Communication Communication NetworksNetworks

Chapter 8 – MultiplexingChapter 8 – Multiplexing

MultiplexingMultiplexing It allows several transmission sources to share a larger It allows several transmission sources to share a larger

transmission capacity on 1 physical line.transmission capacity on 1 physical line. common in long-haul communications.common in long-haul communications. Trunks on long-haul networks are high capacity links.Trunks on long-haul networks are high capacity links. Common forms on multiplexing are FDM, TDM, STDM.Common forms on multiplexing are FDM, TDM, STDM.

Figure 8.1: Multiplexing

Frequency Division MultiplexingFrequency Division Multiplexing FDMFDM can be used with analog signals. can be used with analog signals. A number of signals are carried simultaneously on the A number of signals are carried simultaneously on the

same medium by allocating to each signal a different same medium by allocating to each signal a different frequency band.frequency band.

FDM is possible when the useful bandwidth of the medium FDM is possible when the useful bandwidth of the medium exceeds the required bandwidth of signals to be exceeds the required bandwidth of signals to be transmitted.transmitted.

Each signal is modulated onto a different carrier frequency Each signal is modulated onto a different carrier frequency and the carrier frequencies are sufficiently separated that and the carrier frequencies are sufficiently separated that the bandwidths of the signals do not overlap (the bandwidths of the signals do not overlap (Figure 8.2aFigure 8.2a).).

Each modulated signal requires a certain bandwidth Each modulated signal requires a certain bandwidth centered on its carrier frequency, referred to as a centered on its carrier frequency, referred to as a channelchannel..

To prevent interference, the channels are separated by To prevent interference, the channels are separated by guard bands, which are unused portions of the spectrum.guard bands, which are unused portions of the spectrum.

Frequency Division Multiplexing (contd.)Frequency Division Multiplexing (contd.)

Figure 8.2a: Frequency Division Multiplexing

Frequency Division Multiplexing (contd.)Frequency Division Multiplexing (contd.)

Figure 8.4 shows an FDM system.Figure 8.4 shows an FDM system. A number of analog or digital signals [A number of analog or digital signals [mmii((tt), ), ii = 1, = 1, nn] are to ] are to

be multiplexed onto the same transmission medium.be multiplexed onto the same transmission medium. Each signal Each signal mmii((tt) is modulated onto a carrier ) is modulated onto a carrier ffii; because ; because

multiple carriers are to be used, each is referred to as a multiple carriers are to be used, each is referred to as a subcarriersubcarrier. Any type of modulation may be used.. Any type of modulation may be used.

Modulation equipment is needed to move each signal to Modulation equipment is needed to move each signal to the required frequency band, and multiplexing equipment the required frequency band, and multiplexing equipment is needed to combine the modulated signals.is needed to combine the modulated signals.

The resulting analog, modulated signals are then summed The resulting analog, modulated signals are then summed to produce a composite baseband signal to produce a composite baseband signal mmbb((tt). Figure 8.4b ). Figure 8.4b shows the result.shows the result.

The spectrum of signal The spectrum of signal mmii((tt) is shifted to be centered on ) is shifted to be centered on ffii..

Frequency Division Multiplexing (contd.)Frequency Division Multiplexing (contd.)

ffii must be chosen so that the bandwidths of the must be chosen so that the bandwidths of the various signals do not significantly overlap.various signals do not significantly overlap.

The composite signal is shifted as a whole to The composite signal is shifted as a whole to another carrier frequency by an additional another carrier frequency by an additional modulation step.modulation step.

The FDM signal The FDM signal ss((tt) has a bandwidth ) has a bandwidth BB = Sum = Sum BBii . . At the receiving end, the FDM signal is At the receiving end, the FDM signal is

demodulated to retrieve demodulated to retrieve mmbb((tt), which is then passed ), which is then passed through through nn bandpass filters, each filter centered on bandpass filters, each filter centered on ffii and having a bandwidth and having a bandwidth BBii..

The signal is again split into its component parts. The signal is again split into its component parts. Each component is demodulated to recover the Each component is demodulated to recover the original signal.original signal.

