I.1

MultiplexingReferences

– Frequency-division multiplexing and Time-divisionmultiplexing• Chapter 7, W. Stallings, Data and Computer

Communications, Prentice Hall.

I.2

Introduction

Introduction– Generally, two communicating stations will not utilize the

full capacity of a data link• Example: transmit a voice signal over a optical fiber

– Multiplexing allows multiple users sharing the capacity ofa transmission link

I.3

Introduction

Components– Multiplexer

• combines data from the n input lines

– Link• with n separate channels• example: optical fiber or microwave link

– Demultiplexer• separates the data according to channel• delivers them to the appropriate output lines

I.4

Introduction

Multiplexing TechniquesFrequency-division multiplexing (FDM)

– each channel occupies a fraction of the bandwidth of thelink

– a channel is defined by its center frequency, and itsbandwidth.

– Example: radio and television signal transmission

I.5

Introduction

Time-division multiplexing (TDM)– each channel occupies the entire bandwidth of the link for

a very short period of time– a channel is made up of a sequence of time slots

– Synchronous TDM• time slot are assigned to each channel in a regular

sequence• Example: multiplexing digitalized voice signals and

data streams

I.6

Introduction

– Statistical• improve the efficiency of synchronous TDM by adding

complexity to the multiplexer• time slots are assigned to signals as they arrive at the

multiplexer

Code-division multiplexing (CDM)– each channel occupies the entire bandwidth– signals are differentiated by modulating them with

orthogonal codes– mainly used for mobile communications

I.7

FDM

FDM– FDM is possible when the useful bandwidth of the

medium excess the required bandwidth of signals to betransmission• (BWmedium >> BWsignal)• Example: voice signal is transmitted via an optical

fiber– A number of signals are transmitted simultaneously if

each signal is modulated onto a different carrierfrequency, and the carrier frequencies are sufficientlyseparated that the bandwidths of the signals do notoverlap

I.8

FDM

I.9

FDM

Multiplexing– N sources are fed into a multiplexer

– the multiplexer modulates each signal onto a differentfrequency

– each modulated signal requires a certain bandwidthcentered around its carrier frequency, referred to as achannel

– The modulated signals are summed to produce acomposite signal

I.10

FDM

– The composite signal transmitted across the medium isanalog• The input signals may be either digital or analog• A digital signal must be passed through a modem

I.11

FDM

– to prevent interference, the channels are separated byguard bands, which are unused portions of the spectrum

I.12

FDM

Demultiplexing– At the receiving end, the composite signal is passed

through N bandpass filters, each filter centered atand having a bandwidth

– Each component is then demodulated to recover theoriginal signal

scifsciB

I.13

FDM

I.14

FDM

Example: Transmitting 3 voice signals– the bandwidth of a voice signal is taken to be 4kHz, with

an effective spectrum of 300 to 3400Hz

I.15

FDM

– If this signal is used to amplitude-modulate a 64 kHzcarrier, the spectrum becomes

– The modulated signal has a bandwidth of 8 kHz,extending from 60 to 68kHz.

I.16

FDM

– To make efficient use of bandwidth, we transmit only thelower sideband.

– If three voice signals are used to modulate carriers at 64,68, and 72 kHz, the spectrum is

I.17

FDM

Problems of FDM– Crosstalk

• occur if the spectra of adjacent component signalsoverlap

– Intermodulation noise• on a long link, the nonlinear effects of amplifiers on a

signal in one channel could produce frequencycomponents in other channels

I.18

FDM

Analog Carrier Systems– designed to transmit voiceband signals over high-capacity

transmission links• Example: use coaxial cable and microwave systems

– First level of the hierarchy• 12 voice channels are combined to produce a group

signal with a bandwidth of 12 x 4 kHz = 48 kHz, in therange of 60 to 108 kHz.

I.19

FDM

– Second level• 60-channel supergroup• formed by frequency-division multiplexing five-group

signals.

