Communication Networks

Sanjay K. Bose

Lecture Set II

Data Communications for Networks

2

A Transmission System

Transmitter: Converts digital data into signal suitable for transmission and send the signal over the communication channel

Receiver: Receives the signal and converts it back into digital data to be delivered to the user.

Receiver Communication Channel Transmitter

Depending on the transmission medium, the data may be encoded directly and sent to the medium (Baseband Transmission, Line Coding) or it may be modulated on to an analog carrier and then transmitted (Modulation)

3

Transmission Impairments

Communication Channels

• Copper wires

• Coaxial cables

• Radio

• Microwave

• Light in optical fiber

• Light in air

• Infrared (irDA)

Receiver Communication Channel Transmitter

4

Encoding and Modulation Techniques

Data Encoding: Mapping of information

into sequence of digital signals

Modulation: Embedding of information

into sinusoidal waveforms

Modulation Demodulation

Encoder Decoder

Baseband Transmission

Carrier Frequency Modulation

Use baseband transmission (data encoding) when using a baseband channel and carrier frequency modulation when using a bandpass channel

5

Channel Bandwidth (Baseband Channel)

Channel X(t) = a cos(2πft) Y(t) = aA(f)cos(2πft)

A(f) Frequency Characterization of the Channel

B 0 f

A(f)

Ideal Baseband channel

All frequencies in range (0, B) are passed with same attenuation

Flat Frequency Spectrum Frequency Spectrum is not ideally

flat

B 0 f

A(f)

Real Baseband channel

6

Channel Bandwidth (Bandpass Channel)

Channel X(t) = a cos(2πft) Y(t) = aA(f)cos(2πft)

A(f) Frequency Characterization of the Channel

fc+½B f

A(f)

Ideal Bandpass channel

Flat Frequency Spectrum

fc - ½B fc fc+½B f

A(f)

Real Bandpass channel

Frequency Spectrum is not ideally flat

fc - ½B

fc

7

Why Line Coding and Modulation?

– Need to find a proper digital signal to represent the data bits (0 and 1) in baseband transmission Line Coding

– Need to find a proper analog representation (i.e. modulated carrier) of data bits for bandpass transmission Modulation

8

Design considerations in Line Coding

• Transmitted power Low power consumption desirable

• Bit timing Transitions in signal help timing recovery

• Bandwidth efficiency Excessive transitions wastes bandwidth

• Low frequency content Try to avoid signals with high DC content

Some channels block low frequencies (i.e. at or near DC) In such channels, long periods of +A or of –A causes signal to

“droop”

• Error detection Ability to detect errors helps

• Complexity/cost Low cost implementations (e.g. on a chip) desirable

9

Some Simple Line Coding Schemes

NRZ-inverted

(differential

encoding)

1 0 1 0 1 1 0 0 1

Unipolar

NRZ

Bipolar

encoding

Manchester

encoding

Differential

Manchester

encoding

Polar NRZ

+ +

- -

+ + +

o o + + + + +

+ + +

- -

o o o o

10

Manchester Code

• “1” maps into high-to-low transition (A/2 first T/2, -A/2 last T/2)

• “0” maps into low-to-high transition (-A/2 first T/2, A/2 last T/2)

• Every interval has transition in middle

– Self-clocking feature

– Timing recovery easy

– Uses double the minimum bandwidth

• Simple to implement

• Used in Ethernet & other LAN standards

1 0 1 0 1 1 0 0 1

Manchester

Encoding

11

mB nB codes

• mB nB line code provides increased number of transitions for improved synchronisation

• Maps block of m bits into n bits; n>m

• Manchester code is 1B2B code

• 4B 5B code used in FDDI LAN

• 8B 10B code used in Gigabit Ethernet

• 64B 66B code used in 10G Ethernet

1 0 1 0 1 1 0 0 1 Manchester

Encoding

(1B 2B code)

12

Differential Manchester Coding

• Differential Manchester

– Mid-bit transition is clocking only

– Transition at start of a bit period represents zero

– No transition at start of a bit period represents one

– Used by IEEE 802.5 (Token ring)

Clocking

transitions

Performance similar to Manchester Coding

13

Bipolar Code (Alternate Mark Inversion)

• Three signal levels: {-A, 0, +A}

• “1” maps to +A or –A in alternation

• “0” maps to no pulse – Every +pulse matched by –pulse so

little content at low frequencies

• String of 1s produces a square wave – Spectrum centered at with

no DC component

• Long string of 0s causes receiver to lose timing synchronization – Use Zero-substitution codes to

break long 0s sequence

1 0 1 0 1 1 0 0 1 Bipolar

Encoding

Binary Data 1 1 1 1 1 1 1……

1/f

2 T 1(2 )T

14

Pros & Cons of AMI Codes

• Pros

– Narrower bandwidth as compared with NRZ

Consecutive 1s produce a spectrum of a square wave centered around 1/2T Hz

– Easy error detection

deleting or adding a pulse violates the code property

Code violation can be used to detect code substitution

• Cons

– Not as efficient as NRZ

Each signal element only represents one bit

But a 3 level system should have been able to represent log23 = 1.58 bits

– Receiver must distinguish between three levels instead of two

Absence of transitions in a long sequence of alternate 1s and 0s can results in loss in synchronization

