1、INTERNATIONAL TELECOMMUNICATION UNIONITU-T V.22 bisTELECOMMUNICATIONSTANDARDIZATION SECTOROF ITUDATA COMMUNICATIONOVER THE TELEPHONE NETWORK2400 BITS PER SECOND DUPLEX MODEMUSING THE FREQUENCY DIVISIONTECHNIQUE STANDARDIZED FOR USE ONTHE GENERAL SWITCHED TELEPHONENETWORK AND ON POINT-TO-POINT2-WIRE
2、LEASED TELEPHONE-TYPECIRCUITSITU-T Recommendation V.22 bis(Extract from the Blue Book)NOTES1 ITU-T Recommendation V.22 bis was published in Fascicle VIII.1 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the
3、Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agen
4、cy. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VIII.1 - Rec. V.22 bis 1Recommendation V.22 bisFascicle VII
5、I.1 - Rec. V.22 bis2400 BITS PER SECOND DUPLEX MODEMUSING THE FREQUENCY DIVISION TECHNIQUE STANDARDIZED FOR USEON THE GENERAL SWITCHED TELEPHONE NETWORK AND ONPOINT-TO-POINT 2-WIRELEASED TELEPHONE-TYPE CIRCUITS(Malaga-Torremolinos, 1984; and al Melbourne, 1988)The CCITT,considering(a) that there is
6、a demand for data transmission at 2400 bit/s in the duplex mode over the General SwitchedTelephone Network (GSTN) and on point-to-point 2-wire leased telephone-type circuits;(b) that there is a demand to have the fall-back mode compatibility with modems in accordance withRecommendation V.22;(c) that
7、 in this case the frequency division technique shall be used,(unanimously) declaresthat the characteristics of the modems for this service shall provisionally be as follows:1 IntroductionThese modems are intended for use on connections on the GSTN and on point-to-point 2-wire leased telephone-type c
8、ircuits (see Note). The principal characteristics of these modems are as follows:a) duplex mode of operation on the GSTN and point-to-point leased circuits,b) channel separation by frequency division,c) quadrature amplitude modulation for each channel with synchronous line transmission at 600 baud (
9、nominal),d) inclusion of a scrambler,e) inclusion of an adaptive equalizer and a compromise equalizer,f) inclusion of test facilities,g) data signalling rates of:2400 bit/s synchronous,2400 bit/s start-stop,1200 bit/s synchronous,1200 bit/s start-stop,h) it is compatible with a V.22 modem operating
10、in Modes i) or ii) at the 1200 bit/s signalling rate and includesautomatic bit rate recognition.Note - In certain countries the use of such modems over the GSTN may not be allowed.2 Fascicle VIII.1 - Rec. V.22 bis2 Line signals2.1 Carrier and guard tone frequenciesThe carrier frequencies shall be 12
11、00 0.5 Hz for the low channel and 2400 1 Hz for the high channel. Aguard tone of 1800 20 Hz, to be transmitted only when the modem is transmitting in the high channel, may be disabledas a national option. An alternative guard tone of 550 20 Hz, to be transmitted only when the modem is transmitting i
12、nthe high channel, may be incorporated as a national option.2.2 Data and guard tone line signal levelsThe 1800 Hz or 550 Hz guard tones shall be levels 6 1 dB or 3 1 dB, respectively, below the level of thedata signal power in the high channel. Because of the 1800 Hz guard tone, the power level of d
13、ata signals in the highchannel will be approximately 1 dB lower than that of data signals in the low channel.2.3 Fixed compromise equalizerFixed compromise equalization shall be incorporated in the modem transmitter.2.4 Spectrum and group delay characteristicsThe transmitted line signals, excluding
14、the characteristics of the fixed compromise equalizer, shall have afrequency amplitude spectrum equivalent to the square root of a raised cosine shaping with 75% roll-off and within thelimits shown in Figure 1/V.22 bis. Similarly, the group delay of the transmitter output shall be within the range o
15、f 150microseconds over the frequency ranges 900-1500 Hz (low channel) and 2100-2700 Hz (high channel). These figures areprovisional.FIGURE 1/V.22 bisAmplitude limits for transmitted line signal (unequalized)2.5 Modulation2.5.1 Data signalling ratesThe data rate transmitted to line shall be 2400 bit/
16、s or 1200 bit/s 0.01% with a modulation rate of600 baud 0.01%.Fascicle VIII.1 - Rec. V.22 bis 32.5.2 Encoding of data bits2.5.2.1 2400 bits per secondThe data stream to be transmitted shall be divided into groups of 4 consecutive bits (quadbits). The first two bitsof a quadbit shall be encoded as a
17、phase quadrant change relative to the quadrant occupied by the preceding signalelement. (See Figure 2/V.22 bis and Table 1/V.22 bis.)The last two bits of each quadbit define one of 4 signalling elements associated with the new quadrant (seeFigure 2/V.22 bis). The left hand bits in Table 1/V.22 bis a
18、nd Figure 2/V.22 bis are the first of each pair in the datastream as it enters the modulator portion of the modem after the scrambler.TABLE 1/V.22 bisLine encodingFirst two bits in quabit (2400 bit/s)or dibit values (1200 bit/s)Phasequadrant change001 22 33 44 190011 12 23 34 40111 42 13 24 3270101
