1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 G0TERTELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/.G0G0/6%2G0G04(%G0G04%,%0(/.%.%47/2+G0“)43G0G00%2G0G03%#/.$G0G0-/$%-34!.$!2$):%$G0G0 amended at Geneva, 1980,Malaga-Torremolinos, 1984)The CCITT,considering(a) that there is a demand f
2、or data transmission at 4800 bits per second over the general switchedtelephone network;(b) that a majority of connections over the general switched telephone network within some countries arecapable of carrying data at 4800 bits per second;(c) that a lower proportion of international connections in
3、 the general switched telephone network arecapable of carrying data at 4800 bits per second;(d) that other international connections in the general switched telephone network may still supportoperations at 2400 bits per second using a built-in fallback capability;unanimously declares the viewthat tr
4、ansmission at 4800 bits per second should be allowed on the general switched telephone network.Reliable transmission cannot be guaranteed on every connection or routing and tests should be made between the mostprobable terminal points before a service is provided. The CCITT expects that developments
5、 during the next few yearsin modern technology will bring about modems of more advanced design enabling reliable transmission to be given on amuch higher proportion of connections. The provisions of this Recommendation are to be regarded as provisional inorder to provide service where it is urgently
6、 required and between locations where it is expected that a reasonablysatisfactory service can be given;that the characteristics of the modem for transmission at 4800 bits per second over the general switchedtelephone network shall provisionally be the following:1 Principal characteristicsa) Use of
7、data signalling rate of 4800 bits per second with 8-phase differentially encoded modulation asdescribed in Recommendation V.27.b) Reduced rate capability at 2400 bits per second with 4-phase differentially encoded modulation asdescribed in Recommendation V.26, Alternative A.c) Provision for a backwa
8、rd channel at modulation rates up to 75 bauds, use of this channel being optional.d) Inclusion of an automatic adaptive equalizer.2 Line signals at 4800 and 2400 bits per second operation2.1 Carrier frequencyThe carrier frequency is to be 1800 1 Hz. No separate pilot tones are provided. The power le
9、vels used willconform to Recommendation V.2.2.1.1 Spectrum at 4800 bits per secondThe 50% raised cosine energy spectrum shaping is equally divided between the receiver and transmitter. Theenergy density at 1000 Hz and 2600 Hz shall be attenuated 3.0 dB 2.0 dB with respect to the maximum energy densi
10、tybetween 1000 Hz and 2600 Hz.2 Fascicle VIII.1 - Rec. V.27 ter2.1.2 Spectrum at 2400 bits per secondA minimum of 50% raised cosine energy spectrum shaping is equally divided between the receiver andtransmitter. The energy density at 1200 Hz and 2400 Hz shall be attenuated 3.0 dB 2.0 dB with respect
11、 to themaximum energy density between 1200 Hz and 2400 Hz.2.2 Division of power between the forward and backward channelEqual division between the forward and backward channels is recommended (if provided).2.3 Operation at 4800 bits per second2.3.1 Data signalling and modulation rateThe data signall
12、ing rate shall be 4800 bits per second 0.01%, i.e. the modulation rate is 1600 bauds 0.01%.2.3.2 Encoding data bitsThe data stream to be transmitted is divided into groups of three consecutive bits (tribits). Each tribit is encodedas a phase change relative to the phase of the preceding signal eleme
13、nt (see Table 1/V.27 ter). At the receiver, the tribitsare decoded and the bits are reassembled in correct order. The left-hand digit of the tribit is the one occurring first in thedata stream as it enters the modulator portion of the modem after the scrambler.TABLE 1/V.27 terTribit values Phase cha
14、nge(see Note)00001111001111001001100104590135180225270315Note - The phase change is the actual on-line phase shift inthe transition region from the centre of one signalling elementto the centre of the following signalling element.2.4 Operation at 2400 bits per second2.4.1 Data signalling and modulat
15、ion rateThe data signalling rate shall be 2400 bits per second 0.01% i.e. the modulation rate is 1200 bauds 0.01%.2.4.2 Encoding data bitsAt 2400 bits per second the data stream is divided into groups of two bits (dibits). Each dibit is encoded as aphase change relative to the phase of the immediate
16、ly preceding signal element (see Table 2/V.27 ter). At the receiver,the dibits are decoded and the bits are reassembled in the correct order. The left-hand digit of the dibit is the oneoccurring first in the data stream as it enters the modulator portion of the modem after the scrambler.Fascicle VII
17、I.1 - Rec. V.27 ter 3TABLE 2/V.27 terDibit values Phase change (see Note)00011110090180270Note - The phase change is the actual on-line phase shift inthe transition region from the centre of one signalling elementto the centre of the following signalling element.2.5 Operating sequences2.5.1 Turn-ON
