Lecture Notes on COMPUTER NETWORKS

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Published Date:09-07-2017
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ROEVER ENGINEERING COLLEGE DEPARTMENT OF ECE EC2352-COMPUTER NETWORKS PREPARED BY K.BALAJI, ME., AP/ECE UNIT-I PHYSICAL LAYER 1. Define data communication. It is the exchange of data between two devices via some form of Transmission medium ( such as copper cable,twisted pair cable etc). 2. What are the elements of data communication? The elements of data communication are  Sender   Receiver   Transmission medium   Message   Protocol   3. How we can check the effectiveness of data communication? The effectiveness of data communication can be checked by  Accuracy   Delivery   Timeliness   4. What are the classes of transmission media? The classes of transmission media are  Guided transmission media   Unguided transmission media   5. Define Optical fiber It is a method of transmitting information from one place to another by sending light through an optical fiber. 6. Define distributed processing A task is divided among multiple computers. Instead of single large machine handling all the process, each separate computer handles the subset 7. What do you mean by OSI? Open system interconnection model is a model for understanding and designing a network architecture. It is not a protocol. 8. Define Network. A network is a set of devices connected by physical media links. A network is recursively is a connection of two or more nodes by a physical link or two or more networks connected by one or more nodes 9. What is a Link? At the lowest level, a network can consist of two or more computers directly connected by some physical medium such as coaxial cable or optical fiber. Such a physical medium is called as Link. 10 What is point-point link? If the physical links are limited to a pair of nodes it is said to be point-point link. 11. What is Multiple Access? If the physical links are shared by more than two nodes, it is said to be Multiple Access. 12. Define Switch Switches are hardware or software devices capable of creating temporary Connections between two or more devices 13. What are the types of switching? The types of switching are  Circuit switching    Packet switching    Message switching   14. What do you mean by Crossbar switches? It connects m inputs to n outputs in a grid using electronic micro switches at each cross points. 15. Define Blocking The reduction in the number of cross points result in a phenomenon called Blocking 16. Define packet switching In packet switching data are transmitted in discrete units of potentially variable length blocks called Packets 17. What are the approaches of packet switching? The approaches of packet switching are  Virtual circuit    Datagram   18. What do you mean by Permanent Virtual circuit? The same Virtual circuit is provided between two users on a continuous basis. The circuit is dedicated to the specific user 19. What do you mean by DSL? It is a new technology that uses the existing telecommunication network to accomplish high speed delivery of data, voice & video etc. 20. What is the purpose of Physical layer? The physical layer coordinates the functions required to transmit a bit stream over a physical medium. PART-B DEFINE ISO-OSI LAYER? ISO-OSI 7-Layer Network Architecture This lecture introduces the ISO-OSI layered architecture of Networks. According to the ISO standards, networks have been divided into 7 layers depending on the complexity of the functionality each of these layers provide. The detailed description of each of these layers is given in the notes below. We will first list the layers as defined by the standard in the increasing order of function complexity: 1. Physical Layer 2. Data Link Layer 3. Network Layer 4. Transport Layer 5. Session Layer 6. Presentation Layer 7. Application Layer Physical Layer This layer is the lowest layer in the OSI model. It helps in the transmission of data between two machines that are communicating through a physical medium, which can be optical fibres,copper wire or wireless etc. The following are the main functions of the physical layer: 1. Hardware Specification: The details of the physical cables, network interface cards, wireless radios, etc are a part of this layer. Network Layer Its basic functions are routing and congestion control. Routing: This deals with determining how packets will be routed (transferred) from source to destination. It can be of three types: 15. Static: Routes are based on static tables that are "wired into" the network and are rarely changed. 16. Dynamic: All packets of one application can follow different routes depending upon the topology of the network, the shortest path and the current network load. 17. Semi-Dynamic: A route is chosen at the start of each conversation and then all the packets of the application follow the same route. Transport Layer Its functions are: 1. Multiplexing / Demultiplexing : Normally the transport layer will create distinct network connection for each transport connection required by the session layer. The transport layer may either create multiple network connections (to improve throughput) or it may multiplex several transport connections onto the same network connection (because creating and maintaining networks may be expensive). In the latter case, demultiplexing will be required at the receiving end. A point to note here is that communication is always carried out between two processes and not between two machines. This is also known as process-to-process communication. 