• Each group is treated as a single signal with a 48 kHzbandwidth and is modulated by a subcarrier.

• The subcarrier have frequencies from 420 to 612 kHzin increments of 48 kHz.

• The resulting signal occupies 312 to 552 kHz.

I.20

FDM

– Variation of supergroup formation• Case I

Each of the five inputs to the supergroupmultiplexer may be a group channel containing 12multiplexed voice signals

• Case IIany signal up to 48 kHz wide whose bandwidth is

contained within 60 to 108 kHz may be used asinput to the supergroup multiplexer

• Case IIIdirectly combine 60 voiceband channels into a

supergroup

I.21

FDM

– Third level• 60-channel supergroup• formed by frequency-division multiplexing five-group

signals.

• Each group is treated as a single signal with a 48 kHzbandwidth and is modulated by a subcarrier.

• The subcarrier have frequencies from 420 to 612 kHzin increments of 48 kHz.

• The resulting signal occupies 312 to 552 kHz.

I.22

FDM

No. of voice channel Bandwidth SpectrumGroup 12 48 kHz 60-108 kHzSupergroup 60 240 kHz 312-552 kHzMastergroup 300 2.52 MHz 564-3084 kHzJumbogroup 3600 16.984 MHz 0.564 - 17.584 MHz

I.23

FDM– Note that the original voice or data signal may be

modulated many times• Example

a data signal may be encoded using PSK to forman analog voice signalThe signal could then be used to modulated a 76

kHz carrier to form a component of a group signal.This group signal could then be used to modulate a

516 kHz carrier to form a component of asupergroup signal.

– Each stage can distort the original data if themodulator/multiplexer contains nonlinearities orintroduces noise

I.24

Synchronous TDM

TDM– Synchronous TDM is possible when the achievable data

rate (sometimes, unfortunately, called bandwidth) of themedium exceeds the data rate of digital signals to betransmitted.• rmedium >> rsignal

– TDM is the time interleaving of samples from severalsources so that the information from these source can betransmitted serially over a single communication link

I.25

Synchronous TDM

I.26

Synchronous TDM

Transmitter– A number of signals are to be multiplexed onto the same

transmission medium

I.27

Synchronous TDM

– The incoming data from each source are briefly buffered.• Each buffer is typically one bit or one character in

length• The buffers are scanned sequentially to form a

composite digital data stream• The scan operation is sufficiently rapid so that each

buffer is emptied before more data can arrive

– The composite digital signal stream may be transmitteddirectly or passed through a modem so that an analogsignal is transmitted

I.28

Synchronous TDM

– The data are organized into frames.• Each frame contains a cycle of time slots.• In each frame, one or more slots is dedicated to each

data source.• The sequence of slot dedicated to one source, from

frame to frame, is called a channel

I.29

Synchronous TDM

– Length of slot• The slot length equals the transmitter buffer length,

typically a bit or a character

• The characteristic-interleaving technique is used withasynchronous sources.

Each time slot contains one character of dataThe start and stop bits of each character are

eliminated before transmission and reinserted by thereceiver.

I.30

Synchronous TDM

• The bit-interleaving technique is used withsynchronous sources and may also be used withasynchronous source

Each time slot contains just one bit

I.31

Synchronous TDM

Receiver– The interleaved data are demultiplexed and routed to the

appropriate destination buffer.

I.32

Synchronous TDM

– Synchronous TDM is called synchronous not becausesynchronous transmission is used but because the timeslots are preassigned to sources and fixed.

– The time slots for each source are transmitted whether ornot the source has data to send• capacity is wasted to simplify the hardware

implementation

– It is possible for a synchronous TDM device to handlesources of different data rates. For example, the slowestinput device could be assigned one slot per cycle, whilefaster devices are assigned multiple slots per cycle.

I.33

Synchronous TDM

Pulse Stuffing– The most difficult problem in the design of a synchronous

TDM system is the synchronization of various datasources.

– If each source has a separate clock, any variation amongclocks could cause loss of synchronization.

