1. CSC 535 Communication Networks I
Chapter 4
Multiplexing, Switching,
and Telephone Systems
Dr. Cheer-Sun yang

2. Multiplexing
Several slow speed links sharing one high speed link

3. 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

4. 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.)

5. (a) (b) A A A Trunk group A B B B MUX MUX B C C C C
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.1

6. Multiplexing Techniques
Frequency Division Multiplexing (FDM)
(Synchronous)Time Division Multiplexing (TDM)
(Asynchronous) Statistical Time Division Multiplexing (Statistical TDM)
Wavelength Division Multiplexing (WDM)

7. 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

8. Frequency Division Multiplexing Diagram

9. (a) Individual signals occupy W Hzf B A W
(b) Combined signal fits into channel bandwidth A C B f
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.2

10. FDM System

11. FDM of Three Voiceband Signals

12. 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

13. 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

14. ADSL Design Asymmetric Frequency division multiplexing Range 5.5km
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

15. 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

16. Time Division Multiplexing

17. 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 = * 8 * 8000 = Mbps

18. (a) Each signal transmits 1 unit every 3T secondsB1 B2 C1 C2 3T 0T 6T
(b) Combined signal transmits 1 unit every T seconds t B1 C1 A2 C2 B2 A1
0T 1T 2T 3T 4T 5T 6T
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.3

19. 1 1 2
MUX
MUX 2
. . .
. . . 22 23 24 b 1 2
. . . 24 b 24
frame 24
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.4

20. North American Digital Hierarchy28
M13
Multiplex
M23 x7
Primary
Eg. Digital
Switch
24 chan PCM
M12 x4 1
DS Mbps
DS Mbps
DS Mbps 
European Digital Hierarchy
Primary
Multiplex
Eg. Digital
Switch
30 chan PCM
4th order x4
2nd order
3rd order
Mbps
2.048 Mbps
Mbps
Mbps
CEPT 1
CEPT 4
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.5

21. t 5 4 5 3 2 1 4 3 2 1
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.6

22. TDM System

23. 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

24. Data Link Control on TDM

25. 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 …unlikely on a data channel
Can compare incoming bit patterns on each channel with sync pattern

26. 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

27. TDM of Analog and Digital Sources

28. 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

29. 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

30. 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 Mpbs, whereas T-1 can transmit DS-1 signals at the speed of Mbps.

31. Design Issues SONET multiplexing (WDM) Add-drop multiplexing
Survivability
SONET frame mapping

32. 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

33. 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

34. Frame Structure

35. 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

36. Primary ISDN Frame Formats

37. DS1 DS2 STS-1 CEPT-1 51.84 Mbps DS3 STS-1 44.736 OC-n STS-n STS-3c
Low-Speed
Mapping
Function
DS2
STS-1
CEPT-1
51.84 Mbps
DS3
Medium
Speed
Mapping
Function
STS-1
44.736  
OC-n
STS-n
STS-3c
STS-1
Mux
Scrambler
E/O
CEPT-4
High-
Speed
Mapping
Function
STS-1
STS-1
STS-3c
STS-1
STS-1
High-
Speed
Mapping
Function
ATM
STS-1
150 Mbps
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.8

38. 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 Mbps)
Multiple STS-1 combined into STS-N signal
ITU-T lowest rate is Mbps (STM-1)

39. SONET Frame Format

40. SONET STS-1 Overhead Octets

41. 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

42. Statistical TDM Frame Formats

43. (a) pre-SONET multiplexing
MUX
DEMUX
remove
tributary
insert
tributary
(b) SONET Add-Drop multiplexing
MUX
ADM
DEMUX
remove
tributary
insert
tributary
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.9

44. a b 3 ADMs c physical loop net OC-3n
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.10

45. a a b c b c (b) (a) OC-3n logical fully-connected net
3 ADMs connected in
physical ring topology
OC-3n a b c
logical fully-connected net
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.11

46. (b) Loop-around in response to faultc d a b c d
(a) Dual ring
(b) Loop-around in response to fault
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.12

47. Regional Metro Ring Inter-Office Rings
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.13

48. (a) SONET Terminal Mux reg (b) path line section optical STS PTE LTE
STE
STS-1 Path
STS Line
Section
Mux
reg
SONET
Terminal
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)
optical
section
line
path
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.14

49. 90 bytes B B B 87B Section Overhead 3 rows Information Payload 9 Rows
Line
Overhead
6 rows
125 s
Transport
overhead
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.15

50. first column is path overhead
Pointer
first octet
frame k
87 columns
Synchronous
Payload
Envelope 9
rows
Pointer
last octet
frame
k+1
first column is path overhead
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.16

51. STS-1 STS-1 Map STS-1 STS-1 Interleave Byte STS-1 STS-1 STS-3 STS-1
Incoming
STS-1 Frames
Synchronized New
STS-1 Frames
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.17

52. 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.