2. Fragmentation and Re-assembly: The data accepted by the transport layer from the session layer is split up into smaller units (fragmentation) if needed and then passed to the network layer. Correspondingly, the data provided by the network layer to the transport layer on the receiving side is re-assembled. Presentation Layer This layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate data structures to be exchanged can be defined in abstract way along with standard encoding. It also manages these abstract data structures and allows higher level of data structures to be defined an exchange. It encodes the data in standard agreed way (network format). Suppose there are two machines A and B one follows 'Big Endian' and other 'Little Endian' for data representation. This layer ensures that the data transmitted by one gets converted in the form compatible to other machine. This layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate data structures to be exchanged can be defined in abstract way along with standard encoding. It also manages these abstract data structures and allows higher level of data structures to be defined an exchange. Other functions include compression, encryption etc. Application Layer The seventh layer contains the application protocols with which the user gains access to the network. The choice of which specific protocols and their associated functions are to be used at the application level is up to the individual user. Thus the boundary between the presentation layer and the application layer represents a separation of the protocols imposed by the network designers from those being selected and implemented by the network users. For example commonly used protocols are HTTP(for web browsing), FTP(for file transfer) etc. DEFINE TCP/IP BRIEFLY? TCP/IP PROTOCOL: In most of the networks today, we do not follow the OSI model of seven layers. What is actually implemented is as follows. The functionality of Application layer and Presentation layer is merged into one and is called as the Application Layer. Functionalities of Session Layer is not implemented in most networks today. Also, the Data Link layer is split theoretically into MAC (Medium Access Control) Layer and LLC (Link Layer Control). But again in practice, the LLC layer is not implemented by most networks. So as of today, the network architecture is of 5 layers only. DEFINE GUIDED AND UNGUIDED TRANSMISSION MEDIA? GUIDED AND UNGUIDED TRANSMISSION MEDIA: Physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as 1 bit and not as 0 bit. In physical layer we deal with the communication medium used for transmission. Types of Medium Medium can be classified into 2 categories. 1. Guided Media: Guided media means that signals is guided by the prescence of physical media i.e. signals are under control and remains in the physical wire. For eg. copper wire 2. Unguided Media: Unguided Media means that there is no physical path for the signal to propagate. Unguided media are essentially electro-magnetic waves. There is no control on flow of signal. For eg. radio waves. Communication Links In a network nodes are connected through links. The communication through links can be classified as 1. Simplex: Communication can take place only in one direction. eg. T.V broadcasting. 2. Half-duplex: Communication can take place in one direction at a time. Suppose node A and B are connected then half-duplex communication means that at a time data can flow from A to B or from B to A but not simultaneously. eg. two persons talking to each other such that when speaks the other listens and vice versa. 3. Full-duplex: Communication can take place simultaneously in both directions. eg. A discussion in a group without discipline. Links can be further classified as 1. Point to Point: In this communication only two nodes are connected to each other. When a node sends a packet then it can be received only by the node on the other side and none else. 2. Multipoint: It is a kind of sharing communication, in which signal can be recieved by all nodes. This is also called broadcast. Generally two kind of problems are associated in transmission of signals. 1. Attenuation: When a signal transmits in a network then the quality of signal degrades as the signal travels longer distances in the wire. This is called attenuation. To improve quality of signal amplifiers are used at regular distances. 2. Noise: In a communication channel many signals transmit simultaneously, certain random signals are also present in the medium. Due to interference of these signals our signal gets disrupted a bit. Bandwidth Bandwidth simply means how many bits can be transmitted per second in the communication channel. In technical terms it indicates the width of frequency spectrum. Transmission Media Guided Transmission Media In Guided transmission media generally two kind of materials are used. 1. Copper o Coaxial Cable o Twisted Pair 2. Optical Fiber 1. Coaxial Cable: Coaxial cable consists of an inner conductor and an outer conductor which are seperated by an insulator. The inner conductor is usually copper. The outer conductor is covered by a plastic jacket. It is named coaxial because the two conductors are coaxial. Typical