Figure 8.4: FDMFigure 8.4: FDMSystem overviewSystem overview

Example 8.2: FDM VoicebandExample 8.2: FDM Voiceband Three voice signals are transmitted simultaneously over a medium. The Three voice signals are transmitted simultaneously over a medium. The

bandwidth of a voice signal is generally taken to be 4 kHz, with an bandwidth of a voice signal is generally taken to be 4 kHz, with an effective spectrum of 300 to 3400 Hz (Figure 8.5a).effective spectrum of 300 to 3400 Hz (Figure 8.5a).

If such a signal is used to amplitude-modulate a 64-kHz carrier, the If such a signal is used to amplitude-modulate a 64-kHz carrier, the spectrum of Figure 8.5b results. The modulated signal has a bandwidth spectrum of Figure 8.5b results. The modulated signal has a bandwidth of 8 kHz, extending from 60 to 68 kHz. To make efficient use of of 8 kHz, extending from 60 to 68 kHz. To make efficient use of bandwidth, we elect to transmit only the lower sideband.bandwidth, we elect to transmit only the lower sideband.

If three voice signals are used to modulate carriers at 64, 68, and 72 kHz, If three voice signals are used to modulate carriers at 64, 68, and 72 kHz, and only the lower sideband of each is taken, the spectrum of Figure 8.5c and only the lower sideband of each is taken, the spectrum of Figure 8.5c results.results.

This figure points out two problems that an FDM system must cope withThis figure points out two problems that an FDM system must cope with The first is crosstalk, which may occur if the spectra of adjacent component The first is crosstalk, which may occur if the spectra of adjacent component

signals overlap significantly. In the case of voice signals, with an effective signals overlap significantly. In the case of voice signals, with an effective bandwidth of only 3100 Hz, a 4-kHz bandwidth is adequate.bandwidth of only 3100 Hz, a 4-kHz bandwidth is adequate.

Another problem is intermodulation noise. On a long link, the nonlinear Another problem is intermodulation noise. On a long link, the nonlinear effects of amplifiers on a signal in one channel could produce frequency effects of amplifiers on a signal in one channel could produce frequency components in other channels.components in other channels.

Example 8.2: FDM Voiceband (contd.)Example 8.2: FDM Voiceband (contd.)

Figure 8.5: FDM of Three Voice-band Signals

Analog Carrier SystemsAnalog Carrier Systems long-distance links use an FDM hierarchylong-distance links use an FDM hierarchy GroupGroup

12 voice channels (4kHz each) are combined to 12 voice channels (4kHz each) are combined to produce a group signal with a bandwidth of 48kHz, produce a group signal with a bandwidth of 48kHz, in in the range 60-kHz to 108-kHzthe range 60-kHz to 108-kHz

SupergroupSupergroup FDM of 5 group signals supports 60 channels. The FDM of 5 group signals supports 60 channels. The

subcarriers have frequencies from 420kHz subcarriers have frequencies from 420kHz to 612 kHz. to 612 kHz. The resulting signal occupies 312 to 552 kHz.The resulting signal occupies 312 to 552 kHz.

MastergroupMastergroup FDM of 10 supergroups supports 600 channelsFDM of 10 supergroups supports 600 channels

so original signal can be modulated many timesso original signal can be modulated many times

Wavelength Division MultiplexingWavelength Division Multiplexing

FDM with multiple beams of light at different freqFDM with multiple beams of light at different freq carried over optical fiber linkscarried over optical fiber links

commercial systems with 160 channels of 10 Gbpscommercial systems with 160 channels of 10 Gbps lab demo of 256 channels 39.8 Gbpslab demo of 256 channels 39.8 Gbps

architecture similar to other FDM systemsarchitecture similar to other FDM systems multiplexer consolidates laser sources (1550nm) for multiplexer consolidates laser sources (1550nm) for

transmission over single fibertransmission over single fiber Optical amplifiers amplify all wavelengthsOptical amplifiers amplify all wavelengths Demux separates channels at the destinationDemux separates channels at the destination

also have Dense Wavelength Division also have Dense Wavelength Division Multiplexing (DWDM)Multiplexing (DWDM)

Synchronous time division multiplexingSynchronous time division multiplexing

Can be used with digital signals or analog signals Can be used with digital signals or analog signals carrying digital data. carrying digital data.

Data from various sources are carried in repetitive Data from various sources are carried in repetitive frames.frames.

