صفحه 1:
William Stallings
Data and Computer
Communications
Chapter 8
Multiplexing
صفحه 2:
Multiplexing
Link, n channels
n inputs n outputs
صفحه 3:
Frequency Division Multiplexing
۱
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
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صفحه 4:
Frequency Division Multiplexing
Diagram
3
12
ِ
3
5
bee
صفحه 5:
FDM System
Ww ۱ ۱
bey مق
(Spectrum of composite baseband امرس nel
[napus te
Demos > لتفلنك
صفحه 6:
FDM of Three Voiceband Signals
صفحه 7:
Analog Carrier Systems
T AT&T (USA)
1 Hierarchy of FDM schemes
1 Group
I 12 voice channels (4kHz each) = 48kHz
I Range 60kHz to 108kHz
1 Supergroup
! 60 channel
١ FDM of 5 group signals on carriers between 420kHz
and 612 kHz
0 Mastergroup
I 10 supergroups
صفحه 8:
Synchronous Time Division
Multiplexing
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
ص
وح اوح
ا
صفحه 9:
Time Division Multiplexing
صفحه 10:
TDM System ~
صفحه 11:
TDM Link Control
I No headers and tailers
0 Data link control protocols not needed
0 Flow control
! Data rate of multiplexed line is fixed
I If one channel receiver can not receive data, the
others must carry on
I The corresponding source must be quenched
I This leaves empty slots
0 Error control
| Errors are detected and handled by individual
channel systems
صفحه 12:
Data Link Control on TDM
Input, —
Output,
Input Output,
fo a hg CA Hh a gy dg GA
Cy AQ وا dy dy وه dy dy dy Cz Ay Py fe fe يف
¢b) Input daa streams
و وك رو تاك ره a Fy da dy dy Oy dy رك ول رك sea Fy da fy a fh dy Gy dy dy Cy dy Az Cy Fe AL fy
(c) Multiplexed data stream
Fefag field d= one eeter of data fied
‘akdres field 1'= one octet of PCS field
control tel
صفحه 13:
Framing
I No flag or SYNC characters bracketing TDM
frames
i Must provide synchronizing mechanism
0 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
1 Can compare incoming bit patterns on each
channel with sync pattern
صفحه 14:
Pulse Stuffing
I Problem - Synchronizing data sources
! Clocks in different sources drifting
0 Data rates from different sources not related
by simple rational number
Solution - Pulse Stuffing
I 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
I Stuffed pulses inserted at fixed locations in frame
and removed at demultiplexer
a
صفحه 15:
TDM of Analog and Digital Sources
‘TDM PAM signal TOMPCM signal
Tetons Tips
sedi
ما
ram source 3
TIT, ag
Fram source Pulse
72 005. gta stuffing
Pulse |_ sbops,cigiat
ان
ouput signal
stuffing
From source LL Pulse 5 kbps, digital
72 Kbps digital stuffing
صفحه 16:
Digital Carrier Systems
Hierarchy of TDM
USA/Canada/Japan use one system
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
eas ابحم زاو Sy اس
ص
صفحه 17:
Digital Carrier Systems (2)
0 For voice each channel contains one word of
digitized data (PCM, 8000 samples per sec)
I Data rate 8000x193 = 1.544Mbps
I Five out of six frames have 8 bit PCM samples
I Sixth frame is 7 bit PCM word plus signaling bit
I Signaling bits form stream for each channel
containing control and routing info
1 Same format for digital data
1 23 channels of data
| 7 bits per frame plus indicator bit for data or systems
control
I 24th channel is sync
صفحه 18:
Mixed Data
0 DS-1 can carry mixed voice and data
signals
0 24 channels used
0 No sync byte
0 Can also interleave DS-1 channels
I Ds-2 is four DS-1 giving 6.312Mbps
صفحه 19:
ISDN User Network Interface
0 ISDN allows multiplexing of devices over
single ISDN line
0 Two interfaces
I Basic ISDN Interface
۱ Primary ISDN Interface
صفحه 20:
Basic ISDN Interface (1)
