2. 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.
ReceiverCommunication ChannelTransmitter
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)
4. 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. 5
Channel Bandwidth (Baseband Channel)
ChannelX(t) = a cos(2πft) Y(t) = aA(f)cos(2πft)
A(f) Frequency Characterization of the
Channel
B0
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
B0
f
A(f)
Real Baseband channel
6. 6
Channel Bandwidth (Bandpass Channel)
ChannelX(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. 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. 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. 9
Some Simple Line Coding Schemes
NRZ-inverted
(differential
encoding)
1 0 1 0 1 1 0 01
Unipolar
NRZ
Bipolar
encoding
Manchester
encoding
Differential
Manchester
encoding
Polar NRZ
+ +
- -
+ + +
o o
+ + + + +
+ + +
- -
o o oo
10. 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 01
Manchester
Encoding
11. 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 01
Manchester
Encoding
(1B 2B code)
12. 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. 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 01Bipolar
Encoding
Binary Data 1 1 1 1 1 1 1……
1/f
2 T
1
(2 )T
14. 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. 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. 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. 17
B8ZS
1st violation 2nd violation
2 violations of opposite polarities
equalizes dc component
+ +
- --
18. 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. 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. 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 fc0 f2= fc + Wc/2
Bandwidth,
Wc
Baseband
, B=Wc/2
B
Modulated bandpass
21. 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. 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. 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. 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. 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 CB
f
W0
C
f
B
f
A
fWu
Wu
0
0
0 Wu
Signals separated by guard bands
fit into channel bandwidth,
W>>Wu
26. 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. 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. 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