diameter of coaxial cable lies between 0.4 inch to 1 inch. The most application of coaxial cable is cable T.V. The coaxial cable has high bandwidth, attenuation is less. Twisted Pair: A Twisted pair consists of two insulated copper wires, typically 1mm thick. The wires are twisted togather in a helical form the purpose of twisting is to reduce cross talk interference between several pairs. Twisted Pair is much cheaper then coaxial cable but it is susceptible to noise and electromagnetic interference and attenuation is large. Twisted Pair can be further classified in two categories: Unshielded twisted pair: In this no insulation is provided, hence they are susceptible to interference. Shielded twisted pair: In this a protective thick insulation is provided but shielded twisted pair is expensive and not commonly used. The most common application of twisted pair is the telephone system. Nearly all telephones are connected to the telephone company office by a twisted pair. Twisted pair can run several kilometers without amplification, but for longer distances repeaters are needed. Twisted pairs can be used for both analog and digital transmission. The bandwidth depends on the thickness of wire and the distance travelled. Twisted pairs are generally limited in distance, bandwidth and data rate. 3. Optical Fiber: In optical fiber light is used to send data. In general terms presence of light is taken as bit 1 and its absence as bit 0. Optical fiber consists of inner core of either glass or plastic. Core is surrounded by cladding of the same material but of different refractive index. This cladding is surrounded by a plastic jacket which prevents optical fiber from electromagnetic interference and harshly environments. It uses the principle of total internal reflection to transfer data over optical fibers. Optical fiber is much better in bandwidth as compared to copper wire, since there is hardly any attenuation or electromagnetic interference in optical wires. Hence there is fewer requirements to improve quality of signal, in long distance transmission. Disadvantage of optical fiber is that end points are fairly expensive. (eg. switches) Differences between different kinds of optical fibers: 1. Depending on material Made of glass Made of plastic. 2. Depending on radius Thin optical fiber Thick optical fiber 3. Depending on light source LED (for low bandwidth) Injection lased diode (for high bandwidth) Wireless Transmission 1. Radio: Radio is a general term that is used for any kind of frequency. But higher frequencies are usually termed as microwave and the lower frequency band comes under radio frequency. There are many application of radio. For eg. cordless keyboard, wireless LAN, wireless ethernet but it is limited in range to only a few hundred meters. Depending on frequency radio offers different bandwidths. 2. Terrestrial microwave: In terrestrial microwave two antennas are used for communication. A focused beam emerges from an antenna and is received by the other antenna, provided that antennas 3. 4. should be facing each other with no obstacle in between. For this reason antennas are situated on high towers. Due to curvature of earth terrestrial microwave can be used for long distance communication with high bandwidth. Telecom department is also using this for long distance communication. An advantage of wireless communication is that it is not required to lay down wires in the city hence no permissions are required. 3. Satellite communication: Satellite acts as a switch in sky. On earth VSAT(Very Small Aperture Terminal) are used to transmit and receive data from satellite. Generally one station on earth transmits signal to satellite and it is received by many stations on earth. Satellite communication is generally used in those places where it is very difficult to obtain line of sight i.e. in highly irregular terrestrial regions. In terms of noise wireless media is not as good as the wired media. There are frequency band in wireless communication and two stations should not be allowed to transmit simultaneously in a frequency band. The most promising advantage of satellite is broadcasting. If satellites are used for point to point communication then they are expensive as compared to wired media . NETWORK TOPOLOGIES: A network topology is the basic design of a computer network. It is very much like a map of a road. It details how key network components such as nodes and links are interconnected. A network's topology is comparable to the blueprints of a new home in which components such as the electrical system, heating and air conditioning system, and plumbing are integrated into the overall design. Taken from the Greek work "Topos" meaning "Place," Topology, in relation to networking, describes the configuration of the network; including the location of the workstations and wiring connections. Basically it provides a definition of the components of a Local Area Network (LAN). A topology, which is a pattern of interconnections among nodes, influences a network's cost and performance. There are three primary types of network topologies which refer to the physical and logical layout of the Network cabling. They are: 1. Star Topology: All devices connected with a Star setup communicate through a central Hub by cable segments. Signals are transmitted and received through the Hub. It is the simplest and the oldest