1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 BISTELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/./6%2G0G04(%G0G04%,%0(/.%G0G0.%47/2+G0G0“)43G0G00%2G0G03%#/.$G0G0-/$%-34!.$!2$):%$G0G0 amended at Geneva, 1976 and 1980,Malaga-Torremolinos, 1984)The CCITT,considering(a) that there is a
2、 demand for data transmission at 2400 bit/s over the general switched telephone network;(b) that a majority of connections over the general switched telephone network within some countries arecapable of carrying data at 2400 bit/s;(c) that a much lower proportion of international connections in the
3、general switched telephone service arecapable of carrying data at 2400 bit/s;unanimously declares the view(1) that transmission at 2400 bit/s should be allowed on the general switched telephone network. Reliabletransmission cannot be guaranteed on every connection or routing and tests should be made
4、 between the most probableterminal points before a service is provided.The CCITT expects that developments during the next few years in modern technology will bring aboutmodems of more advanced design enabling reliable transmission to be given on a much higher proportion ofconnections.Note - The pro
5、visions of this Recommendation are to be regarded as provisional in order to provide servicewhere it is urgently required and between locations where it is expected that a reasonably satisfactory service can begiven. The study of improved methods of transmission at 2400 bits/s or above over the gene
6、ral switched telephonenetwork will be urgently continued with the aim of recommending a method of transmission which will enable a morereliable service to be given over a high proportion of the connections encountered in normal service.(2) that the characteristics of the modems for this service shal
7、l provisionally be the following:1 Principal characteristicsa) Use of a data signalling rate of 2400 bit/s with carrier frequency, modulation and coding, according toRecommendation V.26, Alternative B (see Note below) on the communication channel. Administrationsand users should note that the perfor
8、mance of this modem on international connections may not always besuitable for this service without prior testing and conditioning if required.b) Reduced rate capability at 1200 bit/s.c) Inclusion of a backward channel at modulation rates up to 75 bauds, use of this channel being optional.Note - Att
9、ention is drawn to the fact that there are some old-type modems currently in operation for which thecoding method in accordance with Recommendation V.26, Alternative A, is used.2 Fascicle VIII.1 - Rec. V.26 bis2 Line signals at 2400 and 1200 bit/s2.1 The carrier frequency is to be 1800 1 Hz. No sepa
10、rate pilot frequencies are provided. The power levels usedwill conform to Recommendation V.2.2.2 Phase distortion limitsThe transmitted line signal spectrum should have linear phase characteristics (to be obtained by means offilters or equalizers or digital means). The deviation of the phase distort
11、ion characteristic should not exceed the limitsspecified in Figure 1/V.26 bis.FIGURE 1/V.26 bisTolerance limit for phase distortion of the signal transmitted to the line2.3 Division of power between forward and backward channelsEqual division of power between the forward and backward channels is rec
12、ommended provisionally.2.4 Operation at 2400 bit/s2.4.1 The data stream to be transmitted is divided into pairs of consecutive bits (dibits). Each dibit is encoded as aphase change relative to the phase of the immediately preceding signal element (see Table 1/V.26 bis). At the receiverthe dibits are
13、 decoded and the bits are reassembled in correct order. The left-hand digit of the dibit is the one occurringfirst in the data stream.Fascicle VIII.1 - Rec. V.26 bis 3The meaning of phase change is illustrated by the line signal diagram given in Figure 2/V.26 bis.TABLE 1/V.26 bisDibit Phase change (
14、see Note)00011110+ 45+ 135+ 225+ 315Note - The phase change is the actual on-line phase shift in thetransition region from the centre of one signalling element to thecentre of the following signalling element.FIGURE 2/V.26 bis2.4.2 Synchronizing signalFor the whole duration of the interval between t