– Also, in some cases, the data rates of the input datastreams are not related by a simple rational number

I.34

Synchronous TDM

– With pulse stuffing, the outgoing data rate of themultiplexer, excluding framing bits, is higher than thesum of the maximum instantaneous incoming rates.• The extra capacity is used by stuffing extra dummy

bits or pulses into each incoming signal until its rate israised to that of a locally generally clock signal.

• The stuffed pulses (bits) are inserted at fixed locationsin the multiplexer frame format so that they may beidentified and removed at the demultiplexer.

I.35

I.36

Synchronous TDM

Example:

I.37

Synchronous TDM

– Source 1: Analog, 2 kHz bandwidth– Source 2: Analog, 4 kHz bandwidth– Source 3: Analog, 2 kHz bandwidth– Source 4-11: Digital, 7200 bps synchronous

– The analog sources are converted to digital using PCM• Sampling rate = 2 x bandwidth• The samples are quantized using a 4 bit A/D

– For the digital sources, pulse stuffing is used to raise eachsource to a rate of 8 kbps

I.38

Synchronous TDM

– A frame can consist of multiple cycles of 32 bits, eachcontaining 16 PCM bits and two bits from each of theeight digital source.

I.39

Digital Carrier SystemsPlesiochronous Digital Hierarchy (PHD) or TDM Hierarchy

Multiplexing– At each step, the multiplexer has to take into account the

fact that the clocks of the inputs are all slightly different.

– Each clock is allowed a certain range of speeds. Themultiplexer reads each input at the highest allowed clockspeed. When there are no bits in the input buffer as thebits are arriving according to a slower clock, it adds a“stuffing” bit to “stuff” the signal up to the higher clockspeed.

I.40

Digital Carrier Systems

Demultiplexing– It also has a mechanism to signal the demultiplexer that it

has performed stuffing, and the demultiplexer must knowwhich bit to throw it out.

I.41

Digital Carrier Systems

Digital SignalNumber

Number of VoiceChannels

Data rate (Mbps)

DS-1 24 1.544

DS-1C 48 3.152

DS-2 96 6.312

DS-3 672 44.736

DS-4 4032 274.176

DS-5 8064 560.160

(North American hierarchy)

I.42

Digital Carrier Systems

I.43

Digital Carrier Systems

– A single DS-1 signal is usually transmitted over one pairof twisted wires. This types of DS-1 transmission over atwisted-pair medium is known as the T1 carrier system.

I.44

SONET/SDH

Introduction– SONET (Synchronous Optical Network) is an optical

transmission interface originally proposed by BellCoreand standardized by ANSI (American National StandardsInstitute)

– SONET is intended to provide a specification for takingadvantage of the high speed digital transmissioncapability of optical fiber.

– A compatible version, referred to as Synchronous DigitalHierarchy (SDH), has been published by ITU-T.

I.45

SONET/SDH

– SONET specifies a system which integrates andaccommodates all of the different line speeds andcharacteristics of US and European multiplexinghierarchies.• A standardized method of internal operation and

management• multiple speeds of operation, such that as higher and

higher optical speeds are introduced the system canexpand gracefully to operate at the higher speeds.

• Many levels of multiplexing and demultiplexing to beaccomplished in a single step - no need to demultiplexthe higher levels to gain access to the lower levels.

I.46

SONET/SDH

• Many different payloads (different-speed channels) tobe carried through the system

• higher efficiency

– SONET is a synchronous system.• Controlled by a master clock with an accuracy of about

1 part in 109.