Each frame consists of a set of time slots, and each Each frame consists of a set of time slots, and each source is assigned one or more time slots per source is assigned one or more time slots per frame. The effect is to interleave bits of data from frame. The effect is to interleave bits of data from the various sources. The interleaving can be at the the various sources. The interleaving can be at the bit level or in blocks of bytes (Figure 8.2b).bit level or in blocks of bytes (Figure 8.2b).

STDM is called synchronous because the time slots STDM is called synchronous because the time slots are pre-assigned to sources and fixed. The time are pre-assigned to sources and fixed. The time slots for each source are transmitted whether or not slots for each source are transmitted whether or not the source has data to send.the source has data to send.

Synchronous Time Division MultiplexingSynchronous Time Division Multiplexing

Figure 8.2b: Time Division Multiplexing

STDM System OverviewSTDM System Overview Figure 8.6 shows an STDM system.Figure 8.6 shows an STDM system. A number of signals [A number of signals [mmii((tt), ), ii = 1, = 1, nn] are multiplexed onto the same ] are multiplexed onto the same

transmission medium. The signals carry digital data and are generally transmission medium. The signals carry digital data and are generally digital signals.digital signals.

The incoming data from each source are buffered. Each buffer is The incoming data from each source are buffered. Each buffer is typically one bit or one character in length. The buffers are scanned typically one bit or one character in length. The buffers are scanned sequentially to form a composite digital data stream sequentially to form a composite digital data stream mmcc((tt). The scan ). The scan operation is sufficiently rapid so that each buffer is emptied before operation is sufficiently rapid so that each buffer is emptied before more data can arrive. Thus, the data rate of more data can arrive. Thus, the data rate of mmcc((tt) must at least equal ) must at least equal the sum of the data rates of the the sum of the data rates of the mmii((tt).).

The digital signal The digital signal mmcc((tt) may be transmitted directly, or passed through a ) may be transmitted directly, or passed through a modem so that an analog signal is transmitted.modem so that an analog signal is transmitted.

The transmitted data are organized into The transmitted data are organized into framesframes (Figure 8.6b). In each (Figure 8.6b). In each frame, one or more slots are dedicated to each data source. The sequence frame, one or more slots are dedicated to each data source. The sequence of slots dedicated to one source is called a of slots dedicated to one source is called a channelchannel. The slot length equals . The slot length equals the transmitter buffer length.the transmitter buffer length.

At the receiver, the interleaved data are demultiplexed and routed to the At the receiver, the interleaved data are demultiplexed and routed to the appropriate destination buffer. For each input source appropriate destination buffer. For each input source mmii((tt), there is an ), there is an identical output destination that will receive the output data at the same rate identical output destination that will receive the output data at the same rate at which it was generated.at which it was generated.

Figure 8.6: STDMFigure 8.6: STDMSystem OverviewSystem Overview

TDM Link ControlTDM Link Control

The transmitted data stream shown in Figure 8.6b does not contain The transmitted data stream shown in Figure 8.6b does not contain the headers and trailers as the control mechanisms provided by a the headers and trailers as the control mechanisms provided by a data link protocol are not needed.data link protocol are not needed.

As far as the multiplexer and demultiplexer (Figure 8.1) are As far as the multiplexer and demultiplexer (Figure 8.1) are concerned, flow control is not needed.concerned, flow control is not needed.

The data rate on the multiplexed line is fixed, and the multiplexer and The data rate on the multiplexed line is fixed, and the multiplexer and demultiplexer are designed to operate at that rate.demultiplexer are designed to operate at that rate.

If one of the individual output lines attaches to a device that is temporarily If one of the individual output lines attaches to a device that is temporarily unable to accept data, transmission must continue, since the remaining unable to accept data, transmission must continue, since the remaining output lines are expecting to receive data at predetermined times. Thus, output lines are expecting to receive data at predetermined times. Thus, the channel in question will carry empty slots, but the frames as a whole the channel in question will carry empty slots, but the frames as a whole will maintain the same transmission rate.will maintain the same transmission rate.

The reasoning for error control is the same. It would not do to request The reasoning for error control is the same. It would not do to request retransmission of an entire TDM frame because an error occurs on retransmission of an entire TDM frame because an error occurs on one channel. The devices using the other channels do not want a one channel. The devices using the other channels do not want a retransmission nor would they know that a retransmission has been retransmission nor would they know that a retransmission has been requested by some other device on another channel. Again, the requested by some other device on another channel. Again, the solution is to apply error control on a per-channel basis.solution is to apply error control on a per-channel basis.

