CEN 342Introduction to Data Transmission

Chapter 8Multiplexing

Dr. Mostafa Hassan DahshanComputer Engineering DepartmentCollege of Computer and Information SciencesKing Saud University

mdahshan@ccis.ksu.edu.sa

Multiplexing

Communication links are expensive

Using one link/party is inefficient

Many applications require modest data rates

Sharing link is more cost efficient

Link sharing requires multiplexing

Multiplexing Types

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Statistical Time Division Multiplexing

Frequency Division Multiplexing

Possible with large bandwidth

Multiple signals carried simultaneously

Each signal modulated onto different carrier frequency

Carrier frequencies must be sufficiently separated

Bandwidths should not overlap

Frequency Division Multiplexing

Frequency Division Multiplexing

Example: FDM of TV Signals

Example – FDM of Voice Signals

Bandwidth of voice signal (300-3400 Hz)

Generally taken as 4 kHz

Using AM with carrier frequency 64 kHz

Spectrum of modulated signal is 8 kHz60 kHz – 68 kHz

To make efficient use of bandwidthtransmit only lower sideband

FDM Problems

Crosstalkspectrum overlap between adjacent component signals

guard band should be added

e.g. voice 4 kHz instead of 3100 Hz

Intermodulation noiseamplifiers produce frequency components of other channels

Wavelength Division Multiplexing

FDM used in optical fiber

Multiple signals use different frequency

WDM is the commonly used term

Each wavelength carry channel of data

More channels, closely spaceddense wavelength division multiplexing DWDM

160 channels, each 10 Gbps now available

Time Division Multiplexing

Multiple digital signals carried on single path

Portions of each signal interleaved in time

Interleaving: bit level, blocks of bytes, larger

Data from each source is buffered

Buffers scanned sequentially

Data organized into frames

Time Division Multiplexing

Rate of mc(t) must be ≥ Σni mi(t)

Time Division Multiplexing

Synchronous TDM

Time slots pre-assigned to sources

Slot transmitted even if source has no data

May waste capacity but simple to implement

Different data rates are possible

Fast source can be assigned multiple slots

Slots dedicated to source called channel

Framing

Frame sync to identify frame boundaries

Added-digit framingone control bit added to each TDM frame

effectively another channel (control channel)

bit pattern 101010… unlikely on data channel

Synchronizing Multiple Sources

Most difficult problem in TDM design

If each source has separate clockany variation among clock loss of sync

Input data rates not related by simple rational number

Pulse stuffing is used to solve this problem

Pulse Stuffing

Outgoing data rate of multiplexer > sum of max instantaneous incoming rates

Extra capacity used to stuff extra bits

Dummy bits/pulses added to each input until rate matches local clock

Stuffed pulses added at fixed locations

Removed by demultiplexer

Example

Input

Source 1: Analog, 2 kHz

Source 2: Analog, 4 kHz

Source 3: Analog, 2 kHz

Sources 4-11: Digital, 7200 bps

Example

For analog sources

Sources 1, 3 sampled at 4000 samples/s

Source 2 at 8000 samples/s

PCM, quantized using 4 bits/sample

2 sample / scan for source 2 (8 bits)

1 sample / scan for sources 1, 3 (8 bits)

Total sources 1-3 =16 × 4000 = 64 kbps

Example

For digital sources

Pulse stuffing raise each source to 8 kbps

For aggregate data rate = 64 kbps

Frame bit allocation

Suppose frame = 32 bits

16 bits for PCM sources 1-3 (1:4, 2:8, 3:4)

