Unit-1 Introduction to Computer Networks Basic Communication Model Communication model is used to exchange data between two parties. For example communication between a computer, server and telephone (through modem). Source Data to be transmitted is generated by this device, example: telephones, personal computers etc. Transmitter The data generated by the source system are not directly transmitted in the form they are generated. The transmitter transforms and encodes the information in such a form to produce electromagnetic waves or signals. Transmission System A transmission system can be a single transmission line or a complex network connecting source and destination. Receiver Receiver accepts the signal from the transmission system and converts it to a form which is easily managed by the destination device. Destination Destination receives the incoming data from the receiver. Data Communication The exchange of data between two devices through a transmission medium is Data Communication. The data is exchanged in the form of 0’s and 1’s. The transmission medium used is wire cable. For data communication to occur, the communication device must be part of a communication system. Data Communication has two types Local and Remote which are discussed below : Local : Local communication takes place when the communicating devices are in the same geographical area, same building, face-to-face between individuals etc. Remote : Remote communication takes place over a distance i.e. the devices are farther. Effectiveness of a Data Communication can be measured through the following features : 1. Delivery : Delivery should be done to the correct destination. 2. Timeliness : Delivery should be on time. 3. Accuracy : Data delivered should be accurate. 4. Components of Data Communication
1. 2. 3. 4. 5. Message : It is the information to be delivered. Sender : Sender is the person who is sending the message. Receiver : Receiver is the person to him the message is to be delivered. Medium : It is the medium through which message is to be sent for example modem. Protocol : These are some set of rules which govern data communication. Network Topology Network Topology is the schematic description of a network arrangement, connecting various nodes (sender and receiver) through lines of connection. 1. BUS Topology Bus topology is a network type in which every computer and network device is connected to single cable. When it has exactly two endpoints, then it is called Linear Bus topology. Features of Bus Topology 1. It transmits data only in one direction. 2. Every device is connected to a single cable Advantages of Bus Topology 1. 2. 3. 4. 5. It is cost effective. Cable required is least compared to other network topology. Used in small networks. It is easy to understand. Easy to expand joining two cables together. Disadvantages of Bus Topology 1. 2. 3. 4. Cables fails then whole network fails. 1If network traffic is heavy or nodes are more the performance of the network decreases. Cable has a limited length. It is slower than the ring topology. 2. RING Topology It is called ring topology because it forms a ring as each computer is connected to another computer, with the last one connected to the first. Exactly two neighbors for each device. Features of Ring Topology
1. A number of repeaters are used for Ring topology with large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network. 2. The transmission is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology. 3. In Dual Ring Topology, two ring networks are formed, and data flow is in opposite direction in them. Also, if one ring fails, the second ring can act as a backup, to keep the network up. 4. Data is transferred in a sequential manner that is bit by bit. Data transmitted, has to pass through each node of the network, till the destination node. Advantages of Ring Topology 1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data. 2. Cheap to install and expand Disadvantages of Ring Topology 1. Troubleshooting is difficult in ring topology. 2. Adding or deleting the computers disturbs the network activity. 3. Failure of one computer disturbs the whole network. 3. STAR Topology In this type of topology all the computers are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node. Features of Star Topology 1. Every node has its own dedicated connection to the hub. 2. Hub acts as a repeater for data flow.
3. Can be used with twisted pair, Optical Fibre or coaxial cable. Advantages of Star Topology 1. 2. 3. 4. 5. Fast performance with few nodes and low network traffic. Hub can be upgraded easily. Easy to troubleshoot. Easy to setup and modify. Only that node is affected which has failed, rest of the nodes can work smoothly. Disadvantages of Star Topology 1. 2. 3. 4. Cost of installation is high. Expensive to use. If the hub fails then the whole network is stopped because all the nodes depend on the hub. Performance is based on the hub that is it depends on its capacity 4. MESH Topology It is a point-to-point connection to other nodes or devices. All the network nodes are connected to each other. Mesh has n(n-2)/2 physical channels to link n devices. There are two techniques to transmit data over the Mesh topology, they are : 1. Routing 2. Flooding Routing In routing, the nodes have a routing logic, as per the network requirements. Like routing logic to direct the data to reach the destination using the shortest distance. Or, routing logic which has information about the broken links, and it avoids those node etc. We can even have routing logic, to re-configure the failed nodes. Flooding In flooding, the same data is transmitted to all the network nodes, hence no routing logic is required. The network is robust, and the its very unlikely to lose the data. But it leads to unwanted load over the network. Types of Mesh Topology 1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh topology but some devices are only connected to two or three devices. 2. Full Mesh Topology : Each and every nodes or devices are connected to each other.