1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0#/-5.)#!4)/./6%2G04(%G04%,%0(/.%G0.%47/2+G13“!5$G0G0-/$%-G0G034!.$!2$):%$G0G0 amended at Mar del Plata, 1968, Geneva, 1972, 1976 and 1980,Malaga-Torremolinos, 1984 and at Melbourne, 1988)Note - The mode
2、m, designed for use on connections set up by switching in the general telephone network, canobviously be used on leased lines.1 The principal characteristics recommended for a modem to transmit data at medium speed in the generalswitched telephone network are as follows:- use of modulation rates up
3、to 600/1200 bauds on the communication channel (see Recommendation V.5);- frequency modulation with synchronous or asynchronous mode of operation;- inclusion of a backward channel at modulation rates up to 75 bauds for error control, use of this channelbeing optional.2 Modulation rates and character
4、istic frequencies for the forward data-transmission channelF0FZFA(symbol 1, symbol 0,mark) space)Mode 1: up to 600 bauds 1500 Hz 1300 Hz 1700 HzMode 2: up to 1200 bauds 1700 Hz 1300 Hz 2100 HzIt is understood that the modem would be used in mode 1 when the presence of long loaded cables and/or thepr
5、esence on some connections of signalling receivers operating close to 2000 Hz would prevent satisfactory transmissionin mode 2. The modem could be used in mode 2 on suitable connections.3 Tolerances on the characteristic frequencies for the forward channelIt should be possible with all rates of modu
6、lation to permit a tolerance, at the transmitter, of 10 Hz on both theFAand FZfrequencies. This tolerance should be considered as a limit.Acceptance of these tolerances would give a tolerance of 10 Hz for the mean-frequency F0= (FA+ FZ)/2.The tolerance on the frequency difference FA- FZwith regard t
7、o the nominal value would be 20 Hz.A maximum frequency drift of 6 Hz has been assumed in the connection between the modems which mightconsist of several carrier circuits connected in tandem. This would make the tolerances on the mark and spacefrequencies at the receiving modem 16 Hz.2 Fascicle VIII.
8、1 -Rec. V.234 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 rate up to 75 bauds 390 Hz 450 HzIn the absence of any signal on the backwar
9、d channel interface, the condition Z signal is to be transmitted.5 Tolerances on the characteristic frequencies of the backward channelAs the backward channel is a VF telegraph-type channel, the frequency tolerances should be as recommendedin Recommendation R.35 1 for frequency-shift voice-frequency
10、 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.6 Division of power between the forward and backward channelsConsidering the following table whi
11、ch shows the levels of power for total power remaining equal to 1 mW:Forward channel level Backward channel level(dBm) (dBm)0 - - 1 - 7- 2 - 4- 3 - 3equal division of power between the forward and backward channels could be recommended provisionally.7 The following information is provided to assist
12、equipment manufacturers:a) The nominal range of attenuations in subscriber-to-subscriber connections is from 5 to 30 dB at thereference frequency (800 or 1000 Hz), assuming up to 35-dB attenuation at the recommended meanfrequency (F0) of the forward channel.b) A convenient range of sensitivity at th
13、e mean frequency F0for data receivers has been found to be -40 to 0 dBm for the forward channel at the subscribers terminals.c) The data modem should have no adjustment for send level or receive sensitivity under the control ofthe operator.Fascicle VIII.1 - Rec. V.23 38 Interchange circuitsThe confi