and all the telephone switches are based on this. In a star topology, each network device has a home run of cabling back to a network hub, giving each device a separate connection to the network. So, there can be multiple connections in parallel. Advantages o Network administration and error detection is easier because problem is isolated to central node o Networks runs even if one host fails o Expansion becomes easier and scalability of the network increases o More suited for larger networks Disadvantages o Broadcasting and multicasting is not easy because some extra functionality needs to be provided to the central hub o If the central node fails, the whole network goes down; thus making the switch some kind of a bottleneck o Installation costs are high because each node needs to be connected to the central switch  Bus Topology: The simplest and one of the most common of all topologies, Bus consists of a single cable, called a Backbone, that connects all workstations on the network using a single line. All transmissions must pass through each of the connected devices to complete the desired request. Each workstation has its own individual signal that identifies it and allows for the requested data to be returned to the correct originator. In the Bus Network, messages are sent in both directions from a single point and are read by the node (computer or peripheral on the network) identified by the code with the message. Most Local Area Networks (LANs) are Bus Networks because the network will continue to function even if one computer is down. This topology works equally well for either peer to peer or client server. The purpose of the terminators at either end of the network is to stop the signal being reflected back. Advantages o Broadcasting and multicasting is much simpler o Network is redundant in the sense that failure of one node doesn't effect the network. The other part may still function properly o Least expensive since less amount of cabling is required and no network switches are required o Good for smaller networks not requiring higher speeds Disadvantages o Trouble shooting and error detection becomes a problem because, logically, all nodes are equal o Less secure because sniffing is easier o Limited in size and speed  Ring Topology: All the nodes in a Ring Network are connected in a closed circle of cable. Messages that are transmitted travel around the ring until they reach the computer that they are addressed to, the signal being refreshed by each node. In a ring topology, the network signal is passed through each network card of each device and passed on to the next device. Each device processes and retransmits the signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. There is a very nice feature that everybody gets a chance to send a packet and it is guaranteed that every node gets to send a packet in a finite amount of time. Advantages o Broadcasting and multicasting is simple since you just need to send out one message o Less expensive since less cable footage is required o It is guaranteed that each host will be able to transmit within a finite time interval o Very orderly network where every device has access to the token and the opportunity to transmit o Performs better than a star network under heavy network load Disadvantages o Failure of one node brings the whole network down o Error detection and network administration becomes difficult o Moves, adds and changes of devices can effect the network o It is slower than star topology under normal load Generally, a BUS architecture is preferred over the other topologies - ofcourse, this is a very subjective opinion and the final design depends on the requirements of the network more than anything else. Lately, most networks are shifting towards the STAR topology. Ideally we would like to design networks, which physically resemble the STAR topology, but behave like BUS or RING topology. DEFINE TOKEN RING? IEEE 802.4: Token Bus Network In this system, the nodes are physically connected as a bus, but logically form a ring with tokens passed around to determine the turns for sending. It has the robustness of the 802.3 broadcast cable and the known worst case behavior of a ring. The structure of a token bus network is as follows: A 802.4 frame has the following fields: Preamble: The Preamble is used to synchronize the receiver's clock. Starting Delimiter (SD) and End Delimiter (ED): The Starting Delimiter and Ending Delimiter fields are used to mark frame boundaries. Both of them contain analog encoding of symbols other than 1 or 0 so that they cannot occur accidentally in the user data. Hence no length field is needed. Frame Control (FC): This field is used to distinguish data frames from control frames. For data frames, it carries the frame's priority as well as a bit which the destination can set as an acknowledgement. For control frames, the Frame Control field is used to specify the frame type. The allowed types include token passing and various ring maintenance frames. Destination and Source Address: The Destination and Source address fields may be 2 bytes (for a local address) or 6 bytes (for a global address). Data: The Data field carries the actual data and it may be 8182 bytes when 2 byte addresses are used and 8174 bytes for 6 byte addresses. Checksum: A 4-byte checksum calculated for the data. Used in error detection. Mechanism: When