15、he OFF to ON transitions of circuits 105 or 107 and 106, theline signal shall be that corresponding to the continuous transmission of dibit 11. This shall be known as thesynchronizing signal (see 5.2.2 below).Note - Owing to several causes the stability of timing recovery at the receiver is liable t
16、o be data-patternsensitive. The presence of dibit I I provides a stabilizing influence irrespective of the cause of lack of stability. Users areadvised to include sufficient binary ls in the data which will ensure that the dibit 11 will occur frequently.2.4.3 Data signalling and modulation ratesThe
17、data signalling rate shall be 2400 bit/s 0.01%, i.e. the modulation rate is 1200 bauds 0.01%.2.5 Operation at 1200 bit/s2.5.1 Coding and modulation used are 2-phase differential modulation with binary 0 for +90 and binary 1 for+ 270.2.5.2 The data signalling rate shall be 1200 bit/s 0.01%, the modul
18、ation rate remains at 1200 bauds 0.01%.4 Fascicle VIII.1 - Rec. V.26 bis3 Received signal frequency toleranceNoting that the carrier frequency tolerance allowance at the transmitter is 1 Hz and assuming a maximumfrequency drift of 6 Hz in the connection between the modems, then the receiver must be
19、able to accept errors of atleast 7 Hz in the received frequencies.4 Backward channel4.1 Modulation rate and characteristic frequencies for the backward channelThe modulation rate and characteristic frequencies for the backward channel are as follows:FZFA(symbol 1, (symbol 0,mark) space)Modulation ra
20、te up to 75 bauds 390 Hz 450 HzIn the absence of any signal on the backward channel interface, the condition Z signal is to be transmitted.4.2 Tolerances on the characteristic frequencies of the backward channelAs the backward channel is a VF telegraph-type channel, the frequency tolerances should b
21、e as recommendedin Recommendation R.35 1 for frequency-shift voice-frequency telegraphy.The 6 Hz frequency drift in the connection between the modems postulated in 3 above would produceadditional distortion in the backward channel. This should be taken into account in the design.5 Interchange circui
22、ts5.1 List of essential interchange circuitsThe list of interchange circuits essential for the modems when used on the general switched telephone network,including terminals equipped for manual calling or answering or automatic calling or answering is given in Table2/V.26 bis.5.2 Response times of c
23、ircuits 106, 109, 121 and 122 (see Table 3/V.26 bis)5.2.1 Circuit 109 response times are the times that elapse between the connection or removal of the testsynchronizing signal to or from the modem receive line terminals and the appearance of the corresponding ON and OFFcondition on circuit 109.The
24、level of the test synchronizing signal should fall within the level range between 3 dB above the actual OFFto ON threshold of the received line signal detector and the maximum admissible level of the received signal. At alllevels within this range, the measured response times shall be within the spe
25、cified limits.Fascicle VIII.1 - Rec. V.26 bis 55.2.2 Circuit 106 response times are from the connection to 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 not required to initiate the synchronizi
26、ng signal) to the appearance of thecorresponding ON or OFF condition on circuit 106.TABLE 2/V.26 bisInterchange circuitForwad (data) channelone-way system(see Note 1)Forwad (data) channeleither-way system(see Note 1)Withoutbackward channelWithbackward channelWithout WithNo. Designation TransmitendRe
27、ceiveendTransmitendReceiveendbackwardchannelbackwardchannel102103Signal ground or commonreturn. . . . . . . . . . . . . . . . . . . . .Transmitted data. . . . . . . . . . . .XXXXXXXXXX104105106Received data. . . . . . . . . . . . . . .Request to send. . . . . . . . . . . . .Ready for sending. . . .
28、. . . . . . .XXXXXXXXXXXX107108/1 or108/2(see Note 2)109Data set ready. . . . . . . . . . . . . .Connect data set to line. . . . . . .Data terminal ready. . . . . . . . . .Data channel received linesignal detector. . . . . . . . . . . . . .XXXXXXXXXXXXXXXX111113114115118119Data signalling rate selec
29、tor(DTE source). . . . . . . . . . . . . . .Transmitter signal elementtiming (DTE source). . . . . . . . .Transmitter signal elementtiming (DCE source). . . . . . . . .Receiver signal element timing(DCE source). . . . . . . . . . . . . .Transmitted backward channeldata. . . . . . . . . . . . . . . .
