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Introduction to Communications
Data Communications and NetworksMr. Greg VoglUganda Martyrs UniversityLecture 1, 19 March 2003
19 March 2003 Data Communications and Networking: Introduction to Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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Modem Modulator-Demodulator
Sender modem modulates (digitalanalog) Old telephone system uses analog signals Receiver demodulates (analogdigital)
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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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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 Communications
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Other Data Communications Microwave, wireless, satellite The public switched telephone
network The mobile telephone system Cable television