CSC 535
Communication Networks I
Chapter 4
Multiplexing, Switching,
and Telephone Systems
Dr. Cheer-Sun yang
Multiplexing
Several slow speed links sharing one high speed
link
2
Switching
It is costly to establish a fully-connected system
for which there is a direct connection between
any two hosts.
Several transmitters all connecting to a central
location, known as central office, in order to
save the direction connections among all of the
transmitters
Application: telephone system
3
Topics for this chapter
Multiplexing techniques including FDM, TDM,
statistical TDM (asynchronous TDM), WDM(4.1,
4.2, 4.3, 5.5.1, and 5.5.2)
Telephone networks: general concepts (data
transmission, signaling, traffic engineering),
celluar telephone networks.(Sections 4.5, 4.6,
4.7, 4.8)
(Switching will be discussed when network
layer concept is introduced in CSC 581.
Although switching is a physical layer technique
but routing is a network layer concept in which
Wide Area Network is involved.)
4
(a)
(b)
A
A
A
B
B
B
C
C
C
A
Trunk
group
MUX
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MUX
B
C
Figure 4.1
5
Multiplexing Techniques
Frequency Division Multiplexing (FDM)
(Synchronous)Time Division Multiplexing (TDM)
(Asynchronous) Statistical Time Division
Multiplexing (Statistical TDM)
Wavelength Division Multiplexing (WDM)
6
Frequency Division Multiplexing
Introduced in 1930s into telephone network
Useful bandwidth of medium exceeds required
bandwidth of channel
Each signal is modulated to a different carrier
frequency
Carrier frequencies separated so signals do not
overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
7
Frequency Division Multiplexing
Diagram
8
(a) Individual signals occupy W Hz
A
f
W
0
B
0
f
W
C
f
0
W
(b) Combined signal fits into channel bandwidth
A
B
C
f
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Figure 4.2
9
FDM System
10
FDM of Three Voiceband Signals
11
Analog Carrier Systems
AT&T (USA)
Hierarchy of FDM schemes
Group
12 voice channels (4kHz each) = 48kHz
Range 60kHz to 108kHz
Supergroup
60 channel
FDM of 5 group signals on carriers between 420kHz
and 612 kHz
Mastergroup
10 supergroups
12
Asymmetrical Digital
Subscriber Line
ADSL
Link between subscriber and network
Local loop
Uses currently installed twisted pair cable
Can carry broader spectrum
1 MHz or more
13
ADSL Design
Asymmetric
Greater capacity downstream than upstream
Frequency division multiplexing
Lowest 25kHz for voice
Plain old telephone service (POTS)
Use echo cancellation or FDM to give two bands
Use FDM within bands
Range 5.5km
14
Synchronous Time Division
Multiplexing
Introduced into the telephone network in 1960s.
Data rate of medium exceeds data rate of digital
signal to be transmitted
Multiple digital signals interleaved in time
May be at bit level of blocks
Time slots preassigned to sources and fixed
Time slots allocated even if no data
Time slots do not have to be evenly distributed
amongst sources
15
Time Division Multiplexing
16
T-1 Line (DS-n Signal)
The T-1 carrier system carries 24 digital
telephone connections (channels) by sampling a
speech waveform 8000 times/second and by
representing each sample with eight bits.
The T-1 system uses a frame (a data link layer
PDU) that consists of 24 slots of eight bits each.
Each slot carries one PCM sample of a single
channel.
Total speed = 1 + 24 * 8 * 8000 = 1.544 Mbps
17
(a) Each signal transmits 1 unit every 3T
seconds
A1
A2
0T
t
6T
3T
B1
B2
t
6T
3T
0T
C1
C2
0T
t
6T
3T
(b) Combined signal transmits 1 unit every T
seconds
A1 B1
0T 1T 2T
C1
A2
3T 4T
B2
5T
C2
t
6T
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Figure 4.3
1
24
MUX
MUX
22
23
24
b
1
2
...
frame
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24
b
2
...
...
