Communication Networks
P. Demeester
Chapter 9
Telephony
Telephony
9-1
Chapter 9 outline
9.1 Introduction
9.2 Analog Telephony
9.3 Digital Telephony
References
Table of Contents
Telephony
9-2
Introduction
EVOLUTION in the TELEPHONE NETWORK :
 basic service still telephony
 analog ==> digital transmission/switching in core
 analog ==> digital transmission in access (limited)
 advanced control and signalling
 introduction of optical fiber in core
 service integration (ISDN)
•
•
•
•
PSTN
POTS
IDN
ISDN
Telephony
9-3
Analog Telephony
9.1 Introduction
9.2 Analog Telephony
9.2.1 Architecture
9.2.2 Transmission
9.2.3 Switching
9.2.4 Control
9.3 Digital Telephony
References
Table of Contents
Telephony
9-4
General Architecture
TEX
TEX
LEX
access
network
(star)
local
network
transmission
links
LEX
terminals
terminals
LEX
TEX
TEX
core
network
(mesh)
LEX
local
network
access
network
(star)
Telephony
9-5
The Telephone
INVENTION:
March 10, 1876
transmitter
receiver
WATSON’S RECEIVER:
produces varying
resistance
metallic
diaphragm
BELL’s TRANSMITTER:
wire moves
up and down
in acid
tunnel to
concentrate
sound
diaphragm moves
up and down
Resistance across
terminals
varies with sound
EQUIVALENT ELECTRICAL CIRCUIT:
acid
Key Feature: variable resistance transmitter
varying
resistance
Photo of the first electric telephone (circa March 10, 1876).
Note: the transmitter on the left consisted of a small
container of acid in which a wire moved up and down in
response to the acoustic pattern of speech (sound waves).
receiver
constant dc
voltage
Telephony
9-6
The Telephone
Diaphragm
Diaphragm
soundwave
Carbon
granules
Microphone
Electrical current
battery wave form
Telephone
line
(pair of wires)
Recreated
soundwave
Earphone
Telephony
9-7
The Telephone
Talk
Dial : pulses
Push : tones
Technology improved but the basic concepts remained the same
9-8
TRANSFER THE TELEPHONE NUMBER Telephony
and TALK
Mesh-Star Topology
Cost
1 exchange
Total network cost
S
Junction line cost
4 exchange
Exchange cost
S
S
S
S
Subscribers’
line cost
No. of exchanges (S)
Optimum no.
of exchanges
Telephony
9-9
Analog Telephony
9.1 Introduction
9.2 Analog Telephony
9.2.1 Architecture
9.2.2 Transmission
9.2.3 Switching
9.2.4 Control
9.3 Digital Telephony
References
Table of Contents
Telephony 9-10
Frequency Division Multiplexing (FDM)
cable
amplifiers
...
LEX
...
...
...
...
Transmission over wires
(SDM)
TEX ..
.
...
to
other
exchanges
amplifier
LEX
TEX
Transmission over coax
(FDM)
FDM-multiplexers
FDM-demultiplexers
Telephony
9-11
Frequency Division Multiplexing (FDM)
Bandwidth
name
Consists of
Bandwidth
Usual
baseband
Number of
channels
Channel (1
24 telegraph
4 kHz
0-4 kHz
telephone channel) subchannel
120 Hz spacing
Group
12 channels
48 kHz
60-108 kHz
Supergroup
5 groups
240 kHz
312-552 kHz
Basic hypergroup
15 supergroups 3.7 MHz (3.6 MHz used) 312-4028 kHz
(also called a ‘super (3 mastergroups)(240 kHz per supergroup (4 MHz line)
mastergroup)
with 8 kHz spacing
normally between each)
Basic hypergroup
16 supergroups
4 MHz
60-4028 kHz
(alternative)
Mastergroup
5 supergroups
1.2 MHz
312-1548 kHz
Hypergroup
9 mastergroups
12 MHz
312-12336 kHz
(12 MHz)
Hypergroup
36 mastergroups
60 MHz
4404-59580 kHz
(60 MHz)
1
12
60
900
960
300
2700
10800
Telephony 9-12
Analog Telephony
9.1 Introduction
9.2 Analog Telephony
9.2.1 Architecture
9.2.2 Transmission
9.2.3 Switching
9.2.4 Control
9.3 Digital Telephony
References
Table of Contents
Telephony 9-13
Circuit Switching
flexible
connection
LEX
twisted pair