Data Link Control on TDMData Link Control on TDM Flow control and error control can be provided on a per-channel basis by using Flow control and error control can be provided on a per-channel basis by using

a data link control protocol such as HDLC.a data link control protocol such as HDLC. A simplified example is shown in Figure 8.7. Assume two data sources, each A simplified example is shown in Figure 8.7. Assume two data sources, each

using HDLC. One is transmitting a stream of HDLC frames containing three using HDLC. One is transmitting a stream of HDLC frames containing three octets of data each, and the other is transmitting HDLC frames containing four octets of data each, and the other is transmitting HDLC frames containing four octets of data. For clarity, we assume that character-interleaved multiplexing is octets of data. For clarity, we assume that character-interleaved multiplexing is used, although bit interleaving is more typical. The octets of the HDLC frames used, although bit interleaving is more typical. The octets of the HDLC frames from the two sources are shuffled together for transmission over the from the two sources are shuffled together for transmission over the multiplexed line. Note that even though the HDLC frames have lost their multiplexed line. Note that even though the HDLC frames have lost their integrity in some sense, the pieces are reassembled correctly before they are integrity in some sense, the pieces are reassembled correctly before they are seen by the device on the other end of the HDLC protocol. In this sense, the seen by the device on the other end of the HDLC protocol. In this sense, the multiplexing/demultiplexing operation is transparent to the attached stations; to multiplexing/demultiplexing operation is transparent to the attached stations; to each communicating pair of stations, it appears that they have a dedicated link.each communicating pair of stations, it appears that they have a dedicated link.

One refinement is needed in this figure. Both ends of the line need to be a One refinement is needed in this figure. Both ends of the line need to be a combination multiplexer/demultiplexer with a full-duplex line in between. combination multiplexer/demultiplexer with a full-duplex line in between. Then each channel consists of two sets of slots, one traveling in each direction. Then each channel consists of two sets of slots, one traveling in each direction. The individual devices attached at each end can, in pairs, use HDLC to control The individual devices attached at each end can, in pairs, use HDLC to control their own channel. The multiplexer/demultiplexers need not be concerned with their own channel. The multiplexer/demultiplexers need not be concerned with these matters.these matters.

Data Link Control on TDMData Link Control on TDM

Figure 8.7: Use of Data Link Control On TDM Channels

FramingFraming

no flag or SYNC chars bracketing TDM framesno flag or SYNC chars bracketing TDM frames must still provide synchronizing mechanism must still provide synchronizing mechanism

between src and dest clocksbetween src and dest clocks added digit framingadded digit framing

one control bit added to each TDM frameone control bit added to each TDM frame identifiable bit pattern used on control channelidentifiable bit pattern used on control channel eg. alternating 01010101…unlikely on a data channeleg. alternating 01010101…unlikely on a data channel compare incoming bit patterns on each channel with compare incoming bit patterns on each channel with

known sync patternknown sync pattern

Pulse StuffingPulse Stuffing have problem of synchronizing data sourceshave problem of synchronizing data sources with clocks in different sources driftingwith clocks in different sources drifting also issue of data rates from different sources also issue of data rates from different sources

not related by simple rational numbernot related by simple rational number Pulse Stuffing a common solutionPulse Stuffing a common solution

have outgoing data rate (excluding framing bits) have outgoing data rate (excluding framing bits) higher than sum of incoming rateshigher than sum of incoming rates

stuff extra dummy bits or pulses into each incoming stuff extra dummy bits or pulses into each incoming signal until it matches local clocksignal until it matches local clock

stuffed pulses inserted at fixed locations in frame and stuffed pulses inserted at fixed locations in frame and removed at demultiplexerremoved at demultiplexer

Example 8.3: Example 8.3: TDM ExampleTDM Example Figure 8.8 illustrates the use of STDM to multiplex digital and analog sources. Figure 8.8 illustrates the use of STDM to multiplex digital and analog sources.