I Digital data exchanged between subscriber and
NTE - Full Duplex
1 Separate physical line for each direction
1 Pseudoternary coding scheme
I 1=no voltage, 0=positive or negative 750mvV +/-10%
1 Data rate 192kbps
1 Basic access is two 64kbps B channels and one
16kbps D channel
1 This gives 144kbps multiplexed over 192kbps
1 Remaining capacity used for framing and sync
صفحه 21:
Basic ISDN Interface (2)
0 B channel is basic iser channel
0 Data
0 PCM voice
0 Separate logical 64kbps connections o
different destinations
0 D channel used for control or data
LAPD frames
0 Each frame 48 bits long
0 One frame every 250us
صفحه 22:
0
B2
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0
1۳
5
1۳
0
>
Bl
BL
bits
«
Frame Structure
48 bits in 250
سح 2 دص
<8 its
1s channel bits (15 per frame)
1 channel bits (16 per frame)
D channel bits (4 per frame)
spate hits
<
TE t NT
FL BI
NT wo TE
FL BI
<3 bits
= Framing bit
DC halancing bit
= D-echo channel bit
= Activation bit
= ‘Auxiliary Framing bit
Selo opposite oF,
lian
جمدم ودع
صفحه 23:
Primary ISDN
I Point to point
0 Typically supporting PBX
0 1.544Mbps
| Based on US DS-1
I Used on T1 services
1 23 B plus one D channel
0 2.048Mbps
| Based on European standards
1 30 B plus one D channel
I Line coding is AMI usingHDB3
صفحه 24:
Primary ۳ ۹ Formats
3 its; 12
[i Sit, fn
) Interface at 1.544 Mbps
صفحه 25:
Sonet/SDH
I Synchronous Optical Network (ANSI)
0 Synchronous Digital Hierarchy (ITU-T)
0 Compatible
0 Signal Hierarchy
I 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)
I Multiple STS-1 combined into STS-N signal
I ITU-T lowest rate is 155.52Mbps (STM-1)
صفحه 26:
SONET Frame Format
(0) STNEN frame format
صفحه 27:
SONET STS-1 Overhead Octets
Section
Overhead
Line
Overhead
(a) Transport Overhead (b) Path Overhead
صفحه 28:
Statistical TDM
0 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
ص
ص
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صفحه 29:
Statistical TDM Frame Formats
Flag | Address | Controt | Statistical TDM subframe FCS Flag
(a) Overall frame
Address Data ۱
(b) Subframe with one source per frame
Address] Length Data ۱ یماد موه Data |
(c) Subframe with multiple sources per frame
صفحه 30:
Performance
0 Output data rate less than aggregate
input rates
0 May cause problems during peak periods
! Buffer inputs
! Keep buffer size to minimum to reduce delay
صفحه 31:
Buffer Size ۰
and Delay a
صفحه 32:
Asymmetrical Digital Subscriber
Line
1 ADSL
0 Link between subscriber and network
I Local loop
0 Uses currently installed twisted pair cable
| Can carry broader spectrum
1 1 MHz or more
صفحه 33:
ADSL Design
0 Asymmetric
! Greater capacity downstream than upstream
0 Frequency division multiplexing
| Lowest 25kHz for voice
| Plain old telephone service (POTS)
I Use echo cancellation or FDM to give two
bands
I Use FDM within bands
0 Range 5.5km
صفحه 34:
ADSL Channel Configuration
صفحه 35:
Discrete Multitone
۱
Multiple carrier signals at different frequencies
Some bits on each channel
4kHz subchannels
Send test signal and use subchannels with
better signal to noise ratio
1 256 downstream subchannels at 4kHz
(60kbps)
1١ 2
IT Impairments bring this down to 1.5Mbps to 9Mbps
peoeo
صفحه 36:
DMT Transmitter
cos 2n fit
x0)
۱/0۰ 7
watt)
oak bps
(QAM
DMT
signal out
binary
input
08 27 fat
10
R bps
ارول وم
0> >ره | 30
Saj=1 4,R bps {OAM
fis) = fi + 4 KHz
صفحه 37:
xDSL
0 High data rate DSL
0 Single line DSL
0 Very high data rate DSL
صفحه 38:
Required Reading
0 Stallings chapter 8
0 Web sites on
1 ADSL
1 SONET