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Introduction to Communications

Introduction to Communications. Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture 1, 19 March 2003. Overview. Communication principles Asynchronous and synchronous transmission Packet switching, routing, errors, flow control

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Introduction to Communications

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  1. Introduction to Communications Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture 1, 19 March 2003

  2. Overview • Communication principles • Asynchronous and synchronous transmission • Packet switching, routing, errors, flow control • Simplex, half-duplex, duplex operation • Serial vs. parallel transmission • Modems incl. modulation techniques • Multiplexing • Guided transmission media • Other data communications Data Communications and Networking: Introduction to Communications

  3. Communication Principles • Data is sent by sender • Data is received by receiver • Check if data was received correctly • Receiver informs sender of receipt • If not correctly received, attempt error recovery (either repair or resend data) • Various media join sender and receiver Data Communications and Networking: Introduction to Communications

  4. Communication Principles • Standards of communication are used • RS232C, X.21 • Protocols specify who says what when • Digital data sent using analog media like voltages (0=low, 1=high) • Data rate in bps = inverse of signal duration Data Communications and Networking: Introduction to Communications

  5. Types of Connections • Synchronous • Sender and receiver use same data rate • Best for large amounts of data (nearly 100% efficient, % overhead is small) • Asynchronous • Requires start, stop, and parity information • Parity bit checks if data was corrupted • 0=even number of 1’s, 1=odd Data Communications and Networking: Introduction to Communications

  6. Reliable Transmission • Analog signals deteriorate or attenuate with distance • Larger amounts of data increase probability of error and need to resend • (e.g. 10MB Internet download) Data Communications and Networking: Introduction to Communications

  7. Packets • So a large data message is divided into packets which are sent individually • How big should the packets be? • Tradeoff: overhead vs. chances of error • Receiver reassembles packets in order • Each packet has an ID Data Communications and Networking: Introduction to Communications

  8. Routing Across Networks • Connection-based (virtual circuit) • Virtual connection is open, all data sent along same path, then connection closed • Guaranteed correct sequence • Connectionless (datagram) • Each packet routed independently • Packets can arrive out of sequence • Receiver must put in correct sequence Data Communications and Networking: Introduction to Communications

  9. Error Detection/Correction • Detection • Parity bit (only works for one-bit errors) • Checksum or cyclical redundancy check • Uses polynomial to generate remainder • Correction/Recovery • If receiver sends ACK, no need to resend • If receiver sends NAK, repeat data • If no reply after a timeout period, resend data • If timeout a few times, give up Data Communications and Networking: Introduction to Communications

  10. Flow Control • Sender can send multiple packets • Receiver’s ACK tells which it received • Receiver may be busy processing • Receiver can send RNR • Received but not ready for more yet • Data lines can be used for flow control Data Communications and Networking: Introduction to Communications

  11. Types of Connections • Simplex • One direction only • Half-duplex • One direction at a time • Full-duplex • Both directions at the same time • May need two sets of wires Data Communications and Networking: Introduction to Communications

  12. Types of Connections • Serial • Sequentially, one bit at a time (one wire) • Slow (few characters/sec.) • Uses: Modem, mouse, keyboard • Parallel • Several bits at a time (several wires) • Wire signals can skew so cables are short • Uses: Printer Data Communications and Networking: Introduction to Communications

  13. Modem • Modulator-Demodulator • Sender modem modulates (digitalanalog) • Old telephone system uses analog signals • Receiver demodulates (analogdigital) • Carrier signal is modulated • Change wave: amplitude, frequency, phase • Changing two can make bits/sec > baud • Standards for bit rate, baud rate, mod. • V series (V.21, V.90, etc.) Data Communications and Networking: Introduction to Communications

  14. DTE/DCE • Computer->modem->phone lines->modem->computer • DTE->DCE->analog signal->DCE->DTE • Computer=data terminating equipment • Modem=data circuit-terminating equipment Data Communications and Networking: Introduction to Communications

  15. Multiplexing • Several terminals communicate w/ host • Each has its own process/socket • Multiple data inputs into one channel • Data rate divided by no. of channels Data Communications and Networking: Introduction to Communications

  16. Time Division Multiplexing • each terminal polled during its time slot • need start and stop bits if asynchronous • wastes time when no data to send • any terminal can send at any time • include sender’s ID • use input storage buffer to hold >1 input Data Communications and Networking: Introduction to Communications

  17. Frequency Division Multiplexing • Uses analog line; best for asynchronous • Each terminal given frequency channel • Data rate divided by no. of channels • Guard bands waste some of bandwidth • Same as tuning to a radio station Data Communications and Networking: Introduction to Communications

  18. Cable Transmission Limitations • Signal can be damaged • attenuation over long distance; regenerate • interference, line noise, etc. • Maximum cable length depends on: • Signalling methods (analog/digital) • Access method • Data rates/signal speed • Type of cable and its physical properties Data Communications and Networking: Introduction to Communications

  19. Guided Transmission Media • Twisted Pair Wire • Unshielded (UTP): 100 m • Shielded (STP) • Co-axial cable • Thinnet: 200 m • Thicknet: 500 m • Fiber Optic Cable: 2 km Data Communications and Networking: Introduction to Communications

  20. Twisted pair cable categories • Category 1: RS232 telephone cable • Category 2: 4 Mbit/s: token ring • Category 3: 16 Mbit/s: older Ethernet • Category 4: 20 Mbit/s: token ring • Category 5: 100 Mbit/s: LANs, ATM • most common for Ethernet LANs Data Communications and Networking: Introduction to Communications

  21. Connectors • D-Connectors • 25-pin RS232C, <15m • BNC connectors • T and round • used with coax cable by LANs and cable TV • RJ-45 connectors • 8-wire plug four pairs • used with UTP by different types of LANs Data Communications and Networking: Introduction to Communications

  22. EIA/TIA 568B specification • Wire/pin layout for RJ-45 network plug • white/orange • orange/white • white/green • blue/white • white/blue • green/white • white/brown • brown/white Data Communications and Networking: Introduction to Communications

  23. Other Data Communications • Microwave, wireless, satellite • The public switched telephone network • The mobile telephone system • Cable television Data Communications and Networking: Introduction to Communications

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