1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/./6%2G0G04(%G0G04%,%0(/.%G0G0.%47/2+G0G0“)43G0G00%2G0G03%#/.$G0G0$50,%8G0G0-/$%-34!.$!2$):%$G0G0 amended at Mar del Plata, 1968,and at Geneva, 1972, 1976 and 1980,and at Malaga-Torremolinos
2、, 1984)Note - The modem, designed for use on connections set up by switching in the general telephone network, canobviously be used on leased lines.A system of data transmission at a low data signalling rate, such that data could be transmitted over a telephonecircuit operated alternatively for tele
3、phone calls and data transmissions, using simple input/output equipment and easyoperating procedures, would be economical.The data signalling rate must be such as to allow the use of current types of data sources and sinks, especiallyelectromechanical devices.The system for data transmission will be
4、 duplex, either for simultaneous two-way data transmission or for thetransmission of signals sent in the backward direction for error-control purposes. The transmission must be such that usecan be made of normal telephone circuits, and this applies both to the bandwidth available and to the restrict
5、ionsimposed by signalling in the telephone networks.The two correspondents are brought into contact by a telephone call, and the circuit is put into the data-transmission position:a) manually by agreement between the operators, orb) automatically.For these reasons, the CCITTunanimously declares the
6、following view1 Data transmission may take place at low data signalling rates on telephone calls set up on switched telephonecircuits (or on leased telephone circuits).2 The communication circuit for data transmission is a duplex circuit whereby data transmission in bothdirections simultaneously is
7、possible at 300 bit/s or less.The modulation is a binary modulation obtained by frequency shift, resulting in a modulation rate being equalto the data signalling rate.Note - Attention is drawn to the fact that there may be in operation some old-type V.21 modems for which themaximum data signalling r
8、ate is 200 bit/s.3 For channel No. 1, the nominal mean frequency is 1080 Hz.For channel No. 2, it is 1750 Hz.The frequency deviation is 100 Hz. In each channel, the higher characteristic frequency (FA) corresponds to abinary 0._1)See Note under 2 of this Recommendation.2 Fascicle VIII.1 - Rec. V.21T
9、he characteristic frequencies2)as measured at the modulator output must not differ by more than 6 Hz fromthe nominal figures.A maximum drift frequency of 6 Hz is assumed for the line. Hence the demodulation equipment must toleratedrifts of 12 Hz between the frequencies received and their nominal val
10、ues.4 Data may be transmitted by synchronous or asynchronous procedures. With synchronous operation, themodem will not have to provide the signals which would be necessary to maintain synchronism when transmission is notproceeding.5 When echo control device disabling is required, it is recommended t
11、hat the procedures specified inRecommendation V.25 be followed.6 The maximum power output of the modem into the line shall not exceed 1 mW.The power level of the modem should be adjusted to make allowance for loss between this equipment and thepoint of entry to an international circuit, so that the
12、corresponding nominal level of the signal at the international circuitinput shall not exceed - 13 dBm0 (see Recommendation V.2, 2).7 a) When both channels are used for simultaneous both-way data transmission, channel No. 1 is used fortransmission of the callers data (i.e. the person making the telep
13、hone call) towards the called station, while channel No.2 is used for transmission in the other direction.b) When one channel is used for data transmission and the other is used for transmission of check signals,service signals, etc., only, it is channel No. 1 which is used for transmission from the
14、 calling to the called stationregardless of the direction in which the data are transmitted.c) The procedure for the assignment of the channels described under a) and b) above applies in the case ofthe general service of data transmission, making it possible to transmit data or check signal, service
15、 signal, etc.,bilaterally between any two subscribers. In special cases which do not come under this rule, the procedure of assignmentof the channels is determined by the prior agreement between the correspondents, bearing in mind the requirementproper to each service.8 Interchange circuits8.1 List
16、of interchange circuits essential for the modems when used on the general switched telephone network or non switched leased telephone circuits (see Table 1 /V.21)The configurations of interchange circuits are those essential for the particular switched network or leasedcircuit requirement indicated.