the first node on the token bus comes up, it sends a Claim_token packet to initialize the ring. If more than one station sends this packet at the same time, there is a collision. Collision is resolved by a contention mechanism, in which the contending nodes send random data for 1, 2, 3 and 4 units of time depending on the first two bits of their address. The node sending data for the longest time wins. If two nodes have the same first two bits in their addresses, then contention is done again based on the next two bits of their address and so on. After the ring is set up, new nodes which are powered up may wish to join the ring. For this a node sends Solicit_successor_1 packets from time to time, inviting bids from new nodes to join the ring. This packet contains the address of the current node and its current successor, and asks for nodes in between these two addresses to reply. If more than one nodes respond, there will be collision. The node then sends a Resolve_contention packet, and the contention is resolved using a similar mechanism as described previously. Thus at a time only one node gets to enter the ring. The last node in the ring will send a Solicit_successor_2 packet containing the addresses of it and its successor. This packet asks nodes not having addresses in between these two addresses to respond. A question arises that how frequently should a node send a Solicit_successor packet? If it is sent too frequently, then overhead will be too high. Again if it is sent too rarely, nodes will have to wait for a long time before joining the ring. If the channel is not busy, a node will send a Solicit_successor packet after a fixed number of token rotations. This number can be configured by the network administrator. However if there is heavy traffic in the network, then a node would defer the sending of bids for successors to join in. There may be problems in the logical ring due to sudden failure of a node. What happens when a node goes down along with the token? After passing the token, a node, say node A, listens to the channel to see if its successor either transmits the token or passes a frame. If neither happens, it resends a token. Still if nothing happens, A sends a Who_follows packet, containing the address of the down node. The successor of the down node, say node C, will now respond with a Set_successor packet, containing its own address. This causes A to set its successor node to C, and the logical ring is restored. However, if two successive nodes go down suddenly, the ring will be dead and will have to be built afresh, starting from a Claim_token packet. When a node wants to shutdown normally, it sends a Set_successor packet to its predecessor, naming its own successor. Bit stuffing The third method allows data frames to contain an arbitrary number of bits and allows character codes with an arbitrary number of bits per character. At the start and end of each frame is a flag byte consisting of the special bit pattern 01111110. Whenever the sender's data link layer encounters five consecutive 1s in the data, it automatically stuffs a zero bit into the outgoing bit stream. This technique is called bit stuffing. When the receiver sees five consecutive 1s in the incoming data stream, followed by a zero bit, it automatically destuffs the 0 bit. The boundary between two frames can be determined by locating the flag pattern. \ UNIT-II DATA LINK LAYER 1. What do you mean by Automatic Repeat Request (ARQ)? ARQ means retransmission of data in three cases:  Damaged Frame   Lost Frame   Lost Acknowledge   2. What are the responsibilities of Data Link Layer? The Data Link Layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-node delivery.   Framing  Physical Addressing   Flow Control   Error Control   Access Control   3. What are the three protocols used for noisy channels? The three protocols used for noisy channels  Stop – and – Wait ARQ   Go – back – N ARQ   Selective Repeat ARQ   4. What is CSMA/CD? Carrier Sense Multiple Access with Collision Detection is a protocol used to sense whether a medium is busy before transmission and it also has the ability to detect whether the packets has collided with another 5. What are the various types of connecting devices? There are five types of connecting devices  Repeaters   Hubs   Bridges   Routers   Switches.  6. Define Flow control It refers to a set of procedures used to restrict the amount of data the sender can sent before waiting for an acknowledgement 7. What are the categories of Flow control? The categories of Flow control are  Stop& wait   Sliding Window   8. Mention the disadvantages of stop& wait.  Inefficiency   Slow process   9. What are the functions of data link layer? The functions of data link layer are  Flow control   Error control   10. Define Link Discipline It coordinates the link system. It determines which device can send and when it can send. 11. What do you mean by polling? When the primary device is ready to receive data, it asks the secondary to send data. This is called polling. 12.What are the various controlled access methods? The various controlled access methods are  Reservation   Token passing   Polling  13 What are the various Random access methods?\ The various Random access methods are  Slotted ALOHA   CSMA   CSMA/CD,CSMA/CA  14. Define Piconet A Bluetooth network is called Piconet .It can have up to eight stations one of which