TEX
LEX
circuit switched network
fixed bandwidth
low delay
guarantee to finish call
Telephony 9-14
Switch Evolution
Switch
• manual switches (patch panel)
• electromechanical switches
- rotary switches
- cross-bar switches (relay based)
• semiconductor switches
Telephony 9-15
Strowger switch
Handset
Lifted
Digit 4
Dialed
Dial
Released
Digit 4
Pulse Sequence
Next
Digit
Dialed
Next Digit
Pulse Sequence
Loop
disconnect
(10 pps)
signaling
33mS
66mS
Minimum 400mS
inter digit pause
Single motion selector (Strowger)
2
3
1
4
5
electro-magnet
6
7
8
9
ratchet
wheel
outlet 1
outlet 2
outlet 3
wiper
arm
outlet 4
outlet 5
0
pawl
outlet 6
outlet 7
outlet 8
pivot
outlet 9
outlet 10
Telephony 9-16
Multistage Switching
45 x
4 xx
5 xx
495
49 x
55 x
stage 1
stage 2
stage 3
Telephony 9-17
Multistage Switching
Principle of Strowger automatic switching
Telephony 9-18
Crossbar Switch
cross-point :
relay, transistor
cable
twisted
pair
more cables
cross-bar switch
Telephony 9-19
30.000 cross-points
improved functionality
Concentration, distribution,expansion
1
100
901
1000
100 x 1
10
10
100 x 91
10
100
x
100
100
concentrator
1 100 x
10
10
91 100 x
100
901
10
1000
1
1
1000
100
expander
distributor
1
LOCAL
LINES
1
100
1
100
1000
Telephony 9-22
Analog Telephony
9.1 Introduction
9.2 Analog Telephony
9.2.1 Architecture
9.2.2 Transmission
9.2.3 Switching
9.2.4 Control
9.3 Digital Telephony
References
Table of Contents
Telephony 9-23
User and Control Plane
UNI
•
•
•
•
•
overlay network
NNI
TEX
LEX
TEX
LEX
UNI : LD, DMTF
NNI : SS7
U / C -plane
UNI / NNI
Call set-up
Signaling
Routing
LEX
TEX
TEX
LEX
switch
hardware
Telephony 9-24
Call Setup
calling party
exchange A
exchange B
called party
off-hook (seize)
dial tone
address digits
time
seize signal
proceed-to-send signal
address digits
ringing tone
ringing tone
ringing current
answer signal (off-hook) answer signal (off-hook)
CONVERSATION
forward clear (on-hook) forward clear (on-hook) backward clear (on-hook)
Telephony 9-25
Signaling : UNI
UNI
calling party
2
local exchange
• LD
• DTMF
697
697
770 Low-freq. band
852
941
1209 1336
1336 1447 1633
High-freq. band
Example
Digit 2 will cause
tone pair 697Hz
and 1336Hz
All frequencies shown in Hertz (Hz)
Telephony 9-26
Routing : network hierarchy
• routing table set-up
• call forwarding
Submarine
cables
International
network
Satellite
links
International gateway
exchange
National tandem exchanges
Trunk
network
Regional tandem exchanges
Junction network
Local network
Local tandem exchanges
Local exchanges
Customer lines
Note : today telecom operators are reducing the number of layers in the hierarchy
Telephony 9-27
by the use of consolidation (larger and fewer telephone switches)
Digital Telephony
9.1 Introduction
9.2 Analog Telephony
9.3 Digital Telephony
9.3.1 From A to D
9.3.2 Architecture
9.3.3 Transmission
9.3.4 Switching
References
Table of Contents
Telephony 9-28
Analog to Digital Conversion
analog/digital
converter
z(t)
sampling in time +
sampling in amplitude
111
AMPLITUDE :
number of
discrete levels
(number of
bits used)
110
101
100
011
010
001
000
T T T
t
TIME : sampling period T = 1/sampling frequency (=1/f)
100 100 011 001 001 011 101 110 110 011 001 001 011 100 Telephony 9-29
Analog to Digital Conversion
Analog telephony : bandwidth 300 - 3400 Hz
Sampling frequency : f = 8 kHz (T=125 msec period)
Digitization : 8 bits (256 levels)
==> 8 bits at 8 kHz sampling gives : 64 kbit/s
or : 8 bits every 125 msec
Digital telephony : 64 kbit/s
Why sampling at 8 kHz ?
