Chapter 8:

Multiplexing

COE 341: Data & Computer Communications (T061)Dr. Radwan E. Abdel-Aal

2

Contents

1. Introduction

2. Two Multiplexing Techniques1. FDM

2. TDM1. Synchronous

2. Statistical

3. Application: ADSL

(Asymmetric Digital Subscriber Line)

3

Introduction Multiplexing: A generic term used when more

than one application or source share the capacity of one link

Objective is to achieve better utilization of the link bandwidth (channel capacity)

Multiplexer Demultiplexer

4

Motivation High capacity (data rate) links are cost effective. i.e.

it is more economical to go for large capacity links But requirements of individual users are usually fairly

modest…e.g. 9.6 to 64 kbps for non intensive (graphics, video) applications

Solution: Let a number of such users share the high capacity channel (Multiplexing)

Example: Long haul trunk traffic: High capacity links: Optical fiber, terrestrial microwaves, etc.

carrying large number of channels between cities over large distances

5

Multiplexing Types Our three resources:

Space Time Frequency Our channels must be

separated in at least one resource (can overlap in the other two)

The resource in which they are separated is “divided” between them: SDM: Separation in space TDM: Separation in time FDM: Separation in frequency

Space

FrequencyTime

TDM:Time Division Multiplexing

To use the same circuit (line)i.e. sharing space:

Use either TDM or FDM

FDM:Frequency Division Multiplexing

6

Frequency Division Multiplexing (FDM)(With Analog Signals)• Channels exist on the same line (space) at the same time:

• Must be separated in frequency!

f

t

7

FDM Useful bandwidth of medium exceeds required

bandwidth of a channel Signal of each channel is modulated on a different

carrier frequency fc

So, channels are shifted from same base band by different fc’s to occupy different frequency bands

Carrier frequencies separated so that channels do not overlap (also include some guard bands)

Disadvantage: Channel spectrum is allocated even if no data available for transmission in channel (rigid allocation)

8

FDM Multiplexing Process: Frequency-Domain View at TX

All source channels are at (same) base band

ff

f1

f2

f3

0 4 KHz f1 f2 f3

Channels go on the same Link at the same time… butin different frequency bands

9

FDM De-Multiplexing Process: Frequency-Domain View at RX

f1

f2

f3

0 4 KHz

All received channels restored to base band

• Guard bands prevent channel overlap• But represent wasted spectrum

f

Restoration at RX:3 different pass-band filters,each bracketing a channel

10

FDM System – Transmitter

Subcarriers

Main CarrierAny type of modulation:AM, FM, PM

Group of channels

To meet Transmission Requirements

Individual base band channels

11

FDM System – Receiver

fc

Composite base band signal mb(t)

recovered

Individual base band channels

recovered

Subcarriers

Main Carrier

f1

f2

f3

12

FDM of Three Voice band Signals

What is the modulation type ?

3 MUXed channelUsing lower side band only

3 Subcarriers at:64, 68, and 72 KHz

Channel overlap means crosstalk!

Guard bandsTo reduce channel spectrum overlap

BW, allocated: 0 – 4000 Hz

BW, actual : 300 – 3400 Hz

fc

Assume we will keep only the lower side band

for each channel

13

Analog Carrier Systems: How to MUX 4000 Channels?

One-go Vs Hierarchical: (in stages)

…. Channel ….

Group ….

Master super group

Super group ….

4000 channels 4000 channels

... ...

• Modular approach• Easier to implement: e.g. fewer sub carriers• Also, not all channels may be available at one place

Stages

….

….

14

Analog Carrier Systems Devised by AT&T (USA) Hierarchy of FDM schemes: MUXing in stages Group: AM, Lower Side band

12 voice channels = 12 x 4 kHz = 48 kHz BW 12 sub carriers: 64 kHz – 108 kHz in 4 KHz intervals Frequency range for group: 60 kHz – 108 kHz = 48 kHz (lower side band)

Super group: FM 5 groups = 5 x 48 kHz = 240 kHz BW 5 sub carriers: 420 kHz - 612 kHz at 48 KHz intervals (No GBs bet. groups) Frequency range: 312 kHz – 552 kHz = 240 kHz

Master group: FM 10 super groups = 10 x 240 kHz = 2400 kHz BW 10 sub carriers: 1116 kHz - 3396 kHz (Min of 8 KHz GBs between SGs) BW of 2.52 MHz (> 10 x 240 KHz = 2.4 MHz due to GB between SGs)

Jumbo group: FM

6 master groups i.e. total of 6 x 10 x 5 x 12 = 3600 voice channels BW of 16.984 MHz (> 3600 x 4 KHz due to gaud bands between super groups)

Each channel is 300 to 3400 = 3100 Hz. 4000 Hz provides 900 Hz guard band

15

Analog Carrier Systems

12

8 KHz

GroupChannel Super group Master super group

16

Analog FDM Hierarchy

0.24 x 10 Vs 2.52

17

FDM characteristic problems Two potential problems characterize FDM and

all broadband applications Crosstalk:

- Due to overlap between channel spectra and the use of non-ideal filters to separate them

- Use guard bands to reduce it

Inter modulation noise:

- Nonlinearities in amplifiers ‘mix’ channels

- This generates spurious frequency components (sum, difference) which fall within channel BWs!