19、32 43 14 21804 Fascicle VIII.1 - Rec. V.22 bisFIGURE 2/V.22 bisSignal constellation2.5.2.2 1200 bits per secondThe data stream to be transmitted shall be divided into groups of 2 consecutive bits (dibits). The dibits shall beencoded as a phase quadrant change relative to the quadrant occupied by the
20、 preceding signal element (see Table1/V.22 bis). The signalling elements corresponding to 01 in the signal constellation (Figure 2/V.22 bis) shall betransmitted irrespective of the quadrant concerned. This ensure compatibility with Recommendation V.22.2.6 Received signal frequency toleranceThe recei
21、ver shall be able to operate with received frequency offsets of up to 7 Hz.3 Interchange circuits3.1 Essential and optional interchange circuitsThese are listed in Table 2/V.22 bis.3.2 Circuits 106 and 109 response timesAfter the handshaking sequences, circuit 106 will follow OFF to ON or ON to OFF
22、transitions of circuit 105within 3.5 ms. The OFF to ON transition of circuit 109 is part of the handshake sequence specified in 6. Circuit 109shall turn OFF 40 to 65 ms after the level of the received signal appearing at the line terminal of the modem falls belowthe relevant threshold defined in 3.3
23、. In the fall-back mode, the response time may be reduced to a value in the 10 to 24ms range specified in Recommendation V.22. Following a dropout, after the initial handshake, circuit 109 shall turn ON40 to 205 ms after the level of the received signal appearing at the line terminal of the modem ex
24、ceeds the relevantthreshold defined in 3.3.Fascicle VIII.1 - Rec. V.22 bis 53.3 Circuit 109 thresholdHigh channel threshold:- greater than -43 dBmcircuit 109 ON- less than -48 dBmcircuit 109 OFFLow channel threshold:- greater than -43 dBmcircuit 109 ON- less than -48 dBmcircuit 109 OFFThe condition
25、of circuit 109 between the ON and OFF levels is not specified except that the signal detector shallexhibit a hysteresis action, such that the level at which the OFF to ON transition occurs shall be at least 2 dB greater thanthat for the ON to OFF transition.Circuit 109 thresholds are specified at th
26、e input to the modem when receiving scrambled binary 1.Administrations are permitted to change these thresholds where transmission conditions are known.Circuit 109 shall not respond to the 1800 Hz or the 550 Hz guard tones, or the 2100 Hz (nominal) answer toneduring the handshake sequence.3.4 Circui
27、t 111 and data rate controlData rate selection may be by switch (or similar means) or by circuit 111 or a combination of both.The ON condition on circuit 111, where provided, shall select 2400 bit/s operation and the OFF condition shallselect 1200 bit/s operation.TABLE 2/V.22 bisInterchange circuit
28、(Note 1)Interchange circuitNotesNo. Description102103104105106107108/1108/2109111112113114115125140141142Signal ground or common returnTransmitted dataReceived dataRequest to sendReady for sendingData set readyConnect data set to lineData terminal readyData channel received line signal detectorData
29、signalling rate selector (DTE source)Data signalling rate selector (DCE source)Transmitter signal element timing (DTE source)Transmitter signal element timing (DCE source)Receiver signal element timing (DCE source)Calling indicatorLoopback/maintenance testLocal loopbackTest indicatorNote 2Note 3Note
30、 3Note 4Note 5Note 6Note 6Note 7Note 1 - All essential interchange circuits and any others which are provided must comply with the functional and operationalrequirements of Recommendation V.24. All interchange circuits provided must be properly terminated in the data terminal equipmentin accordance
31、with the appropriate Recommendation for electrical characteristics (see 3.5).6 Fascicle VIII.1 - Rec. V.22 bisNote 2 - Some automatic calling equipments are designed to emit a calling tone to line by turning ON circuit 105 to the calling modem.The general switched telephone network (GSTN) constant c
32、arrier handshake is such that no calling tone will be emitted by the V.22 bismodem when used with these equipments.Note 3 - This circuit shall be capable of operation as circuit 108/1 or 108/2 depending on its use.Note 4 - This circuit is optional.Note 5 - When the modem is not operating in a synchr
33、onous mode at the interface, any signals on this circuit shall be disregarded.Many DTEs operating in an asynchronous mode do not have a generator connected to this circuit.Note 6 - When the modem is not operating in a synchronous mode at the interface, this circuit shall be clamped to the OFF condit
34、ion.Many DTEs operating in an asynchronous mode do not terminate this circuit.Note 7 - This circuit is for use with the general switched telephone network only.3.5 Electrical characteristics of interchange circuits3.5.1 Use of electrical characteristics conforming to Recommendation V.28 is recommend
