Modem Modem SynchronizationSynchronization

-DSC-DSC

Medium and High Speed ModemsMedium and High Speed Modems

Transmission rate: > 2400 bpsTransmission rate: > 2400 bps PSK / QAM is usedPSK / QAM is used So requires receiver carrier So requires receiver carrier

oscillators to be atleast frequency oscillators to be atleast frequency coherent (and possibly phase coherent (and possibly phase coherent)coherent)

Modems used synchronous so Modems used synchronous so clock timing recovery to be clock timing recovery to be achieved by receive modemachieved by receive modem

TrainingTraining Clock Recovery Clock Recovery Scrambler and Descrambler Scrambler and Descrambler

circuitscircuits EqualizersEqualizers

TrainingTraining

Modulation technique determines Modulation technique determines number of bits required and number of bits required and hence the duration of the training hence the duration of the training sequencesequence

Example – 208 modem Example – 208 modem

Synchronous, 4800 bps modem that uses 8-DPSKSynchronous, 4800 bps modem that uses 8-DPSK Each symbol is a tribit and duration of each Each symbol is a tribit and duration of each

symbol is 0.625 mssymbol is 0.625 ms 4-phase idle code: sequences through four of the 4-phase idle code: sequences through four of the

eight possible shifts – allows receiver to recover eight possible shifts – allows receiver to recover carrier and clock timing information rapidlycarrier and clock timing information rapidly

4-phase test word: allows adaptive equalizer in 4-phase test word: allows adaptive equalizer in the receive modem to adjust to its final settingsthe receive modem to adjust to its final settings

8-phase initialization period: prepares the 8-phase initialization period: prepares the descrambler circuits for 8-phase operationdescrambler circuits for 8-phase operation

The entire training sequence requires 48.75 ms The entire training sequence requires 48.75 ms (=78 * ? )(=78 * ? )

Clock recovery – Need?Clock recovery – Need?

Timing synchronization Timing synchronization established during training established during training sequence but has to be sequence but has to be maintained during transmissionmaintained during transmission

So we use Clock Recovery circuitSo we use Clock Recovery circuit Example - QPSKExample - QPSK

Example - QPSKExample - QPSK

Clocking information extracted Clocking information extracted from either I or Q channel or from from either I or Q channel or from output of bit combineroutput of bit combiner

Assuming alternating 1 / 0 Assuming alternating 1 / 0 pattern at output of low pass filter pattern at output of low pass filter , clock frequency at the bit rate of , clock frequency at the bit rate of I (or Q) channel can be recoveredI (or Q) channel can be recovered

This clocking information is used This clocking information is used to phase lock loop the receive to phase lock loop the receive clock oscillator onto transmit clock oscillator onto transmit clock frequencyclock frequency

LimitationLimitation

Sufficient transitions between the logic levels Sufficient transitions between the logic levels 1 and 0 required in the received bit stream to 1 and 0 required in the received bit stream to recover clockrecover clock

In QPSK example , if input on transmitter side In QPSK example , if input on transmitter side consists of alternate 1’s and 0’s then one consists of alternate 1’s and 0’s then one channel (I or Q) would contain all 1’s and the channel (I or Q) would contain all 1’s and the opposite channel would contain all 0’sopposite channel would contain all 0’s

Recovered clock lost Recovered clock lost Solution : Restrictions placed on customer’s Solution : Restrictions placed on customer’s

protocol to prevent undesirable sequence.protocol to prevent undesirable sequence.Not a proper solutionNot a proper solution

Scramblers and Scramblers and DescramblersDescramblers

Customer data is scrambled using a Customer data is scrambled using a scrambler circuit according to a particular scrambler circuit according to a particular algorithm on the transmit sidealgorithm on the transmit side

Corresponding descrambling algorithm used Corresponding descrambling algorithm used on receiver side to retrieve customer dataon receiver side to retrieve customer data

Better method : No restrictions placed on Better method : No restrictions placed on customer / usercustomer / user

Purpose of the scrambler : Not to randomize Purpose of the scrambler : Not to randomize the transmitted bit sequence but to detect the transmitted bit sequence but to detect occurrence of undesirable bit sequence and occurrence of undesirable bit sequence and convert it to a more acceptable formconvert it to a more acceptable form

Scrambler circuit inserted prior to the bit Scrambler circuit inserted prior to the bit splitter in QPSK modulator and descrambler splitter in QPSK modulator and descrambler inserted after bit combiner in QPSK inserted after bit combiner in QPSK demodulatordemodulator

Output of Scrambler: ABCD + A’B’C’D’Output of Scrambler: ABCD + A’B’C’D’ ffmm=f=fdd xor (ABCD + A’B’C’D’) xor (ABCD + A’B’C’D’) ffrr=f=fmm xor (ABCD + A’B’C’D’) xor (ABCD + A’B’C’D’) ffrr=f=fdd xor (ABCD +A’B’C’D’) xor (ABCD + xor (ABCD +A’B’C’D’) xor (ABCD +

A’B’C’D’)A’B’C’D’) ffrr=f=fdd

What are we trying to imply?What are we trying to imply?