53. Optical MUX Optical deMUX 1 1 2 1 2. m 2 Optical fiber m m
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.18

54. (a) WDM chain network b c d a (b) WDM ring network a 3 ADMs b c
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.20

55. Asynchronous TDM
Also known as statistical time division multiplexing (sections and 5.5.2)

56. A B C Input lines Output line Buffer Header Data payload
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.42

57. (a) (b) Dedicated Lines A1 A2 B1 B2 C1 C2 Shared Line A1 C1 B1 A2 B2
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.43

58. (a) (b) (c) Dedicated Lines A1 A2 B1 B2 C1 C2 Shared Line A1 C1 B1 A2
N(t)
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.44

59. Average Delay (seconds)
Goodput (bits/second)
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.49

60. Part of this burst is lost
Many
Voice
Calls
Fewer
Trunks
Part of this burst is lost
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.50

61. # connections Trunks Speech loss 48 32 40 24
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.51

62. Buffer Many voice terminals generating voice packets buffer overflowD2 C2 B1 C1 D1 A1
Buffer C3 C2 C1 D3 D2 D1
buffer overflow B2
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.52

63. 1 2 3
Sent t 1 2 3
Received t
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 5.53

64. The Telephone Network Multiplexing
Switching - details will be discussed in CSC581
Signaling

65. Control Connection of inputs to outputs (a) Network Link Switch User n1 2 3 N
Connection
of inputs
to outputs 
(b) Switch
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.21

66. Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.30

67. Source Signal Go Signal Ahead Message Release Signal Destination
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.31

68. (a) Routing in a typical metropolitan area4 C D 2 3 5 A B 1
(b) Routing between two LATAs
net 1
net 2
LATA 1
LATA 2
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.32

69. local telephone office
Pedestal
local telephone office
Serving Area I/f
distribution cable
Distribution Frame
Switch
Serving Area I/f
feeder cable
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.33

70. Transmit pair Original signal Received signal Hybrid transformer
Echoed signal
Hybrid transformer
Receive pair
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.34

71. Channel-switched traffic (digital leased lines) Local analog Tie lines
cross-connect
System
Channel-switched traffic
(digital leased lines)
Local
analog
Tie lines
Foreign exchange
Local
digital
Local
Switch
Digital
trunks
Circuit-switched traffic
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.35

72. Physical SONET Topology using ADMs and DCCs Logical Topology
Switches see this
topology
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.36

73. Basic Rate Interface (BRI): 2B+D
Circuit
Switched
Network
Basic Rate Interface (BRI): 2B+D
Primary Rate Interface (PRI): 23B+D
Channel
Switched
Network
Private
BRI
BRI
Packet
Switched
Networks
PRI
PRI
Signaling
Network
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.37

74. SPC Control Signaling Message
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.39

75. Trunks Signaling Office A Office B Switch Switch Processor Modem Modem
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.39

76. SCP STP STP STP STP SSP SSP Signaling Network Transport Network
SSP = Service switching point (signal to message)
STP = Signal transfer point (message transfer)
SCP = Service control point (processing)
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.40

77. External Database Signaling Network Intelligent Peripheral SSP SSP
Transport Network
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.40

78. Application Layer Presentation Layer TUP TCAP ISUP Session Layer
Transport Layer
SCCP
Network Layer
MTP Level 3
Data Link Layer
MTP Level 2
Physical Layer
MTP Level 1
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.42

79. Many Fewer Lines Trunks
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.43

80. N(t) t all trunks busy 1 2 trunk # 3 4 5 6 7
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.44

81. the offered load a = lamda * expected holding time
TRAFFIC CONTROL
the offered load a = lamda * expected holding time
utilization = a * (1 - blocking probability) / c
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks

82. Blocking Probability # trunks offered load 10 9 8 1 2 3 4 7 5 6
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.45

83. 2 7 3 1 6 4 5 2 2 7 3 7 3 1 1 6 4 6 4 5 5
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.51

84. SS#7 STP MSC HLR VLR wireline terminal EIR PSTN AC BSS BSS
AC = authentication center
BSS = base station subsystem
EIR = equipment identity register
HLR = home location register
MSC = mobile switching center
PSTN = public switched telephone network
STP = signal transfer point
VLR = visitor location register
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.52

85. Um Abis A mobile station base transceiver station
LAPDm
radio
RRM MM CM
SCCP
MTP Level 3
MTP Level 2 CM MM
RRM
64 kbps Um
Abis A
radio
LAPDm
RRM
LAPD
64 kbps
64 kbps
LAPD
RRM
MTP Level 3
MTP Level 2
SCCP
mobile station
base transceiver station
base station controller
MSC
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.53

86. satellite motion (a) (b)
Copyright 2000 McGraw-Hill Leon-Garcia and Widjaja Communication Networks
Figure 4.54

87. 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

88. ISDN User Network Interface
ISDN allows multiplexing of devices over single ISDN line
Two interfaces
Basic ISDN Interface
Primary ISDN Interface

89. Reading Assignment
Chapter 4