Download Digital Communication System

Telecoms Systems (Week 1)
Prof. Laurie Cuthbert
Dr. Michael Chai
Dr Frank Gao
Staff
Prof Laurie Cuthbert – weeks 1 and 4
[email protected]
Dr Michael Chai – week 3
[email protected]
Dr Frank Gao – week 2
[email protected]
2
Changes since last year


Content has not changed
Exam format different – now 4 compulsory
questions in 2 hours:
– One on each week’s material

Remember:
– QM rules on extenuating circumstances apply
3
Assessment


Exam: 88%
Class tests: 12%
–
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–
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Class test every week of teaching on Friday
Each group split into 2
You must be in the right group
Test is a question on anything taught that week
Roughly half an exam question
Each test counts 3%
Open book
4
Emphasis on

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
Why
How
When you come to the lecture, bring:
–
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–

Pen
Paper
Lecture notes
Calculator
You will have to do problems in the class!!
5
Learning Outcomes



Explain the principles of operation and
architectures of circuit-switched and
packet/cell-switched network; wired and
mobile.
Describe the operation of transmission and
switching systems.
Calculate simple numerical problems on
aspects of source coding, error-control coding,
Queuing Theory and Information Theory.
6
Extenuating circumstances


Must be for 'unplanned circumstances that outside of
your control
These include medical and personal circumstances such
as close family being ill, but not events such as:
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–

planned holidays,
job interviews or internships
GRE or IELTS preparation or test
misreading timetables,
computer problems,
not being aware of rules or procedures.
Medical conditions must be sufficiently serious that
they would have a major affect on your examination.
QM rules


ECs for all QM modules will be treated under QM
rules
If you want to claim EC for an exam or class test
you must:
– Complete a form in English (from Jing Liu)
– Add supporting evidence (e.g. medical certificate)
– Give everything back to Jing Liu at least 1 week
before the examination board

Your BUPT tutor does NOT have the authority to
approve ECs for QM modules
MODERN TELECOMMUNICATIONS
9
Telecommunication system – a system for
conveying content

For example, the UK Telecommunications Act 1984, s.4(1)
defines it as:
“a system for the conveyance through the agency of electric,
magnetic, electromagnetic, electro-chemical or electromechanical energy, of
(a) speech, music or other sounds;
(b) visual images;
(c) signals serving the impartation (whether as between persons and
persons, things and things or persons and things) of any matter
otherwise than in the form of sounds or visual images; or
(d) signals serving for the actuation or control of machinery or
apparatus”.
10
“More communications than we know how to
use”


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Many different technologies
Developed in parallel
Lead time to introduce new services
decreasing
“Reliability” of software decreasing
– Online patches for mobile phones

Remote working is now normal
11
Legacy - wired communications using telephony
fax
analogue
digital
Analogue
Dial-up modem
Obsolete !!!!
exchange
Digital
12
Mobile communications
Tablet
often a radio link
to network
WLAN
3G/HSDPA
WLAN
2G/3G/HSDPA/LTE
Cable / radio
(Bluetooth)
13
IP allows competition with telephony
Webcam
Headset
SIP phone
Dual cordless phone that
connects to a normal phone line
and the computer
All telephony is going IP
14
Now telephony is SIP based
15
Transport modes

Traditionally telephony was circuit switched:
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Call set up, conversation and clear-down phases
64 kbit/s (in digital era) allocated in both directions
Much of the capacity wasted
Analogue to digital conversion in local exchange
Control very much centralised
Now IP-based
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SIP sets up and clears down connections
Transport RTP
A-D conversion in the telephone
More distributed
16
Traditional network hierarchy
Local Exchange
Local Exchange
Core network
Digital
Access network
Analogue
Trunk Exchange
Trunk Exchange
17
Transmission

Connections are carrying
little traffic are served with
low capacity links
– Up to 120Mbps in UK for
broadband

