Data Transmission Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture...

Data Transmission

Data Communications and Networks

Mr. Greg Vogl

Uganda Martyrs University

Lecture 4, 28 March 2003

28 March 2003 Data Communications and Networking: Data Transmission

2

Overview

• Signals, noise and errors

• Modulation and modems

• Synchronisation

• Baseband and broadband


3

Sources

• Hodson Ch. 1, 2, 3, 8.2

• Stamper pp. 67-75, Ch. 3

• BITDCO lectures 3-7– Recommended, easily accessible at UMU– Go to DCN website, click Links– HTML and GIF files all fit on one floppy disk


4

Digitising

• Analog digital conversion uses sampling• Amplitude of each sample converted to bits• Accuracy of digitised info. depends on

– Number of bits in sample– How often the samples are taken (period)– Linear or nonlinear digitising of amplitude– Compression/decompression algorithms

• Used for voice, music, photos, video, etc.


5

Signals

• Sinusoidal wave is simplest type of wave

• Complex wave is mixture of sinusoidals

• Square wave with bit rate B can be converted to infinite Fourier series– with frequencies (2n+1)B/2, decreasing amp.

• Bandwidth = highest freq. – lowest freq.

• Phone system 3400 Hz – 300 Hz = 3.1 KHz


6

Fundamental Limits

• Attenuation– amplitude decreases over distance– due to resistance, dispersion, other energy loss– amplifier or repeater restores initial amplitude

• Bit rate limited by – Shannon’s law B = H log2 (1 + S/N)– Bandwidth in Hertz H– Noise or Signal-Noise Ratio S/N


7

Sources of Noise

• Impulse noise: spikes/surges, e.g. lightning– Reduced with power supply/line conditioning

• Crosstalk: interference between channels– Reduced with shielding, insulation, separation

• Echo: signals reflect at connections/ends– Reduced with terminators, repeaters

• White noise: electron thermal energy– Reduced by tuning radio antenna


8

Error Detection and Correction

• Sender– compute a check value based on the data– append check value to data and sends

• Receiver – perform a calculation based on data – compare answer with check value– if they disagree, ask to resend (NAK)

• No error detection scheme finds 100% of errors• Usually easier to detect and resend than correct


9

Redundancy Checks

• Parity Check or Vertical Redundancy Check– Parity bit makes number of 1 bits even/odd– Does not detect even number of bit errors

• Longitudinal Redundancy Check– Used to check a whole block of characters– Check character added to end of block– nth bit checks parity of nth bits of other char’s


10

Cyclic Redundancy Check

• Interpret bits as coefficients of a polynomial• Divide by generator polynomial to get remainder• Subtract remainder from original bits and transmit• Receiver divides received bits by generator

– If remainder is 0, no error occurred

• Versions (no. of bits R): CRC-12, 16, CCITT, 32• Detects almost all errors in block transmissions

– 1 and 2 bit, odd number of bits, burst errors of <R bits


11

Other Error Detection Schemes

• Check sums/digits (sum of fields)• Hash totals (sum group of items)• Byte count (message length)• Character echoing to user (asynch. e.g. terminal)


12

Error Correction

• Usually retransmission of message• Acknowledgment

– ACK, NAK, numbered

• Timeout, retry limit• Forward Error Correction

– Receiver has enough info to reconstruct original– cost of retransmission > cost of reconstruction– E.g. hamming codes (not used in data comm.)


13

Sequence Checks

• Message sequencing– Messages numbered so none go missing– Messages not received can then be re-sent

• Packet sequencing (packet-switched networks)– Sender divides message into numbered packets– Receiver reassembles using packet numbers

• Unnecessary in circuit-switched networks– e.g. phone system uses circuit switching– Connection/session is unbroken stream


14

Connection vs. Connectionless

• Low data rates• High error rates• Error correction• Higher OSI levels• Wide Area Networks

• High data rates• Low error rates• Just transmit data• Lower OSI levels only• Local Area Networks


15

Modulation (Shift Keying)

• digital analog• ac = Ac sin (2fct + )• vary one or more:

– AM/ASK: A=amplitude– FM/FSK: f=frequency– PM/PSK: =phase

• Dibits: 0, 90, 180, -90• Tribits: 0, 45, 90, etc.