17、 Where one or more of such requirements are provided in a modern, then all of theappropriate interchange circuit facilities should be provided._2)The nominal characteristic frequencies;channel No. 1 (FA= 1180 Hz and Fz= 980 Hz);channel No. 2 (FA= 1850 Hz and Fz= 1650 Hz).Fascicle VIII.1 - Rec. V.21
18、3TABLE 1/V.21Note 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 properly terminated in the data terminalequipment and in the data circuit-t
19、erminating equipment in accordance with the appropriate Recommendation for electricalcharacteristics (see 9).Note 2 - Circuit 105 is not required when alternate voice/data service is used on non-switched leased point-to-point circuits.Note 3 - The circuit shall be capable of operation as circuit 108
20、/1 - connect data set to line or circuit 108/2 - data terminalready depending on its use.Note 4 - In the leased point-to-point case, where alternate voice/data service is to be provided, circuit 108/2 may be usedoptionally.8.2 Response times of circuits 106 and 1098.2.1 Definitions8.2.1.1 Circuit 10
21、9 response times are the times that elapse between the connection or removal of a tone to or from themodem receive line terminals and the appearance of the corresponding ON or OFF condition on circuit 109.The test tone should have a frequency corresponding to the characteristic frequency of binary 1
22、 and be derivedfrom a source with an impedance equal to the nominal input impedance of the modem under test.The level of the test tone should fall into the level range between 1 dB above the actual threshold, of thereceived line signal detector and the maximum admissible level of the received signal
23、. At all levels within this range themeasured response times shall be within the specified limits.8.2.1.2 Circuit 106 response times are the times from the connection of an ON or OFF condition on:- circuit 105 (where it is provided) to the appearance of the corresponding OFF or ON condition on circu
24、it106;- circuit 109 (where circuit 105 is not provided) to the appearance of the corresponding ON or OFFcondition on circuit 106.Interchange circuitGeneral switched telephonenetwork including terminalsequipped for manual calling,Non-switched leasedtelephone circuits(Note 1)Number Designationmanual a
25、nswering, automaticcalling, automatic answering(Note 1)Point-to-point Multipoint102103Signal ground or common returnTransmitted dataXXXXXX104105106Received dataRequest to sendReady for sendingX-XXX (Note 2)XXXX107108/1108/2109Data set readyConnect data set to lineData terminal readyData channel rece
26、ived line signal detectorXX (Note 3)X (Note 3)XXXX (Note 4)XXX-X125126Calling indicatorSelect transmit frequencyX-X4 Fascicle VIII.1 - Rec. V.218.2.2 Response timesTABLE 2/V.21Note 1 - These times are used on leased point-to-point networks without alternate voice data facilities and on leased multip
27、ointfacilities.Note 2 - These times are used in the general switched network service and on leased point-to-point circuits with alternate voicedata.8.3 Threshold of data channel received line signal detectorLevel of received line signal at received line signal terminals of modem for all types of con
28、nection, i.e. generalswitched telephone network or non-switched leased telephone circuit:greater than -43 dBm circuit 109 ONless than -48 dBm circuit 109 OFFThe condition of circuit 109 for levels between -43 dBm and -48 dBm is not specified except that the signaldetector shall exhibit a hysteresis
29、action such that the level at which the OFF to ON transition occurs shall be at least2 dB greater than for the ON to OFF transition.Where transmission conditions are known on switched or leased circuits, Administrations should be permittedat the time of modem installation to change these response le
30、vels of the received line signal detector to less sensitivevalues (e.g. -33 dBm and -38 dBm respectively).8.4 Fault condition of interchange circuitsSee Recommendation V.28, 7 for association of the receiver failure detection types).8.4.1 The DTE should interpret a fault condition on circuit 107 as
31、an OFF condition using failure detection type 1.8.4.2 The DCE should interpret a fault condition on circuits 105 and 108 as an OFF condition using failure detectiontype 1.8.4.3 All other circuits not referred to above may use failure detection type 0 or 1.9 Electrical characteristics of interchange
32、circuitsUse of electrical characteristics conforming to Recommendation V.28 is recommended together with theconnector and pin assignment plan specified by ISO 2110.Circuit 106OFF to ONON to OFF20-50 ms (see Note 1) 400-1000 ms (see Note 2) 2msCircuit 109OFF to ONON to OFF 20 ms (see Note 1) 300-700
33、ms (see Note 2)20-80 msFascicle VIII.1 - Rec. V.21 5Note - 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 amore efficient, all-balanced,
34、interface for the V-Series application which minimises the number of interchange circuits.10 The following information is provided to assist equipment manufacturers:a) The nominal range of attenuations in subscriber-to-subscriber connections is from 5 to 30 dB at the reference frequency (800 or 1000 Hz), assuming up to 35 dB attenuation at the frequency 1750 Hz.b) The data modem should have no adjustment for send level or receive sensitivity under the control of the operator.Reference1 CCITT Recommendation Echo suppressors, Vol. III, Rec. G.164.
