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

19 March 2003 Data Communications and Networking: Introduction to Communications2 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

19 March 2003 Data Communications and Networking: Introduction to Communications3 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

19 March 2003 Data Communications and Networking: Introduction to Communications4 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

19 March 2003 Data Communications and Networking: Introduction to Communications5 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

19 March 2003 Data Communications and Networking: Introduction to Communications6 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)

19 March 2003 Data Communications and Networking: Introduction to Communications7 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

19 March 2003 Data Communications and Networking: Introduction to Communications8 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

19 March 2003 Data Communications and Networking: Introduction to Communications9 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

19 March 2003 Data Communications and Networking: Introduction to Communications10 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

19 March 2003 Data Communications and Networking: Introduction to Communications11 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

19 March 2003 Data Communications and Networking: Introduction to Communications12 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

19 March 2003 Data Communications and Networking: Introduction to Communications13 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.)

19 March 2003 Data Communications and Networking: Introduction to Communications14 DTE/DCE Computer->modem->phone lines- >modem->computer DTE->DCE->analog signal->DCE->DTE Computer=data terminating equipment Modem=data circuit-terminating equipment

19 March 2003 Data Communications and Networking: Introduction to Communications15 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

19 March 2003 Data Communications and Networking: Introduction to Communications16 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

19 March 2003 Data Communications and Networking: Introduction to Communications17 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

19 March 2003 Data Communications and Networking: Introduction to Communications18 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

19 March 2003 Data Communications and Networking: Introduction to Communications19 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

19 March 2003 Data Communications and Networking: Introduction to Communications20 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

19 March 2003 Data Communications and Networking: Introduction to Communications21 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

19 March 2003 Data Communications and Networking: Introduction to Communications22 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

19 March 2003 Data Communications and Networking: Introduction to Communications23 Other Data Communications Microwave, wireless, satellite The public switched telephone network The mobile telephone system Cable television