Very high speed optical
fibre links between major
cites
– In excess of 500 Mbps and
support over 7000 voice calls
18
Switching used to be manual
19
Then relays, then electronic – but specialised
Electromechanical exchange
picture courtesy of Nortel
Private electronic exchange
1983
1960s
20
Now just boxes of electronics – high volume
IP router
WLAN AP
Servers
All of these are just “boxes” of
Electronics
IP switch
IP phone
21
Wireless (GSM) network architecture
PSTN
Gateway MSC
Mobile switching centre
Base station controller
BTS
BTS=Base transceiver station
BS
History of wireless communications
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1865 James Clerk Maxwell published his equations
1887 Heinrich Hertz demonstrated EM wave propagation
1893 Nicola Tesla demonstrated communication by radio
1895 Aleksandr Popov demonstrated a wireless system
1896 Guglielmo Marconi demonstrated wireless telegraphy
1901 First wireless signal sent across the Atlantic Ocean
from Cornwall to St. John’s, Newfoundland (Canada)
Marconi was not the ‘inventor’, but appreciated the
commercial opportunities offered by the new medium.
Why wireless?

No more cables
– No cost for installing wires or rewiring
– Wiring is infeasible or costly in some areas, e.g.. rural
areas, old buildings…

Mobility and convenience
– Allows users to access services while moving: walking, in
vehicles…

Flexibility
– Roaming allows connection any where and any time

Scalability
– Easier to expand network coverage compared to wired
networks.
Challenges of wireless

Limited resources: finite radio spectrum
– Frequency reuse, breaking cells into smaller cells, more efficient medium
access technology, e.g. CDMA…

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Supporting mobility - Location management, handover, …
Maintaining Quality of Service (QoS) over unreliable wireless links
– Radio propagation attenuation: path loss, shadowing, multipath fading.

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Connectivity and coverage - roaming and internetworking
Security
– Wireless channels are “open”
– Certification and authentication

Integrated services (voice, data, multimedia, etc.) over a single network
– service differentiation, priorities, resource sharing,...

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Mobile terminal battery life
You will learn more about all of this later
Emerging and existing wireless technology

Mobile Wireless:
– 2G: GSM, TDMA, CDMA
– 2.5G EDGE, GPRS
– 3G W-CDMA, HSDPA,
HSUPA
– 4G - LTE

Fixed Wireless:
– MMDS, LMDS, Satellite
dish, Microwave

Mobile cellular
Point-Point/ Multipoint
Wireless
Wireless LAN:
– IEEE 802.11, Ad-hoc,
Bluetooth,

WiMax
Wireless LAN
Satellite wireless
Types of wireless network

WPAN (Wireless Personal Area Network)
– typically operates within about 30 feet

WLAN (Wireless Local Area Network)
– operates within 300 yards

WMAN (Wireless Metropolitan Area Network )
– operates within tens of miles

WWAN (Wireless Wide Area Network )
– operates over a large geographical area, mobile phone,
…
Features of mobile communications

Mobile phones are portable, convenient, move
with people.
– By their nature, they are location aware.

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Limited frequency bandwidth
Low power: max mobile transmit power
– 125mW for WCDMA
– 2W peak for GSM900
– 1W for GSM1800/1900

Point to multi-point, not broadcast
Cellular concept
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Late 40s: AT&T developed cellular concept for frequency reuse
Break the service area into cells
Shrink the cell size; adopt intensive frequency re-use
Add more cells to add more capacity
Mobility management is required
Evolution of mobile networks
“It is dangerous to put limits on
wireless” Guglielmo Marconi in
1932…….
1970’s
Proposed late 1980’s
GSM launched in 1992
1990’s –present
Proposed in 1998
Launched in UK 2003
1G
2G
2.5 G
3G
Evolution of mobile networks
NTT
TACS
NMT
AMPS
1G
GSM
IS-136
IS-95
PDC
GPRS
HSCSD
EDGE
IS-95B
W-CDMA
(3GSM)
TDSCDMA
cdma2000
4G ?
3G
2.5G
2G
 Higher bit rate ?
 New applications ?
 Speech & low rate data
service
 Digital transmission
 Speech, data, multimedia
 Speech service
services
 Analogue
 Bit rate up to 2 Mbit/s
transmission
 Digital transmission
1G systems