30、 . . . . . .Received backward channeldata. . . . . . . . . . . . . . . . . . . . . .XXXXXXXXXXXXXXXXXXXXXX120121122Transmitted backward channelline signal. . . . . . . . . . . . . . . . . .Backward channel ready. . . . . .Backward channel received linesignal detector. . . . . . . . . . . . . .XXXXX1
31、25 Calling indicator. . . . . . . . . . . . XXXXXX6 Fascicle VIII.1 - Rec. V.26 bisNote 1 - All essential interchange circuits and any others which are provided shall comply with the functional and operationalrequirements of Recommendation V.24. All interchange circuits indicated by X shall be prope
32、rly terminated in the data terminalequipment and in the data circuit-terminating equipment in accordance with the appropriate Recommendation for electricalcharacteristics (see 7).Note 2 - This circuit shall be capable of operation as circuit 108/1 or circuit 108/2 depending on its use.TABLE 3/V.26 b
33、isResponse timesCircuit 106OFF to ON 750 ms to 1400 ms (see Note 1) a) 65 ms to 100 ms (see Note 2)b) 200 ms to 275 ms (see Note 2)ON to OFF 2 msCircuit 109OFF to ON 300 ms to 700 ms (see Note 1) 5 ms to 15 ms (see Note 1)ON to OFF 5 ms to 15 msCircuit 121OFF to ON 80 ms to 160 msON to OFF 2 msCircu
34、it 122OFF to ON 80 msON to OFF 15 ms to 80 msNote 1 - For automatic calling and answering, the longer response times of circuits 106 and 109 are to be used during callestablishment only.Note 2 - The choice of response times depends upon the system application: a) limited protection given against lin
35、e echoes;b) protection given against line echoes.Note 3 - The above parameters and procedures, particularly in the case of automatic calling and answering are provisional and arethe subject of further study. Especially the shorter response times for circuit 109 may need revision to prevent remnants
36、of thesynchronizing signal from appearing on circuit 104.Fascicle VIII.1 - Rec. V.26 bis 75.3 Threshold of data channel and backward channel received line signal detectorsLevel of received line signal at receive line terminals of modem for all types of connections, i.e. general switchedtelephone net
37、work or non-switched leased telephone circuits:- 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
38、 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 desirable at the time of modem installation tochange these response levels of the received line signal detector to less se
39、nsitive values (e.g. -33 dBm and -38 dBmrespectively).5.4 Clamping in half-duplex modeThe DCE, when operating in half-duplex mode on a 2-wire line, shall hold, where implemented:a) circuit 104 in the binary 1 condition and circuit 109 in the OFF condition when circuit 105 is in the ONcondition and,
40、where required to protect circuit 104 from false signals, for a period of 150 25 msfollowing the ON to OFF transition on circuit 105; the use of this additional delay is optional, based onsystem considerations;b) circuit 119 in the binary 1 condition and circuit 122 in the OFF condition when circuit
41、 120 is in the ONcondition and, where required to protect circuit 119 from false signals, for a time interval following theON to OFF transition on circuit 120. The specific duration of this time interval is left for further study.The additional delay is optional, based on system considerations.5.5 F
42、ault condition of interchange circuits(See Recommendation V.28, 7 for association of the receiver failure detection types.)5.5.1 The DTE should interpret a fault condition on circuit 107 as an OFF condition using failure detection type 1.5.5.2 The DCE should interpret a fault condition on circuits 1
43、05 and 108 as an OFF condition using failure detectiontype 1.5.5.3 All other circuits not referred to above may use failure detection type 0 or 1.6 Timing arrangementsClocks should be included in the modem to provide the data terminal equipment with transmitter signal elementtiming, circuit 114 and
44、receiver signal element timing, circuit 115. Alternatively, the transmitter signal element timingmay be originated in the data terminal equipment instead of in the data circuit-terminating equipment and be transferredto the modem via circuit 113.7 Electrical characteristics of interchange circuitsUs
45、e of electrical characteristics conforming to Recommendation V.28 is recommended together with theconnector and pin assignment plan specified by ISO 21 1 0.Note - Manufacturers may wish to note that the long-term objective is to replace electrical characteristicsspecified in Recommendation V.28, and
46、 that Study Group XVII has agreed that the work shall proceed to develop amore efficient, all balanced, interface, for the V-Series application which minimizes the number of interchange circuits.8 Fascicle VIII.1 - Rec. V.26 bis8 The following information is provided to assist equipment manufacturer
47、s:The data modem should have no adjustment for send level or receive sensitivity under the control of theoperator.9 When echo control device disabling is required, it is recommended that the procedures specified inRecommendation V.25 be followed.10 Fixed compromise equalizerA fixed compromise equalizer shall be incorporated into the receiver. The characteristics of this equalizer maybe selected by Administrations but this should be the matter for further study.Reference1 CCITT Recommendation Standardization of FMVFT systems for a modulation rate of 50 bauds, Vol. VII,Rec. R.35.
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