I.47

Signal Hierarchy

SONETDesignation

ITU-TDesignation

Data Rate(Mbps)

Payload rate(Mbps)

STS-1/OC-1 51.84 50.112STS-3/OC-3 STM-1 155.52 150.336STS-9/OC-9 STM-3 466.56 451.008STS-12/OC-12 STM-4 622.08 601.344STS-18/OC-18 STM-6 933.12 902.016STS-24/OC-24 STM-8 1244.16 1202.688STS-36/OC-36 STM-12 1866.24 1804.032STS-48/OC-48 STM-16 2488.32 2405.376

I.48

Signal Hierarchy

– The lowest level, referred to as STS-1 (SynchronousTransport Signal, level 1) or OC-1 (Optical Carrier level1) is 51.84 Mbps.• This rate can be used to carry a single DS-3 signal or a

group of lower-rate signals, such as DS1 and DS2.

– Multiple STS-1 signals can be combined to form an STS-N signal. The signal is created by interleaving bytes fromN STS-1 signals that are mutually synchronized

I.49

STS-1 frame

I.50

STS-1 frame

STS-1 line– between STS-1 multiplexer– consists of several sections that are bounded by repeaters

or the multiplexers that terminate the linePath

– Example: between the terminating M1 multiplexers

I.51

STS-1 Frame

– Signals are divided into fixed frames of 125µs duration.• Each frame consists of 6480 bits or 810 octets.• The octets can be arranged in a frames of 9 rows of 90

columns

I.52

STS-1 Frame

I.53

STS-1 frame

Transport overhead (TOH)– 9 rows x 3 columns (27 octets)– 9 octets of section overhead (SOH) and 18 octets of line

overhead (LOH)Synchronous payload envelope (SPE)

– 9 rows x 87 columns (783 octets) are called the STS-1SPE. Of the 783 octets, 9 octets are reserved for pathoverhead (POH) information.

STS-1 payload– 774 octets

I.54

STS-1 frameSOH

– defines and identifies frames, and monitors section errorsand communication between section terminatingequipment

LOH– locates first octet of SPE, and monitors line errors and

communication between line-terminating equipment

POH– verifies connection, and monitors path errors, receiver

status, and communication between path-terminatingequipment

I.55

STS-3 frame

I.56

STS-3 frame

I.57

Statistical TDM

– In a synchronous time-division multiplexer, it is generallythe case that many of the time slots in a frame are wasted.

– The statistical multiplexer allocates time slotsdynamically on demand.

– The statistical multiplexer has n I/O lines but only k,where k < n, time slots available on he TDM frame.

I.58

Statistical TDM

– For input, the function of the multiplexer is to scan theinput buffers, collecting data until a frame is filled, andthen send the frame.

– On output, the multiplexer receives a frame anddistributes the slots of data to the appropriate outputbuffers.

– Address information is required to assure proper delivery.As a result, there is more overhead per slot for statisticalTDM as each slot carries an address as well as data

I.59

Statistical TDM

I.60

Code division multiplexing

– CDM produces a wideband, noiselike signal, and itoccupies the entire range of frequencies allocated to thesystem.

– Discrimination between the signals is achieved throughthe assignment of unique spreading codes (all codes areorthogonal)

I.61

Code division multiplexing

I.62

Experiment Report

ArrangementLast digit of student ID Report on Experiment

1, 2 or 3 1 or 24, 5, or 6 2 or 37, 8, 9 or 0 1 or 3

Submission deadline: 5:00pm, 11 May 2001(marks will be deduced for late submission)

I.63

Experiment Report

Report ContentOn the cover, it must contains

Your full nameStudent ID no.

IntroductionExperiment setupResultsDiscussions / Answers of questions

MultiplexingIntroductionIntroductionIntroductionIntroductionIntroductionFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMFDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMSynchronous TDMDigital Carrier SystemsDigital Carrier SystemsDigital Carrier SystemsDigital Carrier SystemsDigital Carrier SystemsSONET/SDHSONET/SDHSONET/SDHSignal HierarchySignal HierarchySTS-1 frameSTS-1 frameSTS-1 FrameSTS-1 FrameSTS-1 frameSTS-1 frameSTS-3 frameSTS-3 frameStatistical TDMStatistical TDMStatistical TDMCode division multiplexingCode division multiplexingExperiment ReportExperiment Report