Analogue

– Speech
– Some data at 1.2kbit/s

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Designed for car use
First handportable –
Motorola “Brick”
(DynaTAC 8000X )
–
–
–
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1983
800g
30 mins talk time
USD 3995
Insecure
– Eavesdropping
– Cloning

Almost no roaming
Some 1G systems
System
Band
(MHz)
Example Locations
AMPS
800
US, Canada, Mexico, Australia, New
Zealand, Hong Kong, Brazil, Argentina
TACS
900
UK, Ireland, Spain, Italy, Austria
NMT
450/900
Denmark, Finland, Norway, Sweden,
Belgium, Austria, France, Hungary,
Netherlands, Spain
NTT
800
Japan (First cellular system 1979)
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No real roaming apart from NMT
2G Systems
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Speech and low bit rate data service
Digital transmission
Designed to be more secure
Almost exclusively handportable
2G Systems
System
Band
(MHz)
Example Locations
D-AMPS (IS-54, IS136)
800
North and South America
CDMA (IS-95)
800/1900 North and South America, S Korea,
China
GSM
900/1800 World-wide (except Korea and
Japan)
1900
1900 MHz US and Canada
JDC/PDC
800/1500 Japan
GSM
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Officially launched in 1992
Multiple access: TDMA 8 channels (frames of 8
time slots) on each carrier
FDD (Frequency Division Duplex) – different
frequencies for uplink and downlink
200kHz carrier bandwidth
9.6kb/s net data (13kb/s encoded voice)
(Almost) worldwide availability with multi-band
handset
Useful link www.gsmworld.com
GSM worldwide success
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Over 860 networks in 220 countries/areas
Still growing: No of GSM + 3 GSM subscribers
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11/9/2011 00.55 (CN time) 5,231,269,752
In the next 5 mins an increase of: 7,106 !!!!!
World Population at same time: 6.914 billion
Penetration 59%
System
Uplink (MHz)
Downlink (MHz)
GSM850
824 -849
880 -915
GSM900
890 -915
935 –960
GSM1800 (DCS1800)
1710 –1785
1805 –1880
GSM1900 (PCS1900):
1930 –1990
1850 –1910
GSM network architecture – core components
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BTS: Base Transceiver Station
BSC: Base Station Controller
BSS: Base Station Subsystem
MSC: Mobile Switching
Centre
HLR: Home Location Register
VLR: Visitors Location
Register
AuC: Authentication Centre
GMSC: Gateway MSC
PSTN: Public Switched
Telephone Network
AuC
PSTN
HLR
VLR
MSC
GMSC
BSC
BSC
BTS
MT
BSS
GSM network architecture – other elements

EIR: Equipment Identity Register
– Record of status of phone
– White / grey /black (stolen)