15

Scramble Codes

• Use scrambling to replace sequences that would produce constant voltage – Bipolar With 8 Zeros Substitution (B8ZS) – High Density Bipolar 3 Zeros (HDB3)

• Filling sequence – Must produce enough transitions to sync – Must be recognized by receiver and replace with original – Same length as original sequence

• Design goals – No dc component – No long sequences of zero level line signal – No reduction in data rate – Error detection capability

16

Scrambling - B8ZS

• Bipolar With 8 Zeros Substitution (B8ZS)

• Based on bipolar-AMI

– No dc component, Error detection, Lower bandwidth

• Scrambling method

– If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+

– If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-

• Causes two violations of AMI code

– Unlikely to occur as a result of noise

– Receiver detects and interprets as octet of all zeros

17

B8ZS

1st violation 2nd violation

2 violations of opposite polarities

equalizes dc component

+ +

- - -

18

Scrambling - HDB3

• High Density Bipolar 3 Zeros (HDB3)

– B8ZS only replaces string of 8 zeros

– HDB3 replaces shorter strings of 4 zeros with scramble codes

• Based on bipolar-AMI

• String of four zeros replaced with one or two pulses

– each replacement causes 1 violation of AMI code

Number of Bipolar Pulses

since last substitution

Polarity of preceding Pulse Odd Even

- 000- +00+

+ 000+ -00-

19

HDB3

Number of Bipolar Pulses

since last substitution

Polarity of preceding Pulse Odd Even

- 000- +00+

+ 000+ -00-

1 s

1

2

3 4

5 6

7

8

9 10

odd even even

Odd violation -> cause even number of pulses

Even violation -> alternate polarity of next even violation

20

Modulation for Bandpass Channels

• Bandpass channels pass a range of frequencies around some center frequency fc = ( f1 + f2)/2

– Radio channels, telephone & DSL modems

• Digital modulators embed information into waveform with frequencies passed by bandpass channel

• Sinusoid of frequency fc is centered in middle of bandpass channel

• Modulators embed information into a sinusoid

f1= fc – Wc/2 fc 0 f2= fc + Wc/2

Bandwidth,

Wc Baseband

, B=Wc/2

B

Modulated bandpass

21

Some Simple Modulation Schemes

• Binary Amplitude Shift Keying (ASK)

• Binary Frequency Shift Keying (FSK)

• Binary Phase Shift Keying (BPSK)

• Quadrature Amplitude Modulation (QAM)

• Quadrature Phase Shift Keying (QPSK)

More sophisticated modulation schemes (e.g. OFDM) are used in modern communications, e.g. in wireless networks.

22

Multiplexing

The multiplexer combines data from n input lines and transmits over a higher-

capacity data link.

• Simultaneous sharing of a high capacity link by many input channels

The demultiplexer accepts the multiplexed data stream, separates the data according to channels, and

delivers them to the appropriate output lines.

23

Key Points

• Multiplexing increase link utilization efficiency through medium sharing

• Frequency division multiplexing (FDM)

– Bandwidth sharing through frequency allocation

• Time division multiplexing (TDM)

– Channel capacity sharing through time slot allocation

Synchronous TDM uses fixed assigned time slots

Asynchronous TDM uses available time slots

24

Frequency Division

Each frequency channel occupies a fraction of the transmission

bandwidth all the time

Frequency division of transmission bandwidth into

frequency channels

25

Frequency Division Multiplexing (FDM)

• Useful bandwidth of medium exceeds required bandwidth of signals to be transmitted

• Each signal is modulated to a different carrier frequency

• Carrier frequencies sufficiently separated so signals do not overlap (guard bands)

– Each frequency band is referred as a channel

• Channel allocated even if no data transmission

• Signal transmitted is analogue

A C B

f W 0

C f

B

f

A

f Wu

Wu

0

0

0 Wu

Signals separated by guard bands fit into channel bandwidth,

W>>Wu

26

Synchronous Time Division Multiplexing

Time division of transmission bandwidth into data channels Each data channel occupies the full

transmission bandwidth for an assigned time-slot

27

TDM System -- Frames

Tim

e

• Data are organized into frames

• Each frame contains a cycle of time slots

• In each frame, one or more slots is dedicated to each data

source

• The sequence of slots dedicated to one source, from frame to

frame, is called a channel

• The slot length equals the transmitter buffer length,

typically in units of a bit or a character

N

28

Statistical TDM

• In Synchronous TDM slots are wasted if corresponding buffer is empty (i.e. that source has nothing to send)

• Statistical TDM allocates time slots dynamically based on demand

• Time slots available on the TDM frame is less than the number of input lines

• Multiplexer scans input lines and collects data until frame full

• On the receiver, the multiplexer receives a frame and distributes the slots of data to the appropriate output buffer

• Statistical TDM has more overhead since each slot must carry its own address information as well as data