TUNALIData Communication1 Chapter 8 Multiplexing.

TUNALI Data Communication 1

Chapter 8

Multiplexing


Introduction• In Chapter 7, it was assumed that the

channel capacity is a bottleneck. So, efficient techniques for utilizing data link under load are described.

• Now, we consider the opposite problem. That is, we will assume that the two communicating stations will not utilize the full capacity of a data link.

• For efficiency, it should be possible to share that capacity.


Multiplexing


Facts affecting the widespread use of multiplexing

• The higher the data rate, the more cost effective the transmission facility.

• Most individual data communicating devices require relatively low data rate


Types of multiplexing• FDM: Frequency Division Multiplexing

(Analog)• TDM: Time Division Multiplexing (Digital)

—Synchronous TDM—Statistical (Intelligent) TDM


Frequency Division Multiplexing• FDM• Useful bandwidth of medium exceeds

required bandwidth of signals to be transmitted

• Each signal is modulated to a different carrier frequency called subcarrier

• Carrier frequencies separated so signals do not overlap (guard bands)

• e.g. broadcast radio• Channel allocated even if no data• The composite signal transmitted is analog


Frequency Division MultiplexingDiagram


FDM Procedure (Transmitter)• Each signal is modulated to a subcarrier

frequency. Any type of modulation can be used.• The resulting modulated signals are summed up to

produce the composite baseband signal.• The composite baseband signal may be modulated

to another carrier frequency. n

• The resulting signal has bandwidth B> Bi

• i=1• To prevent interference, the channels are

separated by guard bands, which are unused portions of the spectrum


FDM Procedure (Receiver)• FDM signal is demodulated to retrieve the

composite baseband signal• The composite baseband signal is passed

through n bandpass filters each centered on a different subcarrier frequency and having bandwidth Bi , i=1,..,n.

• Each component is demodulated to recover the original signal.


FDM System


FDM of Three Voiceband Signals


Three voiceband signals • Higher sidebands are suppressed and only

lower sidebands are used.• In general FDM has two problems:

—Crosstalk: Occurs if spectrum of adjacent component signals overlap significantly.

—Intermodulation noise: Due to nonlinearity effects of amplifiers on a signal, signals at unwanted frequencies are produced


Analog Carrier Systems• AT&T (USA) designed a system to transmit

voiceband signals• Hierarchy of FDM schemes to accommodate

transmission systems of various capacities • Group

—12 voice channels (4kHz each) = 48kHz—Range 60kHz to 108kHz

• Supergroup—60 channel—FDM of 5 group signals on carriers between 420kHz and

612 kHz

• Mastergroup—10 supergroups


Wavelength Division Multiplexing• Multiple beams of light at different frequency• Carried by optical fiber• A form of FDM• Each color of lights (wavelengths) carries separate

data channel• 1997 Bell Labs

—100 beams—Each at 10 Gbps—Giving 1 terabit per second (Tbps)

• Commercial systems of 160 channels of 10 Gbps now available

• Lab systems (Alcatel) 256 channels at 39.8 Gbps each—10.1 Tbps—Over 100km


WDM Operation• Same general architecture as other FDM• Number of sources generating laser beams at

different frequencies• Multiplexer consolidates sources for transmission

over single fiber• Optical amplifiers amplify all wavelengths

—Typically tens of km apart

• Demux separates channels at the destination• Mostly 1550nm wavelength range• Was 200MHz per channel• Now 50GHz


Synchronous Time Division Multiplexing• Data rate of medium exceeds data rate of digital

signal to be transmitted• Multiple digital signals interleaved in time• The signals are generally digital signals and carry

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

length• Data rate of TDM streamsum of data rates of

individual channels


Time Division Multiplexing


Synchronous TDM (1)• Time slots preassigned to sources and

fixed• Time slots allocated even if no data• The buffer size is equal to the slot length

so that the buffer is emptied in each cycle• The transmitted data are organized into

frames. Each frame contains a cycle of time slots.

• The sequence of slots dedicated to one source is called a channel


Synchronous TDM (2)• The word ‘synchronous’ is not used to

indicate the actual synchronous transmission. It is there to indicate that the time slots pre-assigned to sources are fixed.

• In general, character interleaving is used for asynchronous devices whereas bit interleaving is used for synchronous devices.

• Capacity is wasted to achieve simplicity of implementation.

• Faster devices can be allocated multiple slots per cycle.


TDM System


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—The corresponding source must be quenched—This leaves empty slots

• Error control—Errors are detected and handled by individual

channel systems


Data Link Control on TDM


Framing (1)• No flag or SYNC characters bracketing

TDM frames• Must provide synchronizing mechanism• 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—Can compare incoming bit patterns on each

channel with sync pattern


Framing (2)• For synchronization, the receiver monitors

the framing bit channel (101010101..)• It compares the incoming bits of a frame

with the expected pattern (10101010..).• If it does not match, successive bits are

searched until the pattern is found.• During monitoring, if pattern breaks down,

the receiver enters to “frame search” mode.