Consider that there are 11 sources to be multiplexed on a single link:Consider that there are 11 sources to be multiplexed on a single link: Source 1:Source 1: Analog, 2-kHz bandwidth, Analog, 2-kHz bandwidth, Source 2: Source 2: Analog, 4-kHz bandwidth, Analog, 4-kHz bandwidth,

Source 3:Source 3: Analog, 2-kHz bandwidth, Analog, 2-kHz bandwidth, Sources 4-11:Sources 4-11: Digital, 7200 bps Digital, 7200 bps synchronoussynchronous

The analog sources are converted to digital using PCM. The required sampling rate The analog sources are converted to digital using PCM. The required sampling rate is 4000 samples per second for sources 1 and 3, and 8000 samples per second for is 4000 samples per second for sources 1 and 3, and 8000 samples per second for source 2.source 2.

These samples, which are analog, must then be quantized or digitized. Let us These samples, which are analog, must then be quantized or digitized. Let us assume that 4 bits are used for each analog sample.assume that 4 bits are used for each analog sample.

For convenience, these three sources will be multiplexed first, as a unit. At a scan For convenience, these three sources will be multiplexed first, as a unit. At a scan rate of 4 kHz, one PAM sample each is taken from sources 1 and 3, and two PAM rate of 4 kHz, one PAM sample each is taken from sources 1 and 3, and two PAM samples are taken from source 2 per scan.samples are taken from source 2 per scan.

These four samples are interleaved and converted to 4-bit PCM samples. Thus, a These four samples are interleaved and converted to 4-bit PCM samples. Thus, a total of 16 bits is generated at a rate of 4000 times per second, for a composite bit total of 16 bits is generated at a rate of 4000 times per second, for a composite bit rate of 64 kbps.rate of 64 kbps.

For the digital sources, pulse stuffing is used to raise each source to a rate of 8 For the digital sources, pulse stuffing is used to raise each source to a rate of 8 kbps, for a total data rate of 64 kbps. A frame can consist of multiple cycles of 32 kbps, for a total data rate of 64 kbps. A frame can consist of multiple cycles of 32 bits, each containing 16 PCM bits and two bits from each of the eight digital bits, each containing 16 PCM bits and two bits from each of the eight digital sources.sources.

TDM ExampleTDM Example

Figure 8.8: TDM of Analog and Digital Sources

Digital Carrier SystemsDigital Carrier Systems

long-distance links use an TDM hierarchylong-distance links use an TDM hierarchy AT&T (USA) and ITU-T (International) variantsAT&T (USA) and ITU-T (International) variants US system based on DS-1 formatUS system based on DS-1 format can carry mixed voice and data signalscan carry mixed voice and data signals 24 channels used for total data rate 1.544Mbps24 channels used for total data rate 1.544Mbps each voice channel contains one word of each voice channel contains one word of

digitized data (PCM, 8000 samples per sec)digitized data (PCM, 8000 samples per sec) same format for 56kbps digital datasame format for 56kbps digital data can interleave DS-1 channels for higher ratescan interleave DS-1 channels for higher rates

DS-2 is four DS-1 at 6.312MbpsDS-2 is four DS-1 at 6.312Mbps

DS-1 Transmission FormatDS-1 Transmission Format

The basis of the TDM hierarchy is the DS-1 transmission The basis of the TDM hierarchy is the DS-1 transmission format (Figure 8.9), which multiplexes 24 channels.format (Figure 8.9), which multiplexes 24 channels.

Each frame contains 8 bits per channel plus a framing bit Each frame contains 8 bits per channel plus a framing bit for 24 for 24 8 + 1 = 193 bits. For voice transmission, the voice 8 + 1 = 193 bits. For voice transmission, the voice signal is digitized using (PCM) at a rate of 8000 samples signal is digitized using (PCM) at a rate of 8000 samples per second.per second.

With a frame length of 193 bits, we have a data rate of With a frame length of 193 bits, we have a data rate of 8000 8000 193 = 1.544 Mbps. 193 = 1.544 Mbps.

For five of every six frames, 8-bit PCM samples are used.For five of every six frames, 8-bit PCM samples are used. For every sixth frame, each channel contains a 7-bit PCM For every sixth frame, each channel contains a 7-bit PCM

word plus a word plus a signaling bitsignaling bit.. The signaling bits form a stream for each voice channel The signaling bits form a stream for each voice channel

that contains network control and routing information.that contains network control and routing information.

DS-1 Transmission Format (contd.)DS-1 Transmission Format (contd.)