- Limits the amount of amplification possible

18

Usually uses synchronous transmission,

but asynchronous is also possible Data rate of medium exceeds data rate of digital

signals to be transmitted for one channel Digital signals of multiple channels interleaved in time Interleaving may be:

At bit level At block level (e.g. bytes)

Two types: Synchronous TDM (Fixed rotation on channels) Statistical or asynchronous TDM (More efficient utilization of

the time slots)

Time Division Multiplexing (TDM)

19

Time Division Multiplexing

300 HzChannels occupy the same frequency band(Base band)and go on the same link

Channels must go on the link atdifferent times

3400 Hz

20

Time Division Multiplexing (TDM)All BasebandSignals- share same Frequency band

Ts = 1/2fmax

Channel sampling interval

Note: MUXing and DeMUXingare transparent to the end stations.Each pair ‘think’ they have a dedicated link !

Time

RotateAt the channel sampling rate

21

TDM Frames

Channel sampling interval = 3TFor each channel, data rate is 1 sample/3T

1

2

3

4

5

6

123456….Sample Number

On the link: Data is sent at a rate of 1 sample/TData rate is 3 times the channel data rate

Sampling Interval

Sampling IntervalChannel data rate = R Link data rate = NR

N Channels

22

Synchronous TDM: (Fixed channel scan arrangement) Time slots pre-assigned to sources and fixed Disadvantage: Time slots allocated even if no data

available (channel capacity waste, as with BW waste in FDM)

But simple to implement, e.g. No need to send ID of source channel

We could assign more than time slot per scan for faster sources- but on a permanent basis

Time Division Multiplexing (TDM)

23

Synchronous TDM – Transmitter

Scanning and link data rate: high enough to prevent channel buffers overflowing

N channels,Sampling rate R sample/s Minimum link

capacity = N R sample/s

Analog Signal

Digital Signalor

Transmitted frames consist of interleaved channel data

Time Slot, T Channel dwell time

T Should be enough to

empty a channel buffer

Channel 2

Bit stream

Channel Buffers

Sample data fills buffer

24

TDM System – Receiver

TDM signal

25

Data Link Control with Sync TDM

Data rate on the link (multiplexed line) is fixed and MUX and DEMUX must operate at it

Data link control protocol not needed on the MUXED line Flow control Channel based

If one channel receiver is not ready to receive data, other channels will carry on

Channel-based flow control would then halt corresponding source channel

This causes transmission of empty slots for that channel in the MUXED data

Error control Channel based

Errors are detected and handled by individual channel systems

26

Framing in TDM So far, no flag or SYNC characters bracketing

composite (MUXED) TDM frames on the link Must provide frame synchronization to allow RX

to keep ‘in step’ with TX Two approaches:

Frame-by-Frame: A synch pattern at the beginning of each assembled frame (similar to the preamble flag)

Frame-to-Frame: Additional control channel with a unique frame-to-frame pattern that can be easily identified by RX (can be just 1 bit, and extends across frames, so less overhead)This is called “added digit framing”

27

Framing in TDM Added digit framing

One control bit added to each TDM frame as an additional “control channel”

Carries an identifiable known bit pattern in time (frame to frame) e.g. alternating 01010101…unlikely to occur on a normal data channel

RX searches frame-to-frame for this pattern until it finds it. This establishes frame sync. Will keep locked to it

28

Frame-to-Frame Sync (added digit framing)

C 1 2 3 4 C 1 2 3 4 C 1

0 1 0

Four data channels

Control Channel, C010101….

…..

A data ChannelUnlikely to have 010101…. over successive frames

Once the position of this control channel is established, RX knows where the channel sequence starts and sync is established with TX

…..

MUXed frame

RX knows the size of the MUXed frame

It can check each frame bit frame-to-frame for the special pattern until it finds it!

29

Digital Carrier Systems Hierarchy for TDM (as with FDM!) US system based on DS format, for example DS-1 (similar to a group in FDM):

Multiplexes 24 PCM voice channels digitized with n = 8 bits + a framing bit (a control channel for frame-to-frame synchronization)

Frame takes a sample of each channel So, frame size is 24 x 8 +1 = 193 bits Channels must be sampled at 2 x 4000 = 8000

sample/s This gives a data rate = 8000 x 193 = 1.544 Mbps for

DS-1 Note: FDM Group needed 48 KHz for 12 channels

30

The DS Hierarchy

42 x 96 = 4032 DS-0(4032 voice channels)

DS-0 is a PCM voice channel:8000 sample/s x 8 b/sample= 64 kbps

Transmission lines used should support the progressivelyincreasing data rate (channel capacity) requirement

FDM Jumbo group: 16.984 MHz for 3600 channels

Which one uses BW more efficiently ?