2
1
24
19
Figure 4.4
North American Digital Hierarchy
Primary
DS1 1.544 Mbps
Multiplex
Eg. Digital
Switch
24 chan PCM
1
M12
Multiplex DS2 6.312 Mbps
x7
x4
M13
Multiplex
M23
Multiplex DS3 44.736 Mbps
DS3 44.736 Mbps



28
European Digital Hierarchy
CEPT 1
Primary
2.048 Mbps
Multiplex
Eg. Digital
Switch
30 chan PCM
2nd order 8.448 Mbps
Multiplex
3rd order 34.368 Mbps
Multiplex
CEPT 4
4th order
Multiplex 139.264 Mbps
x4
x4
x4
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20
Figure 4.5
t
5
4
3
2
1
5
4
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2
1
21
Figure 4.6
TDM System
22
TDM Link Control
No headers and tailers
Data link control protocols not needed
Flow control
Data rate of multiplexed line is fixed
If one channel receiver can not receive data, the
others must carry on
The corresponding source must be quenched
This leaves empty slots
Error control
Errors are detected and handled by individual
channel systems
23
Data Link Control on TDM
24
Framing
No flag or SYNC characters bracketing TDM
frames
Must provide synchronizing mechanism
Added digit framing
One control bit added to each TDM frame
Looks like another channel - “control channel”
Identifiable bit pattern used on control channel
e.g. alternating 01010101…unlikely on a data
channel
Can compare incoming bit patterns on each channel
with sync pattern
25
Pulse Stuffing
Problem - Synchronizing data sources
Clocks in different sources drifting
Data rates from different sources not related by
simple rational number
Solution - Pulse Stuffing
Outgoing data rate (excluding framing bits) higher
than sum of incoming rates
Stuff extra dummy bits or pulses into each incoming
signal until it matches local clock
Stuffed pulses inserted at fixed locations in frame
and removed at demultiplexer
26
TDM of Analog and Digital
Sources
27
Digital Carrier Systems(1)
Hierarchy of TDM
USA/Canada/Japan use one system: T1 Link or
DS-1 signal
ITU-T use a similar (but different) system
US system based on DS-1 format
Multiplexes 24 channels
Each frame has 8 bits per channel plus one
framing bit
193 bits per frame
28
Digital Carrier Systems (2)
For voice each channel contains one word of
digitized data (PCM, 8000 samples per sec)
Data rate 8000x193 = 1.544Mbps
Five out of six frames have 8 bit PCM samples
Sixth frame is 7 bit PCM word plus signaling bit
Signaling bits form stream for each channel
containing control and routing info
Same format for digital data
23 channels of data
7 bits per frame plus indicator bit for data or systems
control
24th channel is sync
29
Synchronous Optical
Network(SONET)
Using optical fiber as the transmission link
Used with ISDN or ATM
Referred to as Synchronous Digital Hierarchy
(SDH) in Europe
The signals are called synchronous transport
signal level-n(STS-n) and Synchronous Transfer
Module-n(STM-n), respectively, in SONET and
SDH.
SONET can transmit STS-1 signals at the speed
of 51.84 Mpbs, whereas T-1 can transmit DS-1
signals at the speed of 1.544 Mbps.
30
Design Issues
SONET multiplexing (WDM)
Add-drop multiplexing
Survivability
SONET frame mapping
31
Basic ISDN Interface (1)
Digital data exchanged between subscriber and
NTE - Full Duplex
Separate physical line for each direction
Pseudoternary coding scheme
1=no voltage, 0=positive or negative 750mV +/-10%
Data rate 192kbps
Basic access is two 64kbps B channels and one
16kbps D channel
This gives 144kbps multiplexed over 192kbps
Remaining capacity used for framing and sync
32
Basic ISDN Interface (2)
B channel is basic iser channel
Data
PCM voice
Separate logical 64kbps connections o different
destinations
D channel used for control or data
LAPD frames
Each frame 48 bits long
One frame every 250s
33
Frame Structure
34
Primary ISDN
Point to point
Typically supporting PBX
1.544Mbps
Based on US DS-1
Used on T1 services
23 B plus one D channel
2.048Mbps
Based on European standards
30 B plus one D channel
Line coding is AMI usingHDB3
35
Primary ISDN Frame Formats
36
DS1
DS2
Low-Speed
Mapping
Function
CEPT-1
DS3
44.736



CEPT-4
139.264
ATM
150 Mbps
STS-1
51.84 Mbps
Medium
Speed
Mapping
Function
HighSpeed
Mapping
Function
HighSpeed
Mapping
Function
STS-1



STS-1
STS-1
STS-1
STS-1
STS-1
STS-1
STS-3c
OC-n
STS-n
Mux
Scrambler
E/O
STS-3c
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37
Figure 4.8
Sonet/SDH
Synchronous Optical Network (ANSI)
Synchronous Digital Hierarchy (ITU-T)
Compatible
Signal Hierarchy