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SMS-C: Short Message Service Centre
OMC: Operation and Maintenance Centre
BS
Locating a Mobile terminal
When a MT moves from one location area to
another:
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MT initiates the location updating procedure.
HLR is notified by the new MSC/VLR.
HLR removes old MSC/VLR information
HLR confirms and updates the new MSC/VLR.
location area update is confirmed with the MT.
Mobile Terminating call
HLR
Location area
PSTN
GMSC
MSC
VLR
traffic
signalling
VLR
MSC
BSC
BT
S
BSC
Roaming incoming call
PSTN
Home network
HLR
Location area
Visited network
GMSC
VLR
MSC
MSC
VLR
MSC
BSC
VLR
BT
S
Roaming leg paid by recipient
BSC
BT
S
Roaming outgoing call
Home network
HLR
Location area
Visited network
GMSC
VLR
MSC
MSC
VLR
MSC
BSC
VLR
Billing centre
BT
S
PSTN
BSC
BT
S
GSM Mobility Management: Authentication
Mobile Terminal
Challenge: RAND Random
number
Key Ks in SIM
A3 algorithm
Key Ki in MSC
A3 algorithm
SRESMSC
Response: SRESMT
If results match, Ks=Ki and the user is genuine
Only information transmitted over the air is RAND and SRESMT
If equal, then
authenticated
How does communications everywhere affect
the global economy?
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Increasingly reliant on communications
technology for business
Variety of actors with competing interests
Communications systems becoming the target
of cyber-terrorist attacks
Communications networks now part of the
national large-scale critical infrastructure.
45
INFORMATION CONVERSION
Transmission of analogue information
Multiplexing
Information:
‘Hello! How are you?’
Demultiplexing
You and I understand but not the telephone!
Analogue signal can be understood by
electrical systems but problematic!
So all new systems digital
47
Information Conversion
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Different sources of information are presented with
different formats at the input of transmitter.
Formatting transforms the source information to a
compatible digital format for digital processing.
Four basic stages of information conversion
48
Overview of Digital Communication System
Format
Source
encode
Encrypt
Channel
encode
Multiplex
Pulse
modulate
Bandpass
modulate
Freq.
spread
Multiple
access
Transmitter
Receiver
Format
Source
decode
Decrypt
Channel
Decode
Demultiplex
Pulse
Detect
Demodulate
Freq.
despread
Multiple
access
49
Transmission side
Format
Source
encode
Encrypt
Channel
encode
Multiplex
Pulse
modulate
Bandpass
modulate
Freq.
spread
Multiple
access
Transmitter
Transform the source information into bits, assuring
compatibility between the information and the signal
processing within the DCS.
Digital
Information
Textual
Information
Analogue
Information
Encoder
Sample
Pulse
modulate
Quantise
50
Formatting Textual Data

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Textual information compromises a sequence of
alphanumeric characters.
Each alphanumeric character is transformed into
binary by character coding. Most popular character
coding method is ASCII
Encoded into sequence of k bits called [symbols]
51
You do not have to remember
this
[1]
Sampling (ideal sampling)
time domain
t
original signal x(t)
t
Sample pulse
xp(t)
t
Sampled signal xs(t)
frequency domain
f
-fm 0 fm
f
-2fs -fs 0
fs 2f
f
-fs-fm 0 fm fs
s
53
Sampling (real sampling)
time domain
t
original signal x(t)
t
Sample pulse
xp(t)
t
Sampled signal xs(t)
frequency domain
f
-fm 0 fm
f
-2fs -fs 0
fs 2f
f
-fs-fm 0 fm fs
s
54
Sampling frequency
fs > 2fmax
fs < 2fmax
55
Aliasing (ideal sampling)
You must remember the
term aliasing
original signal
f
signal sampled with fs > 2 fm
-fm 0 fm
f
-fm 0 fm
fs
signal sampled with fs = 2 fm
f
-fm 0 fm fs
2fs
signal sampled with fs < 2 fm
aliasing occurs
f
-fm 0 fmfs
2fs
56
Aliasing in more detail
original signal
fmax
low-pass filter can recover original signal
fs
signal sampled with fs > 2 fmax
fs
2fs
signal sampled with fs = 2 fmax
recovery not possible - spectra interfere
signal sampled with fs < 2 fmax
aliasing distortion occurs
Oversampling
low-pass filter can recover original signal
original signal
fmax
Easier to implement filter
fs
oversampling
signal sampled with fs > 2 fmax
Needs to be ideal filter
fs
2fs
signal sampled with fs = 2 fmax
recovery not possible - spectra interfere
signal sampled with fs < 2 fmax
aliasing distortion occurs
Sampling Theorem