18、sequenceDuring the interval between the OFF to ON transition of circuit 105 and the OFF to ON transition of circuit106, synchronizing signals for proper conditioning of the receiving modem must be generated by the transmittingmodem. These are signals to establish carrier detect, AGC if required, tim
19、ing synchronization, equalizer convergenceand descrambler synchronization.The synchronizing signals are defined in two separate sequences with the long sequence used once at thebeginning of the established connection and the short sequence used for subsequent turn-around in which the equalizertraini
20、ng pattern is used to update and refine equalizer convergence.Two sequences are defined, i.e.:a) a short one for turn-around operation,b) a longer one for initial establishment of connection.The sequence b) is only used after the first OFF to ON transition of circuit 105 following the OFF to ONtrans
21、ition of circuit 107, or at the OFF to ON transition of circuit 107 if the circuit 105 is already ON. After everysubsequent OFF to ON transition of circuit 105, the sequence a) is used.The sequences, for both data rates, are divided into five segments as in Table 3/V.27 ter.2.5.1.1 The composition o
22、f Segment 3 is continuous 180 phase reversals on line for 14 symbol intervals in the case ofsequence a), for 50 symbol intervals in the case of sequence b).2.5.1.2 Segment 4 is composed of an equalizer conditioning pattern which is derived from a pseudo-random sequencegenerated by the polynomial:1 +
23、 x-6+ x-7For operation at both 4800 bit/s and 2400 bit/s, the equalizer conditioning pattern is derived by using everythird bit of the pseudo-random sequence defined by the polynomial. When the derived pattern contains a ZERO, 0phase change is transmitted; when it contains a ONE, 180 phase change is
24、 transmitted. Segment 4 begins with 0, 180,180, 180, 180, 180, 0, . . . according to the derived pattern and continues for 58 symbol intervals in the case ofsequence a) and for 1074 symbol intervals in the case of sequence b). An example of the detailed sequence generation isdescribed in Appendix I.
25、2.5.1.3 Segment 5 commences transmission according to the encoding described in 2.3 and 2.4 above withcontinuous data ONEs applied to the input of the data scrambler. Segment 5 is 8 symbol intervals. At the end of Segment5, circuit 106 is turned ON and user data are applied to the input of the data
26、scrambler.2.5.1.4 The phase change sequences for Segments 4 and 5 for 4800 bit/s and 2400 bit/s are shown in Table 4/V.27 ter.4 Fascicle VIII.1 - Rec. V.27 terTABLE 3/V.27 terSegment 1 Segment 2 Segment 3 Segment 4 Segment 5 Total of Segments1,2,3,4 and 5Type of line signalUnmodu-NotransmittedContin
27、uous180 phase0-1802-phaseequalizerContinuousscrambledNominal Total“Turn-ON“sequence timelated carrier energy reversals conditioningpatternONEs4800 bit/s 2400 bit/sProtection against talkerecho185 msto200 ms20 msto25 msa) 14 SIb) 50 SIa) 58 SIb) 1074 SI8 SIa) 265 msb) 923 msa) 281 msb) 1158 msWithout
28、 any protection0 ms 0 msa) 14 SIb) 50 SIa) 58 SIb) 1074 SI8 SIa) 50 msb) 708 msa) 66 msb) 943 msSI = symbol intervals. The durations of Segments 3, 4 and 5 are expressed in number of symbol intervals, these numbers being thesame in the fallback operation.TABLE 4/V.27 tera)Data speed Segment 4 Segmen
29、t 54800 bit/s Phase changePRS b)0 180 180 180 180 180 0 . . . . . . . . 180 180 0 0011 101 101 100 100 101 001 . . . . . . . . 110 100 010 001270 225 315 90 45 45 180 180100 110 101 010 000 000 111 1112400 bit/s Phase changePRS b)0 180 180 180 180 180 0 . . . . . . . . 180 180 0 0011 101 101 100 100
30、 101 001 . . . . . . . . 110 100 010 001270 90 270 270 270 270 0 010 01 10 10 10 10 00 00Duration 58 or 1074 symbol intervals (Beginning and ending PRS and symbol sequencesare the same for both lengths) 8 symbol intervals a)For a description of how the alternative sequences for Segment 4 and 5 may b
31、e generated, refer to the Note at the end of AppendixI.b) PRS is the pseudo-random sequence defined in 2.5.1.2. The underlined bits determine the phase changes.2.5.2 Turn-OFF sequenceThe line signal emitted after the ON to OFF transition of circuit 105 is divided into two segments as inTable 5/V.27
32、ter.Fascicle VIII.1 - Rec. V.27 ter 5TABLE 5/V.27 terSegment A Segment B TotalTurn-OFF timeType of line signals Remaining data followed bycontinous scrambled ONEsNo transmitted energy Total of Segments A and BWith or withoutprotection against talkerecho5 ms to 10 ms 20 ms 25 ms to 30 msIf an OFF to
33、ON transition of circuit 105 occurs during the Turn-OFF sequence, it will not be taken intoaccount until the end of the Turn-OFF sequence.In addition, if circuit 105 goes ON during the reception of the Segment A of the Turn-OFF sequence, optionallythe transmission of the Turn-ON sequence shall be st