Synchronous Transport Signal level 1 (STS-1) or
Optical Carrier level 1 (OC-1)
51.84Mbps
Carry DS-3 or group of lower rate signals (DS1 DS1C
DS2) plus ITU-T rates (e.g. 2.048Mbps)
Multiple STS-1 combined into STS-N signal
ITU-T lowest rate is 155.52Mbps (STM-1)
38
SONET Frame Format
39
SONET STS-1 Overhead Octets
40
Statistical TDM
In Synchronous TDM many slots are wasted
Statistical TDM allocates time slots dynamically
based on demand
Multiplexer scans input lines and collects data
until frame full
Data rate on line lower than aggregate rates of
input lines
41
Statistical TDM Frame Formats
42
(a) pre-SONET multiplexing
MUX
DEMUX
MUX
remove
tributary
DEMUX
insert
tributary
(b) SONET Add-Drop multiplexing
MUX
ADM
DEMUX
insert
remove
tributary
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43
Figure 4.9
a
OC-3n
OC-3n
b
3 ADMs
c
OC-3n
physical loop net
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Figure 4.10
(b)
(a)
a
a
OC-3n
OC-3n
b
c
b
c
OC-3n
logical fully-connected net
3 ADMs connected in
physical ring topology
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Figure 4.11
a
a
b
d
c
c
(a) Dual ring
b
d
(b) Loop-around in response to fault
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Figure 4.12
Regional
Ring
Metro
Ring
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Inter-Office
Rings
47
Figure 4.13
(a)
STS
PT
E
SONET
Terminal
LTE
Mux
STE
STE
STE
reg
reg
reg
LTE
Mux
STS
PT
E
SONET
Terminal
Section
Section
STS Line
STS-1 Path
STE: Section Terminating Equipment, e.g. a repeater
LTE: Line Terminating Equipment, e.g. a STS-1 to STS-3 multiplexer
PTE: Path Terminating Equipment, e.g. an STS-1 multiplexer
(b)
path
path
line
section
line
section
section
section
optical
optical
optical
optical
line
section
line
section
section
optical
optical
optical
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48
Figure 4.14
90 bytes
B
Section
Overhead
B
B
87B
3 rows
Information
Payload
Line
Overhead
9 Rows
6 rows
Transport
overhead
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125 s
49
Figure 4.15
Pointer
first octet
frame
k
87 columns
Synchronous
Payload
Envelope
Pointer
9
rows
last octet
frame
k+1
first column is path overhead
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50
Figure 4.16
STS-1 STS-1
STS-1 STS-1
Incoming
STS-1 Frames
Map
Map
STS-1 STS-1
STS-1 STS-1
Byte
Interleave
STS-1 STS-1
STS-1 STS-1
Map
STS-3
Synchronized New
STS-1 Frames
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51
Figure 4.17
Sonet/SDH WDM
WDM can be viewed as an optical-domain
version of FDM in which multiple information
signals modulate optical signals at different
optical wavelengths (colors).
Prisms and diffraction gratings can be used to
combine and split color sigals.
For example, WDM systems are available that
use 16 wavelengths at OC-48 to provide
aggregate rates up to 16 * 2.5 Gbps = 40 Gbps.
52
1
2
m
Optical
MUX
Optical
deMUX
1  2 .
m
1
2
Optical
fiber
m
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Figure 4.18
(a)
WDM chain
network
b
c
a
d
(b) WDM ring network
a
b
3 ADMs
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c
54
Figure 4.20
Asynchronous TDM
Also known as statistical time division
multiplexing (sections 5.5.1 and 5.5.2)
55
Input lines
A
Output line
B
Buffer
C
Header
Data payload
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Figure 5.42
(a)
Dedicated Lines
A1
A2
B1
B2
C1
(b)
Shared Line
A1
C2
C1
B1
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A2
B2
C2
57
Figure 5.43
(a)
Dedicated Lines
A1
A2
B1
B2
C1
(b)
Shared Line
(c)
N(t)
A1
C2
C1
B1
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A2
B2
C2
58
Figure 5.44
Average Delay (seconds)
0.6
0.5
L=1200
0.4
0.3
L=800
L=400
0.2
0.1
L=200
0
0
8000
16000
24000
32000
40000
48000
56000
64000
Goodput (bits/second)
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Figure 5.49
Many
Voice
Calls
Fewer
Trunks
Part of this burst is lost
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Figure 5.50
Trunks
10
12
14
16
20
18
24
22
Speech loss
1
0.1
48
0.01
24
32
40
0.001
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Communication Networks
# connections
61
Figure 5.51
Many voice terminals
generating voice packets
A3
B3
C3
D3
A1
A2
B1
B2
C2
D2
C1
Buffer
B3 C3 A2 D2 C2 B1 C1 D1 A1
D1
buffer overflow
B2
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Figure 5.52
Sent
Received
1
2
3
1
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Communication Networks
t
2
3
t
63