2 bits for each source from 4-11 = 16 bits

Digital Carrier SystemsSynchronous TDM transmission structure

TDM performed at multiple levels

Hierarchy of TDM structuresUS, Canada, Japan use AT&T system

Other countries use ITU-T system

DS-1 Transmission Format

Voice Transmission

Voice is PCM digitized 8000 samples/s

Frame rate must be 8000 frames/s

Frame length = 24 × 8 + 1 = 193 bits

Data rate = 8000 × 193 = 1.544 Mbps

Control bits used in every 6th frame

Data Transmission

Same 1.544 Mbps data rate used

23 channels for data, 1 for sync byte

Within channel7 bits used for data

1 bit indicate channel is user or sys control data

7 × 8000 = 56 kbps max rate / channel

SONET/SDH

SONET (Synchronous Optical Network)optical transmission interface

proposed by BellCore, standardize by ANSI

SDH (Synchronous Digital Hierarchy)compatible version published by ITU-T

few differences from SONET

Signal Hierarchy

SONET defines hierarchy of data ratesLowest STS-1 / OC-1: 51.84 Mbps

STS-1 carry 1 DS-3 or group of DS-1

Multiple STS-1 combined to form STS-N

SDHlowest rate is STM-1: 155.52 = STS-3

Signal Hierarchy

Frame Format

Frame consists of 810 octets

Transmitted every 125 μs = 8000 frame/s

810×8 bit/frame × 8000 frame/s = 51.84Mbps

Frame logically viewed as matrix9 rows, 90 octets each

transmitted one row at time

first 3 cols (27 octets) are overhead

payload includes a column for path overhead

Statistical TDM

TDM does not efficiently utilize capacity

Many times, slots are wasted

Statistical TDM allocates slots on demand

Number of lines n < number of time slots k

Not slots are reserved for specific input line

Multiplexer collects data until frame is filled

Statistical TDM

Output data rate < sum input rates

Can take more sources than TDM at same output rate

or less output rate for same sources as TDM

More overhead than TDMslot positions must be identified

address information must be included with data

Statistical TDM

Frame Structure

Control information is needed

Two possible formats

One data source per frameneed to identify address of source

work well under light load

inefficient under heavy load

Multiple sources per frameneed to identify length of data of each source

Frame Structure

Digital Subscriber Line (DSL)

Subscriber line: customer to central office

Carry voice grade signal: 0 – 4 kHz

Wire can support 1 MHz or more

Provide high speed data over phone line

Asymmetric DSL (ADSL)more downstream rate the upstream

most home user traffic is downstream

ADSL Design

Lowest 25 kHz reserved for voiceknown as plain old telephone service (POTS)

more than 4 kHz to prevent crosstalk

FDM or echo cancellation to allocate bandssmaller upstream, larger downstream

FDM used within each bandbit stream split into multiple parallel bit streams

each portion carried in separate frequency band

Echo Cancellation

Allow simultaneous transmission in both directions on the same band

To recover received signal, transmitter subtracts echo of its own transmission

Frequency band of up/down stream overlap

Echo Cancellation

Advantagesless attenuation in low frequency range

more downstream band in good part of spectrum

more flexible allocation of up/down stream bands

Disadvantageslogic for EC must be installed both sides

more complexity

Discrete Multitone (DMT)

Used in ADSL transmission

Multiple carrier signals different frequencies

Transmission band divided to 4kHz channels

Send some bits on each sub-channel

Substream converted to analog using QAM

QAM can assign different bits / signal

Total data rate = sum of sub-channel rates

Discrete Multitone (DMT)

Initially, DMT modem sends test signals

Test signals sent on all channels to test SNR

More bits assigned to better SNR channels

Each channel carries between 0-60 kbps

ADSL/DMT Transmission

ADSL/DMT Transmission

Design uses 256 downstream sub-channels

Each sub-channel is 4 kHz

Max possible rate 60 kbps× 256 = 15.36 Mbps

In practice, limited by impairments

Actual rates from 1.5 to 9 Mbps

Rate depends on distance and quality

xDSL

ADSL = Asymmetric DSLHDSL = High data rate DSL

SDSL = Single line DSLVDSL = Very high data rate DSL

Additional References

DS0, DS1, DS3, T1, T3 Dedicated FAQ, dedicated-voice-data.alllongdistance.com/dedfaq.shtml

An introduction to ADSL, people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture13/DSL/DSL.html