problem if the entire network shuts down.          Not meant to be used as a stand-alone solution in a large building.

Star A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator.

Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.

Advantages of a Star Topology         Easy to install and wire.          No disruptions to the network then connecting or removing devices.          Easy to detect faults and to remove parts.

Disadvantages of a Star Topology         Requires more cable length than a linear topology.          If the hub or concentrator fails, nodes attached are disabled.          More expensive than linear bus topologies because of the cost of the concentrators.

Ring TopologyRing topologies are used on token ring networks. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. A token, or small data packet, is continuously passed around the network. When a device needs to transmit, it reserves the token for the next trip around, then attaches its data packet to it. The receiving device sends back the packet with an acknowledgment of receipt, then the sending device puts the token back out on the network. The most common type of cabling used for token ring networks is twisted pair, although there are nine different types that can be used. With IBM Type 1 Shielded cable, you can have up to 33 network segments with 260 devices on each. Transmission rates are at either 4 or 16 megabits per second.

Advantages •Very orderly network where every device has access to the token and the opportunity to transmit •Performs better than a star topology under heavy network load •Can create much larger network using Token Ring

Disadvantages•One malfunctioning workstation or bad port in the MAU can create problems for the entire network •Moves, adds and changes of devices can affect the network •Network adapter cards and MAU's are much more expensive than Ethernet cards and hubs •Much slower than an Ethernet network under normal load

TreeA tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages of a Tree Topology         Point-to-point wiring for individual segments.          Supported by several hardware and software venders.

Disadvantages of a Tree Topology         Overall length of each segment is limited by the type of cabling used.          If the backbone line breaks, the entire segment goes down.          More difficult to configure and wire than other topologies.

Mesh Topology

A Mesh topology consists of a network where every device on the network is physically connected to every other device on the network. This provides a great deal of performance and reliability, however the complexity and difficulty of creating one increases as the number of nodes on the network increases. For example, a three or four node mesh network is relatively easy to create, whereas it is impractical to set up a mesh network of 100 nodes. Mesh networks are not used much in local area networks (LANs) but are used in Wide Area Networks (WANs) where reliability is important and the number of sites being connected together is fairly small.

Considerations When Choosing a Topology:

Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators.

Length of cable needed. The linear bus network uses shorter lengths of cable.

Future growth. With a star topology, expanding a network is easily done by adding another concentrator.

Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.

Topology Configuration Advantage Disadvantage

Star All nodes are connected to the central computer

Relatively efficient;Nodes are free to join or quit from the network

Dependent on the central computer

Ring All nodes are connected in a continuous loop

High speed Whole network will break down if one node fails

Bus All nodes are connected to a single cable and share it

Easy to install and cheap;No need to restart the network if a node fails or joins the network

Comparatively low speed