Figure 5.53
The Telephone Network
Multiplexing
Switching - details will be discussed in CSC581
Signaling
64
(a) Network
Link
Switch
User n
User n-1
User 1
Control
(b) Switch
1
2
3



N
Connection
of inputs
to outputs
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1
2
3



N
65
Figure 4.21
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Figure 4.30
Source
Signal
Signal
Go
Ahead
Message
Release
Signal
Destination
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Figure 4.31
(a) Routing in a typical metropolitan area
4
C
2
A
D
3
5
1
B
(b) Routing between two LATAs
net 1
net 2
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2000 McGraw-Hill LATA 2
LATA
1
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Figure 4.32
Pedestal
local telephone office
Distribution Frame
Serving
Area I/f
distribution
cable
Serving
Area I/f
Switch
feeder
cable
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Figure 4.33
Original
signal
Transmit pair
Echoed
signal
Received signal
Hybrid
transformer
Receive pair
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Figure 4.34
Digital
cross-connect
System
Local
analog
Channel-switched traffic
(digital leased lines)
Tie lines
Foreign exchange
Local
digital
Local
Switch
Digital
trunks
Circuitswitched
traffic
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Figure 4.35
ADM
ADM
Physical SONET
Topology using
ADMs and DCCs
ADM
ADM
ADM
DCC
ADM
Logical Topology
Switches see this
topology
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Figure 4.36
Circuit
Switched
Network
BRI
PRI
Channel
Switched
Network
Private
Packet
Switched
Networks
Basic Rate
Interface
(BRI): 2B+D
BRI
Primary Rate
Interface
(PRI): 23B+D
PRI
Signaling
Network
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Figure 4.37
SPC
Control
Signaling Message
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Figure 4.39
Office A
Office B
Trunks
Switch
Processor
Switch
Modem
Modem
Processor
Signaling
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75
Figure 4.39
STP
SCP
STP
STP
Signaling Network
SSP
STP
SSP
Transport Network
SSP = Service switching point (signal to message)
STP = Signal transfer point (message transfer)
SCP = Service control point (processing)
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Figure 4.40
External
Database
Signaling
Network
Intelligent
Peripheral
SSP
SSP
Transport Network
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Figure 4.40
Application Layer
Presentation Layer
TUP
TCAP
ISUP
Session Layer
SCCP
Transport Layer
Network Layer
MTP Level 3
Data Link Layer
MTP Level 2
Physical Layer
MTP Level 1
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Figure 4.42
Many
Lines
Fewer
Trunks
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Figure 4.43
N(t)
all trunks busy
t
1
2
3
4
5
6
7
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Figure 4.44
TRAFFIC CONTROL
the offered load a = lamda * expected holding time
utilization = a * (1 - blocking probability) / c
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# trunks
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Blocking Probability
1
0.1
0.01
10
9
0.001
8
0.0001
1
2
3
4
5
6
7
offered load
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Figure 4.45
2
7
3
1
6
4
5
2
7
2
7
3
1
6
3
1
6
4
4
5
5
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83
Figure 4.51
BSS
BSS
MSC
STP
HLR
VLR
EIR
AC
SS#7
PSTN
AC = authentication center
BSS = base station subsystem
EIR = equipment identity register
HLR = home location register
MSC
PSTN
STP
VLR
wireline
terminal
= mobile switching center
= public switched telephone network
= signal transfer point
= visitor location register
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Figure 4.52
CM
Um
Abis
A
MM
MM
RRM
RRM
RRM
RRM
SCCP
LAPDm
radio
mobile station
CM
LAPDm
LAPD
radio
64
kbps
LAPD
64
kbps
MTP
Level 3
MTP
Level 3
MTP
Level 2
MTP
Level 2
64
kbps
64
kbps
base transceiver
base station
station
controller
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SCCP
MSC
85
Figure 4.53
satellite
motion
(a)
(b)
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Figure 4.54
Mixed Data
DS-1 can carry mixed voice and data signals
24 channels used
No sync byte
Can also interleave DS-1 channels
Ds-2 is four DS-1 giving 6.312Mbps
87
ISDN User Network Interface
ISDN allows multiplexing of devices over single
ISDN line
Two interfaces
Basic ISDN Interface
Primary ISDN Interface
88
Reading Assignment
Chapter 4
89