The same DS-1 format is used to provide digital data The same DS-1 format is used to provide digital data service. In this case, 23 channels of data are provided. service. In this case, 23 channels of data are provided. The twenty-fourth channel position is reserved for a The twenty-fourth channel position is reserved for a special sync byte.special sync byte.

Within each channel, 7 bits per frame are used for data, Within each channel, 7 bits per frame are used for data, with the eighth bit used to indicate whether the channel, with the eighth bit used to indicate whether the channel, for that frame, contains user data or system control data. for that frame, contains user data or system control data. With 7 bits per channel and 8000 samples per second, a With 7 bits per channel and 8000 samples per second, a data rate of 56 kbps can be provided per channel.data rate of 56 kbps can be provided per channel.

Lower data rates are provided using a technique known Lower data rates are provided using a technique known as subrate multiplexing, where an additional bit is as subrate multiplexing, where an additional bit is robbed from each channel. This leaves a total capacity robbed from each channel. This leaves a total capacity per channel of 6 per channel of 6 8000 = 48 kbps. This capacity is 8000 = 48 kbps. This capacity is used to multiplex five 9.6-kbps channels, ten 4.8-kbps used to multiplex five 9.6-kbps channels, ten 4.8-kbps channels, or twenty 2.4-kbps channels.channels, or twenty 2.4-kbps channels.

DS-1 Transmission Format (contd.)DS-1 Transmission Format (contd.)

Figure 8.9: DS-1 Transmission Format

SONET/SDHSONET/SDH

Synchronous Optical Network (ANSI)Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T)Synchronous Digital Hierarchy (ITU-T) have hierarchy of signal rateshave hierarchy of signal rates

Synchronous Transport Signal level 1 (STS-1) Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) is 51.84Mbpsor Optical Carrier level 1 (OC-1) is 51.84Mbps

carries one DS-3 or multiple (DS1 DS1C DS2) carries one DS-3 or multiple (DS1 DS1C DS2) plus ITU-T rates (eg. 2.048Mbps)plus ITU-T rates (eg. 2.048Mbps)

multiple STS-1 combine into STS-N signalmultiple STS-1 combine into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1)ITU-T lowest rate is 155.52Mbps (STM-1)

SONET Frame FormatSONET Frame Format The basic SONET building block is the STS-1 frame, The basic SONET building block is the STS-1 frame,

which consists of 810 octets and is transmitted once which consists of 810 octets and is transmitted once every 125 µs, for an overall data rate of 51.84 Mbps every 125 µs, for an overall data rate of 51.84 Mbps (Figure 8.10a).(Figure 8.10a).

The frame can be viewed as a matrix of 9 rows of 90 The frame can be viewed as a matrix of 9 rows of 90 octets each, with transmission being one row at a time, octets each, with transmission being one row at a time, from left to right and top to bottom.from left to right and top to bottom.

The first three columns (3 octets The first three columns (3 octets 9 rows = 27 octets) 9 rows = 27 octets) of the frame are devoted to overhead octets. Nine of the frame are devoted to overhead octets. Nine octets are devoted to section-related overhead and 18 octets are devoted to section-related overhead and 18 octets are devoted to line overhead.octets are devoted to line overhead.

Figure 8.10b shows the general format for higher-rate Figure 8.10b shows the general format for higher-rate frames, using the ITU-T designation.frames, using the ITU-T designation.

SONET Frame Format (contd.)SONET Frame Format (contd.)

Figure 8.10: SONET/SDH Frame Formats

Statistical TDMStatistical TDM

in Synch TDM many slots are wastedin Synch TDM many slots are wasted Statistical TDM allocates time slots Statistical TDM allocates time slots

dynamically based on demanddynamically based on demand multiplexer scans input lines and collects multiplexer scans input lines and collects

data until frame fulldata until frame full line data rate lower than aggregate input line data rate lower than aggregate input

line rates line rates may have problems during peak periodsmay have problems during peak periods

must buffer inputsmust buffer inputs

STDM Frame FormatSTDM Frame Format

It is desirable to minimize overhead bits to improve throughput.It is desirable to minimize overhead bits to improve throughput. Figure 8.13 shows two possible formats. In the first case, only Figure 8.13 shows two possible formats. In the first case, only

one source of data is included per frame. That source is one source of data is included per frame. That source is identified by an address. The length of the data field is variable, identified by an address. The length of the data field is variable, and its end is marked by the end of the overall frame. This and its end is marked by the end of the overall frame. This scheme can work well under light load but is quite inefficient scheme can work well under light load but is quite inefficient under heavy load.under heavy load.