31

DS & T Lines Rates

Service LineData Rate

(Mbps)No. of Voice

Channels

DS-1DS-1 T-1T-1 1.5441.544 2424

DS-2DS-2 T-2T-2 6.3126.312 9696

DS-3DS-3 T-3T-3 44.73644.736 672672

DS-4DS-4 T-4T-4 274.176274.176 40324032

Transmission line that supports it Corresponding Channel Capacity

Relevant Standards:SONET: Synchronous Optical Network (ANSI)SDH: Synchronous Digital Hierarchy (ITU-T)

32

DS-1 Digital Carrier Systems For voice, each channel contains one byte of digitized

data (PCM, 8000 samples per sec) Data rate 8000 MUXed frames/s x (24x8+1) bits/frame =

1.544Mbps Five out of every six frames have 8 bit PCM user data samples

for each channel Sixth frame has (7 bit PCM user data + 1 signaling bit) for each

channel Signaling bits form a stream for each channel containing

control (e.g. error and flow) 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 for signaling

DS-1 can carry mixed voice and data signals

33

DS-1 Transmission Format

Frame

(frame-to-frame)

125/193

(8000 x 7 bits = 56 kbps)

34

Asymmetric Digital Subscriber Line (ADSL) ADSL is an asymmetric communication technology designed for residential users over ordinary telephone twisted pair wires

High speed digital data transmission Existing subscriber lines (local loops) were installed for base band

speech (0 – 4 kHz), but can actually provide bandwidths of up to 1 MHz (short distances)

ADSL is an adaptive technology, using different data rates based on the condition of the local loop line

Ranges up to 5.5 km (95% of subscriber lines in USA) Two main technologies: - Multi-level encoding, e.g. QAM

- Discrete Multitone (DMT) by FDM

Shorterdistance,Higherdata rates

Q. What is the BE for 2.5 km lines?

35

ADSL Design

Asymmetric: Providing higher capacity down stream (to customer) than upstream (from customer)

Originally targeting the video-on-demand market Now being used for Internet traffic Uses Frequency Division Multiplexing (FDM) in a novel

way to utilize the 1 MHz BW of twisted pair wires

ServiceProvider

Video, graphics

Voice, e-mailADSLSubscriber

Downstream (download)

Upstream (upload)

36

FDM is used at two levels:a. Use FDM to obtain three major bands:

1. POTS band: “Plain Old Telephone Service!” 0 - 20 kHz

(includes 16 KHz Guard band)

2. Upstream band:

25 – 200 kHz

3. Downstream band:

250 – 1000 KHz

4

b. Discrete Multitoning (DMT): Further FDM inside the upstream and the downstream bands: Single fast bit stream is split into multiple bit streams traveling at lower data rates in parallel (simultaneously) in subchannels at different subbands within the upstream and downstream bands.

37

ADSL Hardware Home

Telephone Exchange

Subscriber Loop

Telephone channel operateseven is ADSL line is down or unplugged

38

ADSL Frequency Bands and DMT Channels

Guard bandsBetween voice and data

Channel #

256 x 4 kHz 1 MHz

1 MHz

- 256 4KHz sub channels- DMT distributes data rate load on sub channels, non uniformly

39

Discrete Multitoning (DMT) Multiple subchannels (each 4 KHz wide) within the upstream

and downstream bands Subchannels are modulated with subcarriers

of different frequencies (FDM) (hence “multi-tone”) Bit stream to be transmitted is split into a number of streams

that travel in parallel (simultaneously) at a lower data rate on a number of these limited BW subchannels

1

1011010011Serial to Parallel

Converter

Subchannel 1

Subchannel 5

.

.

1

0

1

1

0

1

0

0

1

1

Data rate for each channel:R/5 bpsOverall data rate: R bps

Data rate R bps

(Each: 4 kHz BW)

15

40

Discrete Multitoning (DMT): Adaptive ADSL adaptive property:

Not all subchannels run at the same data rate! Each subchannel can carry data rate from 0 - 60 kbps In general, channels towards the higher frequency end

would carry lower data rates (larger attenuation lower SNR lower channel capacity!)

Moreover, DMT modem can send out test signals on various subchannels to determine SNR (expected lower for subchannels located at higher frequencies due to larger attenuation)

Then faster data rates are assigned to subchannels having better signal transmission conditions

1

1

41

Discrete Multitoning (DMT) Uses QAM (Quadrature Amplitude Modulation)

multilevel modulation allowing up to 15 bits/baud (L = 15 bits/signal level)(4 KHz B D = 4 kbauds (if filtering coefft. r = 0) R max = 4 kbauds x 15 = 60 kbps per channel)

Ideally, 256 x 60 kbps = 15.36 Mbps maximum (if uniform)

But rate is not uniform over all sub channels, and maximum is not achievable in practice due to various transmission impairments

Practical system operate at 1.5 to 9 Mbps depending on distance and line quality

42

DMT

i: data rate distributing factors (<1) over available sub channels (in either the Upstream or the Downstream bands)

R = Aggregate (total) data rate

R = i R + i R + …. + n R

43

DMT

Demodulators

Modulators