To prevent aliasing and hence to allow the original
signal to be recovered the sampling frequency (fs)
must be given by:
fs ≥ 2 fmax
where fmax is the highest frequency present in the
original signal.
This is the SAMPLING THEOREM and is a
fundamental theorem.
Notice that fmax is the highest frequency present, NOT
the highest frequency of interest.
59
Oversampling



A process of sampling a signal at more than twice the
higher frequency than the highest frequency present
in the original signal.
Oversampled signal is normally expressed with the
oversampled factor of .
fs =  fmax ;  ≥ 2
Oversampling makes it easier to design a simpler
filter to recover the original signal
60
Summary
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

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Different types of network – how they are
linked and different network speeds
Analogue, textual and digital transmission
Character coding – ASCII
Sampling, anti-aliasing and oversampling
61
QUANTISATION
Scope
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Linear quantisation and non-linear quantisation
Companding
Delta modulation
63
Quantisation and PCM

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Quantisation results from mapping continuous
analogue values to the discrete vales that can be
represented digitally.
May be linear or non-linear
Pulse-code modulation (PCM) is a method used
to digitally represent sampled analogue signals –
there are many others.
PCM invented in 1948 by Sir Alec Reeve at STL
Harlow, UK
64
Principle of PCM
time domain
t
original signal x(t)
t
Sample pulse
t
Sampled signal xs(t)
Sample amplitude
represented by N bits
65
Linear quantisation
Quantised signal



Peak-to-Peak Voltage,
Vpp=Vp-(-Vp) = 2Vp
Quantisation interval, q,
(step size) uniformly
distributed over the full
range
Approximation will
result in an error no
larger than ±q/2
q
Quantising level
66
Quantising levels
q
Vp
Vp
q
0
0
-Vp
-Vp
Quantising level
N levels gives a range of (N-1)q
Quantising level
N levels gives a range of Nq
67
Quantising distortion
quantising level
signal changes state
half-way between
quantising levels
error
q
quantised signal
+q/2
error signal
-q/2
Quantising error power
Error is approximately sawtooth over the quantisation
region, apart from the dwell regions.
p(e)
+q/2
1/q
t
error e
-q/2
-q/2
+q/2
A sawtooth waveform has a uniform pdf: all values are equally likely.
The area under the pdf must be 1 so that the amplitude is 1/q.
Note that p(e)=0 outside the range +q/2 to -q/2
Power in quantising distortion =

q 2
2
1
q
Dq   e 2 p e de =  e 2 de =
12
q

q 2
Notes




this holds for reasonable well-behaved signals
without frequent dwell regions.
quantising error leads to distortion, not noise,
because it is causally related to and dependent on
the input: the same input will always produce the
same output.
the statistics of the distortion are independent of
the statistics of the input.
this approximation shows that the distortion
power is constant and depends only on the step
size.
Quantiser SDR
Consider a quantiser with a maximum range of ±V and N bits.
Then we have:
q  2V
2N
4V 2 1
V2
and hence Dq  2 N

2 12 3  2 2 N
If the signal power is S then we can define a signal to distortion ratio (SDR) :
S  3  22 N
SDR 
V2
or in dB:
S 
SDR (dB)  10 log10  2   4.77  6.02N
V 
Quantiser impairment curve

SDR (dB)
increase N
usable

clipping
log (S/V2)
These 2 curves have the same
power (S) but clipping will have
a different effect on each.
As the slope of the clipping line is very
steep, the quantiser must be operated well
away from that boundary
Such an impairment curve is not suitable
for signals such as speech that have a very
wide dynamic range (speech around 30dB).
For instance, if S varies by 30dB this will
not give satisfactory results. A flat
impairment curve is needed.
Dynamic Range (Dy)
 Max possible signal power (no overflow) 
Dy  10 log 10 
 dB.
 Min. power which gives acceptable SQNR 
For uniform quantisati on :
 Max signal power 
 Min power 
Dy  10 log 10 
  10 log 10 