A way to improve efficiency is to allow multiple data sources to A way to improve efficiency is to allow multiple data sources to be packaged in a single frame. Thus, the STDM subframe be packaged in a single frame. Thus, the STDM subframe consists of a sequence of data fields, each labeled with an consists of a sequence of data fields, each labeled with an address and a length.address and a length.

Another refinement is to use a 2-bit label with the length field. Another refinement is to use a 2-bit label with the length field. A value of 00, 01, or 10 corresponds to a data field of 1, 2, or 3 A value of 00, 01, or 10 corresponds to a data field of 1, 2, or 3 bytes; no length field is necessary. A value of 11 indicates that a bytes; no length field is necessary. A value of 11 indicates that a length field is included.length field is included.

STDM Frame Format (CONTD.)STDM Frame Format (CONTD.)

Figure 8.13: STDM Frame Format

Cable ModemCable Modem

dedicate two cable TV channels to data transferdedicate two cable TV channels to data transfer each channel shared by number of subscribers, each channel shared by number of subscribers,

using statistical TDMusing statistical TDM DownstreamDownstream

cable scheduler delivers data in small packetscable scheduler delivers data in small packets active subscribers share downstream capacityactive subscribers share downstream capacity also allocates upstream time slots to subscribersalso allocates upstream time slots to subscribers

UpstreamUpstream user requests timeslots on shared upstream channeluser requests timeslots on shared upstream channel Headend scheduler notifies subscriber of slots to useHeadend scheduler notifies subscriber of slots to use

Cable Modem (contd.)Cable Modem (contd.)

Figure 8.16: Cable Modem Scheme

Asymmetrical Digital Subscriber Asymmetrical Digital Subscriber Line (ADSL)Line (ADSL)

The digital subscriber line is the link between The digital subscriber line is the link between subscriber and network.subscriber and network.

ADSL is designed to provide high-speed digital ADSL is designed to provide high-speed digital data transmission over ordinary telephone wire.data transmission over ordinary telephone wire.

AsymmetricAsymmetric refers to the fact that ADSL provides refers to the fact that ADSL provides more capacity downstream than upstream.more capacity downstream than upstream.

ADSL uses FDM to exploit the 1-MHz capacity ADSL uses FDM to exploit the 1-MHz capacity of twisted pair.of twisted pair.

ADSL provides a range of up to 5.5 km.ADSL provides a range of up to 5.5 km.

ADSL Channel ConfigurationADSL Channel Configuration

There are three elements of the ADSL strategy, There are three elements of the ADSL strategy, shown in shown in Figure 8.17Figure 8.17:: Reserve lowest 25 kHz for voice (POTS). The voice is carried Reserve lowest 25 kHz for voice (POTS). The voice is carried

only in the 0 to 4 kHz band; the additional bandwidth is to only in the 0 to 4 kHz band; the additional bandwidth is to prevent crosstalk between the voice and data channels.prevent crosstalk between the voice and data channels.

Use either echo cancellation or FDM to allocate two bands, a Use either echo cancellation or FDM to allocate two bands, a smaller upstream band and a larger downstream band.smaller upstream band and a larger downstream band.

• Echo cancellation is a signal processing technique that allows Echo cancellation is a signal processing technique that allows transmission of digital signals in both directions on a single transmission of digital signals in both directions on a single transmission line simultaneously. In essence, a transmitter must transmission line simultaneously. In essence, a transmitter must subtract the echo of its own transmission from the incoming subtract the echo of its own transmission from the incoming signal to recover the signal sent by the other side.signal to recover the signal sent by the other side.

Use FDM within the upstream and downstream bands. A Use FDM within the upstream and downstream bands. A single bit stream is split into multiple parallel bit streams and single bit stream is split into multiple parallel bit streams and each portion is carried in a separate frequency band.each portion is carried in a separate frequency band.

ADSL Channel Configuration (contd.)ADSL Channel Configuration (contd.)

When echo cancellation is used, the entire When echo cancellation is used, the entire frequency band for the upstream channel overlaps frequency band for the upstream channel overlaps the lower portion of the downstream channel.the lower portion of the downstream channel.