14、gurations of interchange circuits are those essential for the particular switched network or leasedcircuit requirement as indicated in Tables 1/V.23 and 2/V.23. Where one or more of such requirements are provided in amodem, then all the appropriate interchange circuits should be provided.8.1 List of
15、 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 (see Table 1/V.23).8.2 List of interchange circuits essential for the modems when used on non-switched lea
16、sed telephone circuits (seeTable 2/V.23)8.3 Response times of circuits 106 and 109, 121 and 1228.3.1 Definitions8.3.1.1 Circuits 109 and 122 response times are the times that elapse between the connection or removal of a tone toor from the modem receive line terminals and the appearance of the corre
17、sponding ON or OFF condition on circuits 109and 122.The test tone should have a frequency corresponding to the characteristic frequency of binary 1 and be derivedfrom a source with an impedance equal to the nominal input impedance of the modem.The level of the test tone should fall within the level
18、range between 3 dB above the actual threshold of thereceived line signal detector and the maximum admissible level of the received signal. At all levels within this range themeasured response times shall be within the specified limits.4 Fascicle VIII.1 -Rec. V.23TABLE 1/V.23Interchange circuitForwar
19、d (data) channelone-way system(Note 1)Forward (data) channeleither way system(Note 1)Without backward channel With backward channel Without WithNo. Designation TransmitendReceiveendTransmitendReceiveendbackwardchannelbackwardchannel102103Signal ground or commonreturnTransmitted dataXXX-XXX-XXXX10410
20、5106Received dataRequest to sendReady for sending-XX-XX-XXXXXX107108/1or108/2(Note 2)109Data set readyConnect data set to lineData terminal readyData channel received linesignal detectorXX-XXXXX-XXXXXXXXX111114(Note 3)115(Note 3)Data signalling rate selector(DTE)Transmitter signal elementtiming (DCE
21、)Receiver signal element timing(DCE)XX-X-XXX-X-XXXXXXX118119120Transmitted backward channeldataReceiver backward channeldataTransmit backward channelline signal-X-X-XXX121(Note 4)122(Note 4)125Backward channel readyBackward channel received linesignal detectorCalling indicator-X-X-XXX-X-XXXXNote 1 -
22、 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 properly terminated in the data terminalequipment and in the data circuit-terminating equipment
23、in accordance with the appropriate Recommendation for electricalcharacteristics (see 9).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.Note 3 - These circuits are required when the optional cl
24、ock is implemented in the modem.Note 4 - These circuits are not required if the modem is operating in an asymmetrical duplex mode.Fascicle VIII.1 - Rec. V.23 5TABLE 2/V.23Interchange circuitForward (data) channelone-way system(Note 1)Forward datachannel eitherway or both wayssimultaneously system(No
25、te 1)Without backward channel With backward channel Without WithNo. Designation TransmitendReceiveendTransmitendReceiveendbackwardchannelbackwardchannel102103Signal ground or commonreturnTransmitted dataXXX-XXX-XXXX104105106Received dataRequest to sendReady for sending-XXX-XXX-XXXXXX107108/1109Data
26、set readyConnect data set to lineData channel received linesignal detectorXX-XXXXX-XXXXXXXXX111114(Note 2)115(Note 2)Data signalling rate selector(DTE)Transmitter signal elementtiming (DCE)Receiver signal element timing(DCE)XX-X-XXX-X-XXXXXXX118119120Transmitted backward channeldataReceived backward
27、 channeldataTransmit backward channelline signal-X-X-X-XXX121122Backward channel readyBackward channel received linesignal detector-XX-XXNote 1 - All essential interchange circuits and any others which are provided shall comply with the functional and operationalrequirements of Recommendation V.24.