Pulse Stuffing• Problem - Synchronizing data sources• Clocks in different sources drifting• Data rates from different sources not

related by simple rational number• Solution - Pulse Stuffing

—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

—Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer


TDM of Analog and Digital Sources


Digital Carrier Systems• The long-distance carrier system was designed to

transmit voice signals over high capacity transmission links

• AT&T developed a 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


Digital Carrier Systems (2)• For voice each channel slot contains one word of

digitized data (PCM, 8000 samples per sec)—Data rate 8000x193 = 1.544Mbps—Five out of six frames have 8 bit PCM samples—Sixth frame is 7 bit PCM word plus signaling bit—Signaling bits form stream for each channel containing

control and routing info

• Same format for digital data—23 channels of data

• 7 bits per frame plus indicator bit for data or system control

• Each frame is repeated 8000 times/sec = 56 kbps—24th channel is sync

• Reliable reframing following a framing error


Mixed Data• DS-1 can carry mixed voice and data signals• 24 channels used• No sync byte• Can also interleave DS-1 channels

—DS-2 is four DS-1 giving 6.312Mbps

• Subrate multiplexing is used to allow transmit of slower rates. In this case, the first bit is allocated to indicate the subrate multiplexing and the other 6 bits are used to multiplex slower data (E.g. 6X8000=48kbps can be used as 5X9.6=48 kbps


DS-1 Transmission Format


SONET/SDH• Synchronous Optical Network (ANSI)• Synchronous Digital Hierarchy (ITU-T)• SONET provides taking advantage of the high

speed digital transmission capability of optical fiber• Signal Hierarchy

—Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1)

—Data rate: 51.84Mbps—Carry DS-3 or group of lower rate signals (DS1 DS1C DS2)

plus ITU-T rates (e.g. 2.048Mbps)—Multiple STS-1 combined into STS-N signal—ITU-T lowest rate is 155.52Mbps (STM-1)


SONET Frame Format


Statistical TDM• 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


Statistical TDM Characteristics• Since there are no fixed slots per channel,

addressing information is required• In general, HDLC is used. HDLC frames

contain multiplexing information.• Since the frames are not in fixed size,

frame length information is required• Address and frame fields increase the

overhead in Statistical TDM• Methods to reduce the size of these fields

are devised.


Statistical v Synchronous TDM• Fig 8.14


Statistical TDM Frame Formats


Improving Efficiency• The address field can be reduced by using

relative addressing—Each address specifies the current address

relative to previous address

• Refinement in length field—A value of 00, 01, 10 corresponds to a data

field of 1,2 or 3 bytes—11 indicates that a length field is included


Performance• Output data rate less than aggregate

input rates• May cause problems during peak periods

—Buffer inputs—Keep buffer size to minimum to reduce delay

• There is a tradeoff between system response time and the speed of the multiplexed line


Performance Math (1)• Let

—I = number of input sources—R = data rate of each source, bps—M = effective capacity of multiplexed line, bps = mean fraction of time each source is

transmitting, 0 < < 1—K = M / I R ratio of multiplexed line capacity to

total maximum input—Then the value of K is bounded by < K < 1

– If K = 1 then synchronous TDM– If K < then input exceed multiplexing capacity


Performance Math (2)• Define further the following:

= average arrival rate in bps

— Ts = time it takes to transmit one bit = fraction of total link capacity being used—N = amount of buffer space being used

—Tr = average delay encountered by an input source

• Then, using queuing theory, we have, = I R

—Ts = 1 / M

= Ts = / M

—N = 2 / (2(1-)) +—Tr = Ts (2 - ) / (2 (1 - ))


Buffer Size and Delay


Probability of Overflow as a Function of Buffer Size• Fig 8-17


Performance Comments• The figures suggest that the utilization

should not go above 80 percent since buffer requirements and delay increase

• There is always a finite probability that the buffer will overflow.

• For constant utilization, average delay decreases as line capacity increases


Cable Modem Outline• Two channels from cable TV provider dedicated

to data transfer—One in each direction

• Each channel shared by number of subscribers—Scheme needed to allocate capacity—Statistical TDM


Cable Modem Operation• Downstream

—Cable scheduler delivers data in small packets—If more than one subscriber active, each gets fraction of

downstream capacity• May get 500kbps to 1.5Mbps

—Also used to allocate upstream time slots to subscribers

• Upstream—User requests timeslots on shared upstream channel

• Dedicated slots for this

—Headend scheduler sends back assignment of future time slots to subscriber


Cable Modem Scheme


Asymmetrical Digital Subscriber Line• ADSL• Link between subscriber and network• Uses currently installed twisted pair cable

—Can carry broader spectrum—Links installed to carry voice grade signals in a

bandwidth from 0 to 4 kHz —1 MHz or more


ADSL Design• Asymmetric

—Greater capacity downstream than upstream

• Frequency division multiplexing—Lowest 25kHz for voice

• Plain old telephone service (POTS)

—Use echo cancellation or FDM to give two bands

—Use FDM within the downstream and upstream bands


ADSL Channel Configuration


FDM v Echo Cancellation in ADSL• Higher frequency has higher attenuation.

With echo cancellation, more of the downstream bandwidth is in good portion of the spectrum as compared to FDM

• In Echo Cancellation, upstream capacity can be expanded

• Echo Cancellation is more complicated


Discrete Multitone• DMT• 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• 256 downstream subchannels at 4kHz

(60kbps)—15.36MHz—Impairments bring this down to 1.5Mbps to 9Mbps


DTM Bits Per Channel Allocation


DMT Transmitter


xDSL• High data rate DSL

—Data rate 1.544 or 2.048 Mbps (with different coding schemes)

• Single line DSL—HDSL requires two twisted pair—SDSL was developed to provide the same type

of service as HDSL

• Very high data rate DSL—Provides a scheme similar to ADSL at a much

higher data rate by sacrificing distance


Required Reading• Stallings chapter 8• Web sites on

—ADSL —SONET

TUNALIData Communication1 Chapter 8 Multiplexing.

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