The output of either OR gate is 1 if The output of either OR gate is 1 if the 4 – bit register contains all 1’s or the 4 – bit register contains all 1’s or all 0’s. Neither of these is a all 0’s. Neither of these is a desirable sequence.desirable sequence.

If OR gate output is 1, fIf OR gate output is 1, fmm is is complement of fcomplement of fd d else it is equal to felse it is equal to fd.d.

Intent is to create transitions in a Intent is to create transitions in a prolonged bit stream of either all 1’s prolonged bit stream of either all 1’s or 0’s.or 0’s.

There is a flaw in the above There is a flaw in the above scrambler logic.Can you spot it?scrambler logic.Can you spot it?

EqualizationEqualization

Compensation for the phase delay distortion and Compensation for the phase delay distortion and amplitude distortion of a telephone lineamplitude distortion of a telephone line

Compromise Equalizers: Compromise Equalizers: – contained in transmit section of modem and provide contained in transmit section of modem and provide

pre-equalizationpre-equalization– shape the transmitted signal by altering its delay and shape the transmitted signal by altering its delay and

gain characteristics before it reaches the telephone gain characteristics before it reaches the telephone lineline

Adaptive Equalizers:Adaptive Equalizers:– located in receiver section of modem and provide post located in receiver section of modem and provide post

equalization to the received analog signalequalization to the received analog signal– automatically adjust their gain and delay automatically adjust their gain and delay

characteristics to compensate for telephone line characteristics to compensate for telephone line impairments impairments

MODEM CONTROL – AT MODEM CONTROL – AT COMMAND SETCOMMAND SET First generation modems called dumb First generation modems called dumb

modemsmodems– Consisted of little more than a modulator and Consisted of little more than a modulator and

demodulatordemodulator– Required operators on both ends of circuit to Required operators on both ends of circuit to

perform basic functions like initiating , completing perform basic functions like initiating , completing and terminating callsand terminating calls

Intelligent modems Intelligent modems – Introduced in the early 1970sIntroduced in the early 1970s– Built in microprocessors to perform routine Built in microprocessors to perform routine

functions like answering a call, call initiating , functions like answering a call, call initiating , dialling etc.dialling etc.

Smart modems controlled by Smart modems controlled by large computers through a large computers through a system of commandssystem of commands

The most common set of modem The most common set of modem commands is the AT COMMAND commands is the AT COMMAND SET also known as HAYES SET also known as HAYES COMMAND SET.COMMAND SET.

Two AT modes:Two AT modes:– AT Command modeAT Command mode– AT Online modeAT Online mode

AT Command ModeAT Command Mode

All modem commands in the AT All modem commands in the AT command set begin with ASCII command set begin with ASCII characters AT(Attention)characters AT(Attention)

Whenever a modem is not Whenever a modem is not communicating directly with another communicating directly with another mode it is in the command modemode it is in the command mode

Modem takes commands from local Modem takes commands from local terminalterminal

Valid command is preceded by ATValid command is preceded by AT Example: ATDT16024617777Example: ATDT16024617777

AT Online ModeAT Online Mode

Once communication is established with a remote Once communication is established with a remote modem, local modem switches to the online mode.modem, local modem switches to the online mode.

In online mode modem becomes transparent and In online mode modem becomes transparent and interprets characters received from DTE, including AT interprets characters received from DTE, including AT characters as data.characters as data.

The local modem simply accepts the characters and The local modem simply accepts the characters and allows them to modulate the carrier before sending allows them to modulate the carrier before sending them to a remote location.them to a remote location.

Possible to switch modem from online to command Possible to switch modem from online to command mode : momentary pause followed by (+++) followed mode : momentary pause followed by (+++) followed by momentary pause again.by momentary pause again.

(+++) is the escape sequence. (+++) is the escape sequence.

Difference between Tone Dialing and Pulse Dialing?Difference between Tone Dialing and Pulse Dialing?