Telephony 9-30
Sampling in time : Nyquist criterium
How fast should we sample in order to
be able to reproduce the original analog signal ?
if the bandwidth of the signal is B,
we should sample at least with a
frequency f = 2B
(Nyquist criterium)
Telephony 9-31
Telephony: 8000 Hz sampling >2x3100 Hz bandwidth
=> OK
Digital Telephony
9.1 Introduction
9.2 Analog Telephony
9.3 Digital Telephony
9.3.1 From A to D
9.3.2 Architecture
9.3.3 Transmission
9.3.4 Switching
References
Table of Contents
Telephony 9-32
General Architecture
local and core network : digital transmission and switching
access network : analog transmission
evolution from analog to digital network (1 single network!)
IDN : Integrated Digital Network
ANALOG
DIGITAL
TEX
LEX
TEX
LEX
access
network
(star)
local
network
ANALOG
transmission
links
LEX
terminals
terminals
•
•
•
•
TEX
TEX
core
network
(mesh)
LEX
local
network
access
network
(star)
Telephony 9-33
Digital Telephony
9.1 Introduction
9.2 Analog Telephony
9.3 Digital Telephony
9.3.1 From A to D
9.3.2 Architecture
9.3.3 Transmission
9.3.4 Switching
References
Table of Contents
Telephony 9-34
Guided Transmission Media:
twisted pair, coax, fiber
twist
length
Twisted pair
• separately insulated
• twisted together
• often “bundled” into cables
Insulation
Coaxial cable
•
•
•
•
•
•
•
•
Inner
conductor
outer conductor is braided shield
inner conductor is solid metal
separated by insulating material
covered by padding
Optical fiber
Outer sheath
Outer
conductor
Core
glass or plastic core
laser or light emitting diode
specially designed jacket
small size and weight
Jacket
Cladding
Angle of
Angle of
Light at less than incidence
reflection
critical angle is
absorbed in jacket
Telephony 9-35
Time Division Multiplexing (TDM)
gate
1 bit
A
4 bits
B
INPUT
C
OUTPUT
D
bit-TDM multiplexer
1 bit
TRIB A
TRIB B
TRIB C
TRIB D
GATE A
GATE B
GATE C
BIT INTERLEAVED
GATE D
OUTPUT
4 bits
Telephony 9-36
Time Division Multiplexing (TDM)
input
signals
frame
frame
1 byte
A
B
C
D
125
msec
8 bits
64 kbit/s
M
U
X
4 bytes
output
signal
125 msec
32 bits
256 kbit/s
byte-TDM
multiplexer
BYTE INTERLEAVED
compress and add
Telephony 9-37
Timing in TDM
FAS
FRAME
BYTE
BIT
CLOCK
(synchronization)
Telephony 9-38
PDH: Plesiochronous Digital Hierarchy
Europe
USA
Japan
bitrate
multiplex bitrate
multiplex
bitrate
multiplex
kbit/s
factor
factor
kbit/s
factor
64
kbit/s
64
64
2048
30/31
1544
24
1544
24
8448
4
6312
4
6312
4
34368
4
44736
7
32064
5
139264
4
139264
3
97728
3
higher bitrates are proprietary
• from 64 kbit/s to 2.048 Mbit/s : byte interleaved
• higher multiplex levels : bit interleaved
Telephony 9-39
SDH: Synchronous Digital Hierarchy
Synchronous
Transport
Module
(SDH)
Line Rate
Mbit/s
Synchronous
Transport
Signal
(SONET)
51.84
STS - 1
STM - 1
155.52
STS - 3
STM - 4
622.08
STS - 12
STM - 16
2488.32
STS - 48
STM - 64
9953.28
STS - 192
SDH : Synchronous Digital Hierarchy (Europe)
SONET : Synchronous Optical Network (USA)
Telephony 9-40
Digital Telephony
9.1 Introduction
9.2 Analog Telephony
9.3 Digital Telephony
9.3.1 From A to D
9.3.2 Architecture
9.3.3 Transmission
9.3.4 Switching
References
Table of Contents
Telephony 9-41
Concept
TEX
PDH
transmission
links
TRANSIT EXCHANGE (TEX)
LEX
1
8
TEX
1
TEX
34
1
..