28、All interchange circuits indicated by X shall be properly terminated in the data terminalequipment and in the data circuit terminating equipment in accordance with the appropriate Recommendation for electricalcharacteristics (see 9).Note 2 - These circuits are required when the optional clock is imp
29、lemented in the modem.8.3.1.2 Circuit 106 response times are from the connection of an ON or OFF condition on:- circuit 105 (where it is provided) to the appearance of the corresponding ON or OFF condition on circuit106;- circuit 122 (where circuit 105 is not provided) to the appearance of the corre
30、sponding ON or OFFcondition on circuit 106 in a configuration having a single data channel together with a single backwardchannel only;- circuit 107 (where circuits 105 and 122 are not provided) to the appearance of the corresponding ON orOFF condition on circuit 106;8.3.1.3 Circuit 121 response tim
31、es are from the connection of an ON or OFF condition on:6 Fascicle VIII.1 -Rec. V.23- circuit 120 (where it is provided) to the appearance of the corresponding ON or OFF condition oncircuit 121;- circuit 109 (where circuit 120 is not provided) to the appearance of the corresponding ON or OFFconditio
32、n on circuit 121.8.3.2 Response timesTABLE 3/V.23Circuit 106OFF to ONON to OFF750 ms to 1400 ms (see Note 1) a) 20 ms to 40 ms(see Note 2)b) 200 ms to 275 ms(see Note 2) 2 msCircuit 109OFF to ONON to OFF300 ms to 700 ms (see Note 1) 10 ms to 20 ms (see Note 1)5 ms to 15 msCircuit 121OFF to ONON to O
33、FF80 ms to 160 ms 2 msCircuit 122OFF to ONON to OFF 80 ms15 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) no protecti
34、on given against line echoes;b) protection given against line echoes.Note 3 - The above parameters are provisional and are the subject of further study.8.4 Threshold of data channel and backward channel received line signal detectorsLevel of received line signal at receive line terminals of modern f
35、or all types of connections, i.e. generalswitched telephone network or non-switched leased telephone circuits:greater than - 43 dBm circuits 109/122 ONless 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 thes
36、ignal 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 on switched or leased circuits, Administrations should be permittedat the time of mode
37、m installation to change these response levels of the received line signal detectors to less sensitivevalues (e.g. -33 dBm and -38 dBm respectively).Fascicle VIII.1 - Rec. V.23 78.5 Clamping in half-duplex modeThe DCE, when operating in half-duplex mode on a 2-wire line, shall hold, where implemente
38、d:i) circuit 104 in the binary 1 condition and circuit 109 in the OFF condition when circuit 105 is in the ONcondition and, where required to protect circuit 104 from false signals, for a period of 150 25milliseconds following the ON to OFF transition on circuit 105. The use of this additional delay
39、 isoptional, based on system considerations;ii) circuit 119 in the binary 1 condition and circuit 122 in the OFF condition when circuit 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 s
40、pecific duration of this time interval is left for further study.The additional delay is optional, based on system considerations.8.6 Fault condition of interchange circuits(See Recommendation V.28, 7 for association of the receiver failure detection types).8.6.1 The DTE should interpret a fault con
41、dition on circuit 107 as an OFF condition using failure detection type 1.8.6.2 The DCE should interpret a fault condition on circuits 105 and 108 as an OFF condition using failure detectiontype 1.8.6.3 All other circuits not referred to above may use failure detection types 0 or 1.9 Electrical chara
42、cteristics of interchange circuitsUse of electrical characteristics conforming to Recommendation V.28 is recommended together with theconnector pin assignment plan specified by ISO 2110.Note - Manufacturers may wish to note that the long-term objective is to replace electrical characteristicsspecifi
43、ed in Recommendation V.28, and 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.10 Equipment for the disablement of echo suppressorsWhen echo control device
44、 disabling is required, it is recommended that the procedures specified inRecommendation V.25 be followed.11 Inclusion of a clock in the modemA clock is not an essential item in the standardized modem. However, the modem may conveniently include aclock when used primarily for synchronous transmissio
45、n.8 Fascicle VIII.1 -Rec. V.23If such a clock is included in the modem, a synchronizing pattern consisting of alternate binary 0 and binary 1 atclock rate should be transmitted for the whole interval between the OFF to ON transitions of interchange circuits 105and 106. Users should note that part of
46、 this synchronizing pattern may appear at the distant receiver on circuit 104 afterthe OFF to ON transition of circuit 109. The data terminal equipment should make provision to differentiate betweenthese false signals and true data.Reference1 CCITT Recommendation Standardization of FMVFT systems for a modulation rate of 50 bauds, Vol. VII,Rec. R.35.