35、ed together with theconnector and pin assignment plan specified by ISO 2110.Note - Manufacturers may wish to note that the long-term objective is to replace electrical characteristicsspecified in Recommendation V.28, and that Study Group XVII has agreed that the work shall proceed to develop a moree
36、fficient, all-balanced, interface for the V-Series application which minimizes the number of interchange circuits.3.6 Fault condition of interchange circuitsSee Recommendation V.28, 7 for association of the receiver failure detection types.)3.6.1 The DTE should interpret a fault condition on circuit
37、 107 as an OFF condition using failure detection type 1.3.6.2 The DCE should interpret a fault condition on circuits 105 and 108 as an OFF condition using failure detectiontype 1.3.6.3 All other circuits not referred to above may use failure detection types 0 or 1.4 Modes of operationThe modem can b
38、e configured for the following modes of operation:Mode 1 2400 bit/s 0.01% synchronousMode 2 2400 bit/s start-stop 8, 9, 10 or 11 bits per characterMode 3 1200 bit/s 0.01% synchronousMode 4 1200 bit/s start-stop 8, 9, 10 or 11 bits per character.4.1 Transmitter4.1.1 In the synchronous modes of operat
39、ion, the modem shall accept synchronous data from the DTE on circuit 103under control of circuit 113 or circuit 114. The data shall then be scrambled in accordance with 5 and then passed to themodulator for encoding in accordance with 2.5.4.1.2 In the start-stop modes, the modem shall accept a data
40、stream of start-stop characters from the DTE at anominal rate of 2400 or 1200 bits per second. The start-stop data to be transmitted shall be converted in conformity withRecommendation V.14 to a synchronous data stream suitable for transmission in accordance with 4.1.14.2 ReceiverDemodulated data sh
41、all be decoded in accordance with 2.5.2, then descrambled in accordance with 5.2 andthen passed to the converter in conformity with Recommendation V.14 for regaining the data stream of start-stopcharacters.The intracharacter signalling rate provided to the DTE over circuit 104 shall be in the ranges
42、 given in Table3/V.22 bis when operating in the basic or in the extended signalling rate ranges, respectively.Fascicle VIII.1 - Rec. V.22 bis 7TABLE 3/V.22 bisIntracharacter signalling rate rangeData rate Signalling rate rangeBasic Extended2400 bit/s1200 bit/s2400 to 2424 bit/s1200 to 1212 bit/s2400
43、 to 2455 bit/s1200 to 1227 bit/s5 Scrambler and descrambler5.1 ScramblerA self synchronizing scrambler having the generating polynomial 1 x-14 x-17shall be included in the modemtransmitter. The message data sequence applied to the scrambler shall be effectively divided by the generatingpolynomial. T
44、he coefficients of the quotients of this division, taken in descending order, form the data sequence whichshall appear at the output of the scrambler. The scrambler output data sequence shall thus be:whereDsis the data sequence at the output of the scramblerDiis the data sequence applied to the scra
45、mbler denotes module 2 addition. denotes binary multiplication.Figure 3/V.22 bis shows a suitable implementation.To prevent occasional inadvertent instigation of remote loop 2 caused by scrambler lockup, circuitry shall beincluded to detect a sequence of 64 consecutive ones at the scrambler output (
46、Ds) and, if detected, invert the next input tothe scrambler (Di) and reset the counter of 64 consecutive ones. This circuitry shall operate whenever the scrambler isoperational. No scrambler initialization is required during the handshake or retrain sequence.FIGURE 3/V.22 bisScrambler8 Fascicle VIII
47、.1 - Rec. V.22 bis5.2 DescramblerA self synchronizing descrambler having the polynomial 1 x-14 x-17Shall be provided in the modemreceiver. The message data sequence produced after demodulation shall be effectively multiplied by the generatingpolynomial 1 x-14 x-17to form the descrambled message. The
48、 coefficients of the recovered message sequence takenin descending order form the output data sequence Dowhich is given bywhere the notation is as defined in 5.1.Circuitry may be included to detect a sequence of 64 consecutive ones at the input to the descrambler (Ds) and, ifdetected, invert the nex
49、t output from the descrambler (Do) This detector shall operate whenever the descrambler isoperational.Figure 4/V.22 bis shows a suitable implementation.FIGURE 4/V.22 bisDescrambler6 Operating sequences6.1 Channel allocation and signalling rate selection6.1.1 GSTNOn the general switched telephone network, the modem at the calling data station shall transmit in the lowchannel and receive in the high channel (call mode). The modem at the answering data station shall receive in the lowchannel and transmit in the high chan