.
2
3
4
2
2
3
3
4
4
2
TIME-switching
.
e.g.: TS8 from input (2, 3, 15)
N
34 Mb/s
port
1
34
2
3
8 Mb/s 2 Mb/s
port
port
4
TS20 from output (N, 1, 3)
Mb/s
DEMUX (PDH)
1
3
4
SPACE-switching
1
..
.
2
64 kbit/s circuits
2
.
..
.
OUTPUT
.
1
2
2
3
3
4
4
1
8
.
34
2
1
2
4
8
to
2
3
INPUT
.
8
34
2
..
.
1
2
1
.
2
2
1
2
2
3
3
4
4
DIGITAL SWITCH BLOCK
N
Telephony 9-42
MUX (PDH)
Space Switching
OUTLETS
1
2
3
1
2
3
1
2
3
1
2
3
321
INLETS
321
321
321
Telephony 9-43
Time Switching
TSI
4321 4321
frame
frame
Time Slot Interchange
4321 4321
frame
frame
TS1
TS2
4321
TSI
TS3
TS4
4321
Telephony 9-44
Summary
8 bit
125 msec
Switch &
Multiplex
frame
32 bit
125 msec
LEX
TEX
LEX
circuit : a timeslot (8 bits) in every frame (125 msec)
is allocated between two phones during a call
Telephony 9-45
exchange : multiplex calls, switch calls
Time-Space Switching
A1
A2
A3
B1
TSI
TS10
TS11
TSI
B2
?
TS20
TS20
TSI
B3
time switches
Connection 1 :
A2/TS10 -> B1/TS20
space switches
Connection 2 :
A2/TS11 -> B2/TS20
Telephony 9-46
Time-Space-Time Switching
A1
A2
A3
TSI
TS10
TS11
TSI
TS14
TS14
TS15
TS15
TSI
TSI
B1
TS20
B2
TS20
TSI
TSI
time switches
time switches
Connection 1 :
A2/TS10 -> B1/TS20
B3
space switches
Connection 2 :
A2/TS11 -> B2/TS20
Telephony 9-47
Evolution
Switch technology over the years for a typical industrialized country
# Switches
installed per
Year
Crossbar
Digital Broadband
(electronic)
SPC
Broadband
(photonic)
Analog
Step-by-step SPC
Panel
1950
1960
1970
1980
1990
SPC =
Stored Program
2000 2010 2020
Control
Telephony 9-48
Evolution
Percentage
of Total
Access Lines
in US
100
99
97
90
92
80
Digital SPC
80
70
60
50
40
30
20
Analog SPC
62
58
59
60
56
52
50
48
44
42
38
36
33
Electromechanical
23
19
16
10
0
1980
0
3
10
84
88
1
5
92
7
96
2
0
2000
Year
Telephony 9-49
References
“Networks and Telecommunications, Design and Operation” by Martin P. Clark
(1997,Second Edition, John Wiley & Sons Ltd., ISBN 0-471-97346-7)
“Telecommunications switching, traffic and networks”, by J.E. Flood (1994,
Prentice Hall, ISBN-0-13-033309-3)
“SPC Digital Telephone Exchanges”, F.J. Redmill & A.R. Valdar, Peter Peregrinus
Ltd, 1990, ISBN 0 86341 298 x
“Integrated Digital Networks”, L.S. Lawton, Sigma Press, 1993,
ISBN 1-85058-181-9
Telephony 9-50
Table of Contents
9.1 Introduction
2
9.2 Analog Telephony
9.2.1 Architecture
4
9.2.2 Transmission
10
9.2.3 Switching
13
9.2.4 Control
23
9.3 Digital Telephony
9.3.1 From A to D
28
9.3.2 Architecture
32
9.3.3 Transmission
34
9.3.4 Switching
References
Table of Contents
41
50
51
Telephony 9-51