Computer Networks UNIT I

The factors to be considered In deciding a network is a good network or not are Performance Performance can be measured using transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time Is the elapsed time between an Inquiry and a response. Other factors include no. Of users, the type of transmission medium, capability of hardware and efficiency of software.

Reliability Network Reliability is measured by the frequency of failure, the time it takes to link to recover from a failure. SecurityNetwork security Includes protecting the data from unauthorized access. Types of Networks based on physical Connection A link Is a communications pathway that transfers data from one device to another. There are two types of connections: Point-to-Point A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices.

Workstation Multiplier A Multicolumn(also called Multiword) connection Is one In which more than two specific devices share a single link. If several devices can use the link simultaneously, it is aMainframe The Physical topology is the way in which the devices are physically connected in a network. The topology of a network is the geometric representation of the relationship of all the links and linking devices called nodes. There are 4 basic topologies: 1.

Mesh 2. Star 3. Us 4. Ring 1 . Mesh topology In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects.

A fully connected mesh network has n(n-1)/2 physical channels to link n devices.Every device in the network must have n-1 1/0 ports. Advantages The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems A mesh topology is robust. If one link becomes unusable, the network do not fails It is more secure because the link is not shared. Point-to-point links make fault identification and fault isolation easy Disadvantages Amount of cabling and no.

Of ports required is more Installation and reconnection are difficult since every device must be connected to every other device. Wiring occupies more spaceThe hardware required to connect each link can be expensive. A mesh topology can be used in a limited fashion. For example connecting the main computers of a network. - as a backbone In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub.

Star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device. Advantages Less expensive than mesh topologyEach device needs only one link and one 1/0 port to connect any number of devices Easy to install and reconfigure It is robust. If one link fails, only that link is affected. More cabling is required to link each device to central hub but is lesser than that of mesh topology 3.

Bus Topology Bus topology is multiplier. One long cable acts as a backbone to link all the devices in a network. Nodes are connected to the bus cable by drop lines and taps. Drop Line Tap cable end Drop Drop Drop line Tap Cable end A drop line is a connection running between the device and the main cable. A tap is injector.

This is a limit on the number of taps a bus can support and on the distance between those taps. Ease of Installation Less cabling than mesh or star topologies Only the backbone cable stretches through the entire network Disadvantages Difficult to add new devices Signal reflection at the taps can cause degradation in quality which can be controlled by limiting the number and spacing of devices. 4. Ring Topology In a ring topology, each device has a dedicated point-to-point connection only with the two devices on either side of it.

A signal is passed along the ring in one direction, room device to device, until it reaches its destination.Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. Easy to install and reconfigure To add or delete a device requires changing only two connections Fault isolation is simplified A signal is circulating at all times Unidirectional traffic A break in the ring can disable the entire network. Categories of Networks Three primary categories of network are 2.

Metropolitan area network (MAN) 3. Wide area network (WAN) Local Area Network (LANA) A LANA is privately owned and links the devices in a single office, building, or campus.Lana are designed to allow resources to be shared between personal computers or workstations. The resources to be shared can include hardware, software or data. One of the computers may be given a large capacity disk drive and may become a server to the client. Software can be stored on this central server and used as needed by the whole group.

In addition to size, Lana are distinguished from other types of networks by their transmission media and topology. Lana have data rates in the 4 to 16 Mbps range. Metropolitan Area Network (MAN) A metropolitan Area network (MAN) is designed to extend over an entire city.It may be a single network such as a cable television network, or it may be a means of connecting a number of Lana into a larger network so that resources may be shared LANA-to-LANA as well as device-to-device. For example, a company can use a MAN to connect the Lana in all its offices throughout the city.

Wide Area Network (WAN) A Wide area Network (WAN) provides long-distance transmission of data, voice, image and video information over large geographic areas that may comprise a country, a continent, or even the whole world. In contrast to Lana, Wants may be utilize public, leased or private communication equipment usually in combinations.A WAN that is wholly owned and used by a single company is referred to an enterprise network. When two or more networks are connected, they become an Networking, or internet.