2
2
q / 12


 q / 12 
 Dy = max possible SQNR (dB)  min acceptable SQNR (dB)
This final expression for Dy is well worth remembering,
but it only works for uniform quantisation!
73
ITU Recommendation – what we need

A better impairment curve is obtained by non-liner quantisation
known as companding (compressing and expanding). This is done in
a codec (coder - decoder). here are two standardised coding laws: the
(generally 7-bit) µ-law used in N. America and Japan and the 8-bit
A-law used in the rest of the world (including Europe)
approx. 30dB
33.2dB
SDR(dB)
CCITT (now ITU-T)
recommendation
input level (power)
Speech and Linear Quantisation



The higher values of the quantisation are rarely
used for speech.
SNR is worse for lower signals as quantisation
noise is the same for all signal magnitudes.
Non-uniform quantisation can provide fine
quantisation of the weak signals and coarse
quantisation of the strong signals.
75
Non-linear quantisation
♦ 8-bits per sample not sufficient for good speech encoding
with uniform quantisation.
♦ Problem lies with setting a suitable quantisation step-size.
♦ One solution is to use non-linear quantisation.
♦ Step-size adjusted according to amplitude of sample.
♦ For larger amplitudes, larger step-sizes used as
illustrated next.
♦ ‘Non-linear’ because step-size changes from sample to
sample.
76
Linear Quantisation and Non-linear Quantisation
15
15
0
0
77
Non-linear quantisation




Non linear quantisation uses the logarithmic compression and
expansion function.
 Compress at the transmitter and expand at the receiver.
Compression process changes the distribution of the signal
amplitude.
 Lower amplitude signals strength to higher values of
quantisation.
As the result the compressed speech signal is now more suitable
for linear quantisation.
The logarithmic compression and expansion function is also
called Companding.
78
Implementation of companding (in principle)




Pass x(t) thro’ compressor to produce y(t).
y(t) is quantised uniformly to give y’(t) which is transmitted or
stored digitally.
At receiver, y’(t) passed thro’ expander which reverses effect of
compressor.
analogue implementation uncommon but shows concept well.
x(t)
Compressor
y(t)
Uniform
quantiser
y’(t)
Expander
x’(t)
Transmit
or store
79
Companding
 There are two compading standards: A-law compression (used mainly in
Europe) and µ-law compression (used in North America and Japan).
 The A-law is given by the mathematical expression:
sgn x  A x
FA  x  
1  ln A
and
x 1 A

sgn x  1  ln A x 
1  ln A

1 A  x 1
 However, it is not used like this, but as a segmented, piece-wise linear
approximation. The segmented A-law uses 13 segments (0, ±17) with
A=87.6
 8-bit code consist of
i) polarity bit P (range is ±V)
ii) 3 segment decoding bits XYZ
iii) 4 bits (abcd) specifying intra segment value on a linear scale
80
Compression law derivation
+27
-211
+211
-27
m-law
A-law
Variation of SQNR with amplitude of sample
SQNR dB
48
Uniform
36
A-law
24
12
Amplitude
of sample
0
V/16
V/4
V/2
3V/4
V
82
A-law encoding table
Segment Coder input range
0
1
2
3
4
5
6
7
0-V/128
V/128-V/64
V/64-V/32
V/32-V/16
V/16-V/8
V/8-V/4
V/4-V/2
V/2-V
output code
P 000 abcd
P 001 abcd
P 010 abcd
P 011 abcd
P 100 abcd
P 101 abcd
P 110 abcd
P 111 abcd
P is a polarity bit
abcd is a 4-digit intra-segment code
quantum interval
V/2048
V/2048
V/1024
V/512
V/256
V/128
V/64
V/32
PCM for telephony
 8kHz
sampling to satisfy Sampling Theorem
 8 bits per sample with A-law encoding
64kbit/s digital speech signal
Note that the sampling rate of 8kHz leads to a period of
125ms between speech samples
Sampling theorem applied to telephony (PCM)
Sampling frequency of
8kHz equivalent to a
sample every 125ms
low-pass anti-aliasing filter
v
v
3.4kHz
f
f
v
v
3.4kHz
t
t
By bandlimiting the incoming speech signal to 3.4kHz
and sampling at 8kHz, the sampling theorem is satisfied
8-bit A-law
compander
Delta modulation (DM)