34、arted within a time period of less than 20 ms after the end ofreception of Segment A.3 Received signal frequency toleranceNoting that the carrier frequency tolerance allowance of the transmitter is 1 Hz and assuming a maximumdrift of 6 Hz in the connection between the modems, then the receiver must
35、be able to accept errors of at least 7 Hzin the received frequencies.4 Backward channelThe modulation rate, characteristic frequencies, tolerances, etc. to be as recommended for backward channel inRecommendation V.23. This does not preclude the use of a higher speed backward channel with operational
36、 capabilityof 75 bauds or higher, bearing the same characteristic frequencies as the V.23 backward channel.5 Interchange circuits5.1 List of interchange circuitsInterchange circuits essential for the modem when used on the general switched telephone network, includingterminals equipped for manual ca
37、lling or automatic calling or answering are as in Table 6/V.27 ter.5.2 Response times of circuits 106, 109, 121 and 122 (see Tables 7/V.27 ter and 8/V.27 ter)5.2.1 Circuit 109Circuit 109 must turn ON after synchronizing is completed and prior to user data appearing on circuit 104.Circuit 109 is prev
38、ented from turning ON during reception of unmodulated carrier when the optional protection againsttalker echo is used.6 Fascicle VIII.1 - Rec. V.27 terTABLE 6/V.27 terInterchange circuit Forward (data) channelone-way system(see Note 1)Forward (data) channeleither-way system(see Note 1)No. Designatio
39、n Withoutbackward channelWithbackward channel Without WithTransmitendReceiveendTransmitendReceiveendbackwardchannelbackwardchannel102103Signal ground or commonreturnTransmitted data.XXXXXXXXX104105106Received data.Request to sendReady for sending.XXXXXXXXXX107108/1 or108/2(see Note 2)109Data set rea
40、dy Connect data set to lineData terminal readyData channel received linesignal detectorXXXXXXXXXXXXXXXX111113114115118119Data signalling rate selector(DTE source)Transmitter signal elementtiming (DTE source).Transmitter signal elementtiming (DCE source).Receiver signal element timing(DCE source)Tran
41、smitted backward channeldata.Received backward channeldata.XXXXXXXXXXXXXXXXXXXXXX120121122Transmit backward channelline signal.Backward channel readyBackward channel received linesignal detectorXXXXX125 Calling indicator. XXXXXXNote 1 - All essential interchange circuits and any others which are pro
42、vided shall comply with the functional and operationalrequirements of Recommendation V.24. All interchange circuits indicated by X shall be properly terminated in the data terminalFascicle VIII.1 - Rec. V.27 ter 7equipment and in the data circuit-terminating equipment in accordance with the appropri
43、ate Recommendation for electricalcharacteristics (see 6).Note 2 - This circuit shall be capable of operation as circuit 108/1 - connect data set to line or circuit 108/2 - data terminal readydepending on its use.TABLE 7/V.27 terResponse times for operation at 4800 bits per secondCircuit 106 With pro
44、tection againsttalker echoWithout protection againsttalker echoOFF to ON a) 215 10 ms + 50 msb) 215 10ms + 708 msa) 50 msb) 708 msON to OFF 2 ms 2 msCircuit 109OFF to ON See 5.2.1 See 5.2.1ON to OFF 5 to 15 ms 5 to 15 msCircuit 121OFF to ON 80 to 160 ms 80 to 160 msON to OFF 2 ms 2 msCircuit 122OFF
45、to ON 80 ms 80 msON to OFF 15 to 80 ms 15 to 80 ms8 Fascicle VIII.1 - Rec. V.27 terTABLE 8/V.27 terResponse times for operation of 2400 bits per secondCircuit 106 With protection againsttalker echoWithout protection againsttalker echoOFF to ON a) 215 10 ms + 67 msb) 215 10 ms + 944 msa) 67 msb) 944
46、msON to OFF 2 ms 2 msCircuit 109OFF to ON See 5.2.1 See 5.2.1ON to OFF 5 to 15 ms 5 to 15 msNote 1 - a) and b) refer to sequence a) and sequence b) as defined to 2.5.1.Note 2 - The parameter and procedures, particularly in the case of automatic calling and answering,are provisional and are the subje
47、ct of further study.5.2.2 Circuit 106Circuit 106 response times are from the connection of an ON or OFF condition on:- circuit 105 to the appearance of the corresponding ON or OFF condition on circuit 106; or,- circuit 107 (where circuit 105 is already ON) to the appearance of the corresponding ON o
48、r OFF conditionon circuit 106.5.3 Threshold of data channel and backward channel received line signal detectorsLevel of received line signal at the receive line terminals of the modem for all types of connections, i.e. thegeneral switched telephone network or non-switched 2-wire leased telephone cir
49、cuits:- greater than - 43 dBm: circuits 109/122 ON- less than -48 dBm: circuits 109/122 OFFThe condition of circuits 109 and 122 for levels between -43 dBm and -48 dBm is not specified except that thesignal detectors shall exhibit a hysteresis action, such that the level at which the OFF to ON transition occurs is at least2 dB greater than that for the ON to OFF transition.Where transmission conditions are known and allowed, it may be desirab