A protocol is a set of rules that governs data communication. A protocol defines what is communicated, how it is communicated and when it is communicated. The key elements of a protocol are: 1 . Syntax 2. Semantics 3. Timing.

Syntax: It refers to the structure or format of data, 'e the order in which they should be represented. Egg-a simple protocol might expect the first 8 bits of the data to be the address of the sender.The second 8 bit for the receiver address and the rest is the information. Semantics: It refers to the meaning of each section of bits. How is the particular pattern to be interpreted and what action is to be taken based on that interpretation. Timing: It refers to two characteristics.

1. When data should be send. 2. How fast it should be send.

Standards Standards are essential in creating and maintaining an open and competitive market for equipment manufactures and in guaranteeing national and international interoperability of data and telecommunication technology. Some of the SST committees are- 1.ISO- International SST Organization 2. TU-T-lnternationalTelecommunication Union-Telecommunication American National Standard Institute 4. IEEE- Institute of Electrical and Electronics Engineers 5. EIA- Electronics Industries Association OSI MODEL SST.

3. ANSI- Systems Interconnection (SO') model. An Open System is a model that allows any two different systems to communicate regardless of their underlying architecture. The purpose of OSI model is to communicate between different systems without squiring changes to the logic of the underlying hardware and software. Layered Architecture The OSI model is built of 7 ordered layers: 1.Physical layer (layer 1) 2.

Data link layer (layer 2) 3. Network layer (layer 3) 4. Transport layer (layer 4) 5. Session layer (layer 5) 6.

Presentation layer (layer 6) 7. Application layer (layer 7) As the message travels from A to B, it may pass through many intermediate nodes. These nodes involve only the first 3 layers of the OSI model as shown in the figure. Peer-to-Peer Process Within a single machine, each layer calls upon the services of the layer Just below it.

The processes on each machine that communicate at a given layer are called peer-to- stream of bits to Machine B.At the higher layers, communication must move down through the layers on machine A, over to machine B, and then back up through the layers. Each layer in the sending machine adds its own information to the message it receives from the layer Just above it and passes it to the layer Just below it. This information is added in the form of headers or trailers. Interfaces between layers The passing of the data and network information down through the layers of the ending machine and back up through the layers of the receiving machine is made possible by an interface between each pair of adjacent layers.

Organization of the layers The 7 layers can be grouped into 3 subgroups 1 . Network Support Layers Layers - Physical, Data link and Network are the network support layers. They deal with the physical aspects of moving data from one device to another such as electrical specifications, physical addressing, transport timing and reliability. 2. Transport Layer Layers, transport layer, ensures end-to-end reliable data transmission on a single link. 3.

User Support Layers Layers 5,6,7 - Session, presentation and application are the user support layers.They allow interoperability among unrelated software systems Functions of the Layers 1. Physical Layer The physical layer coordinates the functions required to transmit a bit stream over a physical medium. The physical layer is concerned with the following: Physical characteristics of interfaces and media The physical layer defines the characteristics of the interface between the devices and the transmission medium. Representation of bits To transmit the stream of bits, it must be encoded to signals. The physical layer fines the type of encoding.

Data Rate The transmission rate-the number of bits sent each second - is also defined by the physical layer. Synchronization of bits The sender and receiver must be synchronized at the bit level. Their clocks must be synchronized. Line Configuration In a point-to-point configuration, two devices are connected together through a dedicated link. In a multiplier configuration, a link is shared between several devices. Physical Topology can be connected using a mesh, bus, star or ring topology.

Transmission Mode The physical layer also defines the direction of transmission between two devices: implied, half-duplex or full-duplex. . Data Link Layer The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery. At this layer, data packets are encoded and decoded into bits. The data link layer is divided into two subleases: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

The MAC sublease controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking. The duties are:Framing- the ODL divides the stream of bits received from the n/w layer into data units called frames. Physical addressing- if frames are to be distributed to different systems on the n/w , the ODL adds a header to the frame to define the sender and receiver. Flow control- if the rate at which the data are absorbed by the receiver is less than the rate produced in the sender ,the ODL imposes a flow CTR mechanism. Error control- used for detecting and retransmitting damaged or lost frames and to prevent duplication of frames.

This is achieved through a trailer added at the end of he frame.