A simpler way than PCM
Provides a staircase version of the message signal by
referring to the difference between the input signal
and its approximation
Quantization is done using 2 levels:
– Positive difference: +
– Negative difference: -

Provided that the input signal does not change too
rapidly from sample to sample, this approximation
works well
87
DM illustration
mq(t)
Staircase approximation of m(t)
m(t)
Input signal
Binary sequence at
modulator output
111111010000000000101111110101010
88
DM Discrete-time relations

Let
– Ts be the sampling period
– e(nTs) be the error signal
– eq(nTs) be the quantised error signal
89
DM transmitter
Comparator
Sampled Input
m(nTs)
+
e(nTs)
∑
Quantiser
eq(nTs)
Encoder
DM data
sequence
+
∑
mq(nTs-Ts)
+
Delay Ts
mq(nTs)
Accumulator
90
DM quantisation error


Slope overload distortion
Granular noise
Slope overload
distortion
mq(t)
Granular noise
m(t)

To minimise slope overload distortion

dm ( t )
 max
Ts
dt
91
Summary






Sampling process is restricted by Nyquist criterion – Aliasing
will occur during undersampling.
The non-linear quantisation is more effective than the linear
quantisation at lower quantisation values.
A-Law and µ-Law compression are used in the non-linear
quantisation.
Delta Modulation is a simpler way than PCM.
DM is efficient technique for signals that changes less rapid.
Slope overload distortion at DM
92
ATM
93
ATM - Main features






Legacy now – but structure and principles appear in
modern systems
Connection-oriented transfer mode
The cells are much shorter than in a conventional
packet network to get reasonable delay variance.
Overhead is minimised to maximise efficiency (e.g.
there is no error correction mechanism for payload)
Cells are transported at regular intervals; there is no
space between cells, idle periods on the link carry
unassigned cells.
ATM provides cell sequence integrity.
ATM cell stream
unassigned cell
cell from source 1
cell from source 2
cell from source 3
ATM cell stream
cell-based transfer mode
this means that information from is transferred as fixed-length cells
Principle of ATM

cell-based transfer mode
– this means that information from the source is
transferred as fixed-length cells
– no “white space” between cells - if no information
an empty cell is sent instead.
ATM cell structure
5 octet header
48 octet payload
Details are given later
Cell assembly delay
with short blocks of data it takes time to assemble one cell
for transmission
payload
ATM header
this is cell assembly delay: for 64kbit/s it is 6ms
Segmentation
Large data packets need to be segmented:
data segment
cell stream
This is done in the ATM Adaptation Layer (AAL) - considered later
ATM relative merits

Advantages of ATM
– ease of handling VBR services
– inherent multiplexing of the cells with ATM offers
ease of integration of sources onto one link
– the network operator only has to provide one
connection (one access link) to the customer and all
the services can be provided over this link.

Disadvantages
– cell delay variation
– cell assembly delay.
Cell delay variation caused by queueing delays
cells from source being tracked
this cell delayed by 1 slot because of queueing delays
expected delay
through the network
increased gap
reduced gap
B-ISDN protocol reference model
Management plane
Control Plane User Plane
Higher Layers Higher Layers
ATM Adaptation Layer
ATM Layer
Plane management
Physical Layer
Layer management
Normal OSI 7-layer model does not apply - separate B-ISDN model
Original physical layer interfaces (ITU)



