Multiplexing allows multiple channels to be transmitted over a single physical line. There are different multiplexing techniques including Frequency Division Multiplexing (FDM) which allocates different frequency bands to different channels, and Time Division Multiplexing (TDM) which allocates time slots to different channels in a repeating cycle. FDM was commonly used for analog carrier systems while TDM is used for digital transmission standards like SONET/SDH. Technologies like ADSL use FDM to transmit data over existing twisted pair phone lines, allocating different frequency bands for voice, upstream data, and downstream data.

1. Multiplexing
CEN 220/CIS 192 Advanced Data Communications and Networking
Data and Computer Communications, W. Stallings 9/E, Chapter 8

2. 2
Multiplexing
“It was impossible to get a conversation going,
everybody was talking too much.”
—Yogi Berra

3. 3
Multiplexing
multiple links on 1 physical line
common on long-haul, high capacity, links
have FDM, TDM, STDM alternatives

4. 4
Frequency Division Multiplexing (FDM)

5. 5
FDM
System
Overview

6. 6
FDM Voiceband Example

7. 7
Analog Carrier Systems
long-distance links use an FDM hierarchy
AT&T (USA) and ITU-T (International) variants
Group

12 voice channels (4 kHz each) = 48 kHz

Range 60 kHz to 108 kHz
Supergroup

FDM of 5 group signals supports 60 channels

carriers between 420 kHz and 612 kHz
Mastergroup

FDM of 10 supergroups supports 600 channels
original signal can be modulated many times

8. 8
North American and International FDM
Carrier Standards

9. 9
Wavelength Division Multiplexing (WDM)

10. 10
ITU WDM Channel Spacing (G.692)

11. 11
Synchronous Time Division Multiplexing

12. 12
TDM System
Overview

13. 13
TDM Link Control
no headers and trailers
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
 corresponding source must be quenched
 leaving empty slots
error control
 errors detected & handled on individual channel

14. 14
Data Link Control on TDM

15. 15
Framing
no flag or SYNC chars bracketing TDM frames
must still provide synchronizing mechanism between
source and destination clocks
added digit framing is most common

one control bit added to each TDM frame

identifiable bit pattern used as control channel

alternating pattern 101010…unlikely to be sustained on a
data channel

receivers compare incoming bits of frame position to the
expected pattern

16. 16
Pulse Stuffing
– problem of synchronizing various data sources
– variation among clocks could cause loss of
synchronization
– issue of data rates from different sources not related by a
simple rational number

17. 17
TDM Example

18. 18
Digital Carrier Systems

19. 19
North American and International
TDM Carrier Standards

20. 20
DS-1 Transmission Format

21. 21
SONET/SDH
Synchronous Optical Network (ANSI)
Synchronous Digital Hierarchy (ITU-T)
high speed capability of optical fiber
defines hierarchy of signal rates
– Synchronous Transport Signal level 1 (STS-1) or
– Optical Carrier level 1 (OC-1) is 51.84 Mbps
– carries one DS-3 or multiple (DS1 DS1C DS2)
plus ITU-T rates (e.g., 2.048 Mbps)
– multiple STS-1 combine into STS-N signal
– ITU-T lowest rate is 155.52 Mbps (STM-1)

22. 22
SONET/SDH Signal Hierarchy

23. 23
SONET Frame Format

24. 24
Statistical TDM

25. 25
Statistical TDM Frame Format

26. 26
Single-Server Queues with Constant Service Times
and Poisson (Random) Arrivals

27. 27
Cable Modems
– dedicate two cable TV channels to data transfer
– each channel shared by number of subscribers using
statistical TDM

28. 28
Cable Spectrum Division
To support both cable television programming and data
channels, the cable spectrum is divided in to three ranges:

user-to-network data (upstream): 5 - 40 MHz

television delivery (downstream): 50 - 550 MHz

network to user data (downstream): 550 - 750 MHz

29. 29
Cable Modem Scheme

30. 30
Asymmetrical Digital Subscriber Line (ADSL)
link between subscriber and network
uses currently installed twisted pair cable
is Asymmetric - bigger downstream than up
uses Frequency Division Multiplexing

reserve lowest 25 kHz for voice (POTS)

uses echo cancellation or FDM to give two bands
has a range of up to 5.5 km

31. 31
ADSL Channel Configuration

32. 32
Discrete Multitone (DMT)
multiple carrier signals at different frequencies
divide into 4 kHz subchannels
test and use subchannels with better SNR
256 downstream subchannels at 4 kHz (60 kbps)

in theory 15.36 Mbps, in practice 1.5-9 Mbps

33. 33
DMT Transmitter

34. 34
Broadband – Customer Side
DSL link is between provider and customer
a splitter allows simultaneous telephone and data
service
data services use a DSL modem

sometimes referred to as G.DMT modem
DSL data signal can be divided into a video stream and
a data stream

the data stream connects the modem to a router which
enables a customer to support a wireless local area
network

35. 35
Broadband – Provider Side
a splitter separates telephone from Internet
voice traffic is connected to public switched
telephone network (PSTN)
data traffic connects to a DSL multiplexer (DSLAM)
which multiplexes multiple customer DSL
connections to a single high-speed ATM line.
ATM line connects ATM switches to a router which
provides entry to the Internet

36. 36
xDSL
high data rate DSL (HDSL)

2B1Q coding on dual twisted pairs

up to 2Mbps over 3.7km
single line DSL

2B1Q coding on single twisted pair (residential) with
echo cancelling

up to 2Mbps over 3.7km
very high data rate DSL

DMT/QAM for very high data rates

separate bands for separate services

37. 37
Comparison of xDSL Alternatives

38. 38
Summary
Multiplexing multiple channels on single link
FDM

analog carrier systems

wavelength division multiplexing
TDM

TDM link control

pulse stuffing
statistical TDM
broadband
ADSL and xDSL