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Note for Data Communications - DC by Sunil Reddy

  • Data Communications - DC
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Data Communication Model • Data transmission deals with transmission of signals in reliable & efficient manner. Communication Model • Simple model of communication is shown Example: Communication between workstation & server over telephone network • • I. Another example is exchange of voice signals between two telephone over same network. Key elements of model are: Source Device generates data to be transmitted Ex:-telephone or pcs II. Transmitter Data generated by source system are not as it is transmitted Rather, a transmitter transforms & encodes information in such a way to produce electromagnetic signals that can be transmitted across transmission system Ex:-Modem takes digital bit stream from attached device such as PC & transforms bit stream into analog signals that can be handled by telephone networks. III. Transmission System This can be single or complex network connecting source & destination. IV. Receiver Receiver accepts signal from transmission system and converts it into a form that can be handled by destination device. Ex: - Modem will accept an analog signal coming from network or transmission line & convert it into digital bit stream.

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V. Destination Takes incoming data from receiver key tasks that must be performed in data communication streams are: Communication task are • Transmission system utilization Make efficient use of transmission facilities that are typically shared among a no. of communicating device. Various techniques (multiplexing) are used to allocate total capacity of transmission medium among users. • Interface To communicate, a device must interface with transmission system • Signal Generation Once, the interface is established, signal generation is required for communication. Properties of signal such as form & intensity must be such that signal is capable of being propagated through transmission system and it should be interpretable as data at receiver. • Synchronization There should be some form of synchronization between transmitter & receiver. The receiver must be able to determine when a signal begins to arrive & when it ends. • Exchange Management If data are to be exchanged in both directions over a period of time, the two parties must cooperate. Ex:- Telephonic conversation • Error Detection & Correction In all communication systems, there is potential for error, transmitted signals are distorted to some extent before reaching their destination. Error detection & correction are necessary as errors cannot be tolerated. Ex:- in file transfer, it is not acceptable for contents of file to be altered. • Flow Control Flow control is required to assure that source does not overwhelm the destination by sending data faster than they can be processed & absorbed. • Addressing and Routing When more than two devices share a transmission facility, a source system must indicate the identity of intended destination Transmission system must assure that the destination system & only that system, receives data. A specific route through this network must be chosen. • Recovery Recovery concept is different from error correction Recovery techniques are needed in situations in which an information exchange such as database transaction or file transfer is interrupted due to a fault somewhere in the system Objective is to either resume activity at the pint of interruption or atleast to restore the state of systems involved to condition prior to beginning of exchange • Message Formatting It is the agreement between two parties as to the form of data to be exchanged or transmitted, such as binary code for characters • Security The sender of the data may wish to be assured that only intended receiver actually receives the data. Receiver of data may wish to be assured that received data have not been altered in transit And data is from the proposed sender. • Network Management Network Management is needed to configure the system, monitor its status, react to failures and overloads and plan intelligently for future growth.

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Simplified Data Communication Model Fig: Simplified data communication model Let us consider, the user of pc wishes to send a message m to another user... We can view it as a sequence of bits of memory… PC is connected to some transmission medium such as local network or telephone line by a receiving device such as local network transreceiver or modem. Input data are transferred to transmitter as sequence of voltage shifts [g (t)] representing bits on some communication bus or cable. Transmitter is directly connected to medium and converts incoming stream g (t) into signal s (t) suitable for transmission. As the transmitted signal is subject to impairments before it reaches receiver, it may be different from received signal. Receiver will attempt to estimate the original signal s (t) based on r (t) & its knowledge of medium, producing a sequence of bits g’(t). These bits are sent to output personal computer, where they are buffered in memory as block of bits (g’). The destination stream will try to determine & rectify if any error has occurred, by co-operating with source These data are sent to intended user via an output device such as printer, screen in the form of message (m’). DATA COMMUNICATION NETWORKING Sometimes, it is not possible for two communication devices to be point to point connected because of following contingencies • The devices are very far apart, it would be expensive to connect them. Ex:-To string a dedicated link between two devices thousands of kilometres apart • There is set of devices, each of which may require a link to many of others at various times. Ex:All the telephones in the world and all the terminals and computers owned by single organizations it is impractical to provide a dedicated wire between each pair of devices. Solution to this problem is to attach each device to communication network. There are two categories of communication network which are classified as 1. WAN(Wide Area Network) 2. LAN(Local Area Network) WAN(Wide Area Network) It covers large geographical area; typically WAN consists of number of interconnected switching modes. A transmission from any device is routed through these internal nodes to specified destination device. These nodes are not concern with the content of data; rather their purpose is to provide a switching facility that will move data from node to node until they reach a destination WANs are implemented using various technologies: i. ii. iii. Circuit switching & Packet Switching Frame Relay ATM Networks

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Circuit Switching In this dedicated communication path is established between two stations through nodes of network Path is connected in sequence of physical links between nodes On each link, logical channel is dedicated to connection Data generated by source station is transmitted along dedicated path as rapidly as possible At each node, incoming data are routed or switched to appropriate outgoing channel without delay Ex: Telephone network Packet Switching It is not necessary to dedicate communication path through the network Data are sent in sequence of small chunks, called packets Each packet is passed through network from node to node along some path to ensure from source to destination At each node entire packet is received stored briefly and they are transmitted to real node Ex: Terminal to computer and computer to computer connection Frame Relay There is considerable amount of overhead built into packet switching schemes to compensate for errors. The overhead includes additional bits added to each packet to introduce redundancy & additional processing at the end stations and intermediate switching nodes to detect and recover from errors. With the modern high speed tele-communication systems, this overhead is unnecessary and counterproductive. It is unnecessary because the rate of errors has been dramatically lowered and any remaining errors can easily be caught in end system by logic that operates above the level of packet switching logic. Frame relay was developed to take advantage of this high data rates and low error rates Original packet switching networks were designed with a data rate to end user of about 64KBps, frame relay network are designed to operate efficiency at user data rates of upto 2MBps Key to achieve these high data rates is to strip out more of overhead involved with error control. ATM (Asynchronous Transfer Mode) Asynchronous Transfer Mode (ATM), sometimes referred to as cell relay. ATM can be viewed as an evolution from frame relay Difference between frame relay and ATM is that Frame relay uses variable length packets, called frames and ATM uses fixed length packets called cells As with frame relay, ATM provides little overhead for error control, depending on internet reliability of transmission system and on higher layers of logic in end system to catch and correct errors By using fixed length packet, processing overhead is reduced even further for ATM compared to frame relay. The result is ATM is designed to work in range of 10s & 100s of Mbps ATM can also be viewed as an evolution from circuit switching, with circuit switching only fixed data rate circuits are available to end systems. ATM allows definition of multiple virtual channels with data rates that dynamically defined at the time virtual channel is overhead created. ATM offers constant data rate channel even though it is using packet switching technique. LAN-Local Area Networks LAN is communication network that interconnects a variety of devices & provides a means for information exchange among those devices. These are several key distinctions between LANs & WANs. i. The scope of LAN is small, typically a single building or a cluster of buildings. ii. LAN is owned by some organization that owns attached devices.

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