This has two advantages compared to the use of This has two advantages compared to the use of distinct frequency bands for upstream and distinct frequency bands for upstream and downstream:downstream: the higher the frequency, the greater the attenuation, and the higher the frequency, the greater the attenuation, and echo cancellation design is more flexible for changing echo cancellation design is more flexible for changing

upstream capacity.upstream capacity. The disadvantage of the use of echo cancellation The disadvantage of the use of echo cancellation

is the need for echo cancellation logic on both is the need for echo cancellation logic on both ends of the line.ends of the line.

ADSL Channel Configuration (contd.)ADSL Channel Configuration (contd.)

Figure 8.17: ADSL Channel Configuration

Discrete Multitone (DMT)Discrete Multitone (DMT) multiple carrier signals at different frequenciesmultiple carrier signals at different frequencies divide into 4kHz subchannelsdivide into 4kHz subchannels test and use subchannels with better SNRtest and use subchannels with better SNR 256 downstream subchannels at 4kHz (60kbps)256 downstream subchannels at 4kHz (60kbps)

in theory 15.36Mbps, in practice 1.5-9Mbpsin theory 15.36Mbps, in practice 1.5-9Mbps

Figure 8.18: DMT Bits per Channel Allocation

DMT TransmitterDMT Transmitter

Figure 8.19 Figure 8.19 provides a general block diagram for provides a general block diagram for DMT transmission.DMT transmission. After initialization, the bit stream to be transmitted is After initialization, the bit stream to be transmitted is

divided into a number of substreams, one for each divided into a number of substreams, one for each subchannel that will carry data. The sum of the data subchannel that will carry data. The sum of the data rates of the substreams is equal to the total data rate.rates of the substreams is equal to the total data rate.

Each substream is then converted to an analog signal Each substream is then converted to an analog signal using QAM. This scheme works easily because of using QAM. This scheme works easily because of QAM’s ability to assign different numbers of bits per QAM’s ability to assign different numbers of bits per transmitted signal.transmitted signal.

Each QAM signal occupies a distinct frequency band, Each QAM signal occupies a distinct frequency band, so these signals can be combined by simple addition to so these signals can be combined by simple addition to produce the composite signal for transmission.produce the composite signal for transmission.

DMT Transmitter (contd.)DMT Transmitter (contd.)

Figure 8.19: DMT Transmitter

xDSLxDSL A number of schemes for providing high-speed digital A number of schemes for providing high-speed digital

transmission of the subscriber line are referred to as xDSL.transmission of the subscriber line are referred to as xDSL. High Data Rate DSL (HDSL).High Data Rate DSL (HDSL).

• was developed as a was developed as a T1T1 replacement. replacement.• HDSL uses the HDSL uses the 2B1Q2B1Q coding to provide a data rate of up to 2 Mbps over coding to provide a data rate of up to 2 Mbps over

two twisted-pair lines within a bandwidth up to about 196 kHz.two twisted-pair lines within a bandwidth up to about 196 kHz.• This enables a range of about 3.7 km to be achieved.This enables a range of about 3.7 km to be achieved.

Single Line DSL (SDSL)Single Line DSL (SDSL)• Similar to HDSL but over a single twisted-pair line for residential Similar to HDSL but over a single twisted-pair line for residential

subscribers.subscribers.• As with As with HDSLHDSL, , 2B1Q2B1Q coding is used. coding is used.• Echo cancellation is used to achieve full-duplex transmission over a Echo cancellation is used to achieve full-duplex transmission over a

single pair.single pair. Very High Data Rate DSL (VDSL)Very High Data Rate DSL (VDSL)..

• Similar to Similar to ADSLADSL at a much higher data rate by sacrificing distance. at a much higher data rate by sacrificing distance.• The likely signaling technique is The likely signaling technique is DMT/QAMDMT/QAM..• VDSL does not use echo cancellation but provides separate bands for VDSL does not use echo cancellation but provides separate bands for

different services, with the following allocation: different services, with the following allocation: POTSPOTS: 0 – 4 kHz, : 0 – 4 kHz, ISDNISDN: : 4 – 80 kHz, 4 – 80 kHz, UpstreamUpstream: 300 – 700 kHz, : 300 – 700 kHz, DownstreamDownstream: ≥ 1 MHz.: ≥ 1 MHz.