Optical or electrical
SDH framed or cell-based
Bitrates:
– 155.52 Mbit/s upstream and downstream
– 622.08 Mbit/s in at least one direction (symmetry
of this interfaces not yet defined)
ATM layer -1



characteristics of ATM layer independent of
physical medium.
ATM uses virtual connections for information
transport: the virtual path and the virtual channel
All functions of ATM layer are supported by the
ATM cell header.
– Cell multiplexing and demultiplexing
– Cell Virtual Path Identifier (VPI) and Virtual Channel
Identifier (VCI) translation
– Cell Header generation/extraction
– Generic Flow Control
Cell structure
NNI header
bit 8
5 octet
header
bit 1
VPI
VPI
octet 1
VCI
VCI
VCI
48 octet
payload
CLP
PT
HEC
1st octet, UNI header
GFC
VPI
NNI: network node interface
UNI: user network interface
VPs and VCs
Virtual Path (VP)
Physical layer
Virtual Channel (VC)
Each VP within the physical layer has its own distinct Virtual Path
Identifier (VPI); each VC within a VP has its distinct Virtual
Channel Identifier (VCI)
VPI & VCIs: specific to a link
routeing information
in switch
VPIa, VCIb
input
output
port VPI VCI port VPI VCI
....... ...... ...... ....... ..... ......
P
a
b
Q x y
....... ...... ...... ....... ..... ......
VPIx, VCIy
input port P
output port Q
VC & VP Switching
VC switch
Endpoint of VPC
VP switch
representation of VC and
VP switching
VP switch
representation of VP switching
Virtual Paths (VPs)





VP generic name for a bundle of VC links, all VC
links in the bundle having the same endpoints
Virtual path links concatenated to form Virtual
Path Connection (VPC)
VPs provide logical direct routes between
switching nodes via intermediate cross-connects.
VP identified by VPI - routeing translation tables
in each node provide VPI translation.
VP concept may also be used in access network to
provide virtual leased lines, or to allow access to
competing operators.
AAL introduction



ATM Adaptation Layer (AAL) performs the
necessary mapping between the ATM layer and
the next higher layer at the edge of the network.
Functions of AAL depend upon higher layers (ie
on services as well)
Examples of service provided by AAL:
–
–
–
–
–
handling of quantization effect from cell payload size
handling of transmission errors
handling of lost and mis-inserted cell conditions
flow control and timing control
segmentation and reassembly
AAL - principles of segmentation
user information
addition of Convergence
Sublayer header and
trailer protocol
AAL
information to the user
information
addition of Segmentation and
Reassembly Sublayer header and
trailer protocol information to every
segment
Segmentation
payload for
the cell
heade
r
cell payload
cell
addition of the ATM header
ATM layer
Segmentation without end segment indication
data units
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5 data segmented
into cells
received cells - with one missing because of cell loss
1 2 3
5
1 2 3 4 5
1 2 3 4 5
reconstructed segments - all in error because of the slip of 1 cell
1 2 3 5 1
2 3 4 5 1
2 3 4 5
Segmentation with end segment indication
data units
end segment indicator
1 2 3 4 5 data segmented
into cells
received cells - with one missing because of cell loss
1 2 3 4 5
1 2 3 4 5
1 2 3
1 2 3 4 5
5
1 2 3 4 5
reconstructed segments - only 1 in error because reconstruction
starts again after end segment received
1 2 3 5
this data unit in error
1 2 3 4 5
1 2 3 4 5
these data units are correct
Connection admission control (CAC)

Network decides at call set-up whether to accept a
(VP or VC) connection request. Criteria:
– sufficient resources (QoS) available for connection
request across network
– agreed QOS of existing calls not affected


Call can have more than one connection: CAC
procedures should be performed for each
CAC needs the following information:
– source traffic characteristics
– required QOS class.
Connection admission control (CAC)

CAC uses this information to determine:
– whether the connection can be accepted or not
– the traffic parameters needed by usage parameter
control
– the allocation of network resources.
UPC continued

Functions
– checking validity of VCIs and VPIs
– checking traffic volume per VPC and VCC to
ensure contract not violated

Actions on violation
–
–
–
–
Discarding of cells
Dropping connection
Tagging of cells
(Punitive charging)