16

Constellation Diagram

• Quadrature (QAM)

• Modulation of:– Amplitude (radius)– Phase (angle)

• V.32, 32 points

• 2400 baud, 9600 bps

0º

90º

-90º

180º


17

RS232C/V.24 Modem Interface


18

RS232C/V.24 Modem Interface

• TxD, RxD - Transmit and receive data pins • RTS - Request To Send, from DTE (computer)• CTS - Clear To Send, from DCE (modem)

now used for flow control/hardware handshaking• DTR - Data Terminal Ready - computer enabled• DSR - Data Set Ready - modem is turned on• CD - Carrier Detect - this modem is receiving a

signal from remote modem


19

Modem Features and Types

• Speed (56Kbps is most common)• Auto/manual dial/redial/answer/disconnect• Fax, digitised voice (for Internet phone calls)• Speaker, indicator lights (external), self-testing• MNP 1-4 error correction, 5 compression• Standards (ITU V.90, EIA, USR, Hayes AT cmds)• Mobile (PCMCIA, cellular, RF)• HDSL, Cable, ISDN, DSL, Fibre optic


20

The Synchronisation Problem

• How to synchronise Sender and Receiver?• Asynchronous

– Bit (clock) level– Byte (character) level– Frame level

• Synchronous - Character and Bit Oriented– Bit level

• Clock encoding and extraction• Data encoding and clock synchronisation

– Byte level– Frame (block) level


21

Asynchronous Bit Synch.


22

Asynchronous Bit Synch.

• Resynchronise for each byte (character)

• Isolated characters received at random times

• Transition between voltage levels is a bit

• 1 start bit, 8 bits for ASCII char., 1 stop bit

• Efficiency only 80% (2 bits of overhead)

• Used in low bit rate, low volume links

• Used by RS232 ports on PCs


23

ASCII vs. EBCDIC CodesCharacter

ASCIIRepresentation

EBCDICRepresentation

0 00110000 111100001 00110001 111100012 00110010 111100103 00110011 111100114 00110100 111101005 00110101 111101016 00110110 111101107 00110111 111101118 00111000 111110009 00111001 11111001A 01000001 11000001B 01000010 11000010C 01000011 11000011D 01000100 11000100E 01000101 11000101F 01000110 11000110G 01000111 11000111H 01001000 11001000I 01001001 11001001J 01001010 11010001K 01001011 11010011L 01001100 11010011M 01001101 11010100N 01001110 11010101O 01001111 11010110P 01010000 11010111Q 01010001 11011000R 01010010 11011001S 01010011 11100010T 01010100 11100011U 01010101 11100100V 01010110 11100101W 01010111 11100110X 01011000 11100111Y 01011001 11101000Z 01011010 11101001


24

Synchronous Bit Synch.

• A frame (block of bits) is sent

• Preamble uses unique bit sequence


25

HDLC Synchronisation

• High-level Data Link Control Protocol

• Use 01111110 as start/stop flags

• If 1111111 appears anywhere in data,– Sender inserts a 0 after fifth 1 (“bit stuffing”)– Receiver removes the 0


26

Baseband

• Data and clocking signal combined• Single channel (takes whole bandwidth)• Usually bi-directional (two way)• Uses an encoding scheme such as:

– Manchester– Diff. Manchester– Phase modulation– 4/5 bit– 5/6 bit


27

Manchester Encoding

• Transmitted signal is Data XOR Clock

• Transition in middle of each cell

• Lohi = 1, hilo = 0

• Often used in Ethernet

• Preamble for synchronising and start of data

• If polarity reversal, data misrepresented


28


29

Differential Manchester Encoding

• Transitions still occur at centre of each bit– 1=no transition at data start boundary– 0=transition at data start boundary

• Typically used in token ring systems

• No need for long preamble

• J and K signals have no midpoint transition– used to signal start and end of data


30

Phase Modulation Encoding

• Twisted pair provides two channels

• Phase difference used to signal data values– 0 if a transition in one channel– 1 if a transition in both channels


31

4 bit/5 bit Encoding

• Represent each 4 bits as 5 bits• Ensure each 5 bit pattern has signal change

– Not use all the same bit (00000 or 11111)

• Use fixed translation/mapping table• Avoids problem of many repeated 1’s or 0’s• 100 Mbit/s data rate requires 125 Mbit/s

signal rate• Used in high-speed fibre optic LANs


32

5 bit/6 bit Encoding

• High speed LANs on UTP cable

• Copper wire must use low frequencies– US/int’l interference and emission regulations

• Two steps used– Quintet scrambler– 5B/6B two-level non-return-to-zero encoder

• result is 3 1’s and 3 0’s; allows error check


33

Broadband

• Modulated using modem (digital analog)

• Phase coherent so unidirectional

• Multiple channels divide bandwidth: FDM

• Channels often 6MHz wide, 10 Mbit/s

• The cable uses low freq. 0-50 Hz for power

• Used for cable TV or data (cable modems)

• Carrierband: one channel=whole bandwidth


34

Single vs. Dual Cable Broadband

• Single cable– traffic to/from head end (amp/freq translator)– high=outbound 216-300 MHz– low=inbound 5-110 MHz

• Dual cable– Each cable has 5-300 MHz bandwidth– Simpler headend (no amp/freq translation)– Higher cost for cabling

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