1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/./6%2G0G04(%G0G04%,%0(/.%G0G0.%47/2+G0G0“)43G0G00%2G0G03%#/.$G0G0-/$%-34!.$!2$):%$G0G0 amended at Geneva, 1972, 1976 and 1980,Malaga-Torremolinos, 1984)On leased circuits, considering that
2、there exist and will come into being many modems with features designedto meet the requirements of the Administrations and users, this Recommendation in no way restricts the use of any othermodems.1 The principal characteristics for this recommended modem for transmitting data at 2400 bits per secon
3、d on4-wire leased point-to-point and multipoint circuits conforming to Recommendation M.1020 1 are as follows:a) it is capable of operating in a full-duplex mode;b) four-phase modulation with synchronous mode of operation;c) inclusion of a backward (supervisory) channel at modulation rates up to 75
4、bauds in each direction oftransmission, the use of these channels being optional.2 Line signals2.1 The carrier frequency is to be 1800 1 Hz. No separate pilot frequencies are provided. The power levels usedwill conform to Recommendation V.2.2.2 Division of power between the forward and backward chan
5、nelsIf simultaneous transmission of the forward and backward channels occurs in the same direction, a backwardchannel shall be 6 dB lower in power level than the data channel.2.3 The data stream to be transmitted is divided into pairs of consecutive bits (dibits). Each dibit is encoded as aphase cha
6、nge relative to the phase of the immediately preceding signal element. At the receiver the dibits are decodedand the bits are reassembled in correct order. Two alternative arrangements of coding are listed in Table 1/V.26. Theleft-hand digit of the dibit is the one occurring first in the data stream
7、.TABLE 1/V.26Dibit Phase change (see Note)Alternative A Alternative B000111100+ 90+ 180+ 270+ 45+ 135+ 225+ 315Note - The phase change is the actual on-line phase shift in thetransition region from the centre of one signalling clement to thecentre of the following signalling element.2 Fascicle VIII.
8、1 - Rec. V.26The meaning of phase change for alternatives A and B is illustrated by the line signal diagram in Figure 1/V.26.2.4 Synchronizing signalFor the whole duration of the interval between the OFF to ON transitions of circuits 105 and 106, the line signalshall be that corresponding to the con
9、tinuous transmission of dibit 11. This shall be known as the synchronizing signal.FIGURE 1/V.26Note - Owing to several causes, the stability of timing recovery at the receiver is liable to be data-patternsensitive. The presence of dibit 11 provides a stabilizing influence irrespective of the cause o
10、f lack of stability. Users areadvised to include sufficient binary 1s in the data which will ensure that the dibit 11 will occur frequently. In certaincases, the use of a scrambling method may also facilitate timing recovery problems. However, prior agreement isrequired between users of a circuit.3
11、Data signalling and modulation ratesThe data signalling rate shall be 2400 bits per second 0.01%, i.e. the modulation rate is 1200 bauds 0.01%.4 Received signal frequency toleranceNoting that the carrier frequency tolerance allowance at the transmitter is 1 Hz and assuming a maximumfrequency drift o
12、f 6 Hz in the connection between the modems, then the receiver must be able to accept errors of atleast 7 Hz in the received frequencies.5 Backward channelThe modulation rate, characteristic frequencies, tolerances, etc., to he as recommended for backward channel inRecommendation V.23.Fascicle VIII.
13、1 - Rec. V.26 36 Interchange circuits6.1 List of interchange circuits concerned (see Table 2/V.26)6.2 Threshold and response times of circuit 109A fall in level of the incoming line signal to -31 dBm or lower for more than 10 5 ms will cause circuit 109 tobe turned OFF. An increase in level to -26 d
14、Bm or higher will, within 10 5 ms, turn this circuit ON. The condition ofcircuit 109 for levels between -26 dBm and -31 dBm is not specified except that the signal level detector shall exhibit ahysteresis action such that the level at which the OFF to ON transition occurs is at least 2 dB greater th
15、an that for the ONto OFF transition. These values shall be measured when the synchronizing signal as defined in 2.4 above is beingtransmitted. It should be noted that the aforementioned times relate only to the defined function of circuit 109 and do notnecessarily include the time for the modem to a
16、chieve bit synchronism.Note - The signal levels specified above shall apply unless completion of Recommendation M.1020 1indicates otherwise.TABLE 2/V.26Interchange circuitForward (data) channelhalf-duplex or full-duplex(see Note)No. DesignationWithoutbackwardchannelWith backwardchannel102 Signal gro
17、und or common return. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X103 Transmitted data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X104 Received data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18、. . . . . . . . . . . X X105 Request to send. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X106 Ready for sending. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X107 Data set ready. . . . . . . . . . . . . . . .
19、 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X108/1 Connect data set to line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X X109 Data channel received line signal detector . . . . . . . . . . . . . . . . . . . . . . . . . . X X113 Transmitter signa
20、l element timing(DTE source). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .XX114 Transmitter signal element timing(DCE source). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .XX115 Receiver signal eleme
21、nt timing(DCE source) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .XX118 Transmitted backward channel data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - X119 Received backward channel data. . . . . . . . . . . . . . . . . . . . . .
22、. . . . . . . . . . . . - X120 Transmit backward channel line signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - X121 Backward channel ready. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - X122 Backward channel received line signal detector. . . .
23、. . . . . . . . . . . . . . . . . . . - X4 Fascicle VIII.1 - Rec. V.26Note - All essential interchange circuits and any others which are provided shall comply with the functional andoperational requirements of Recommendation V.24. All interchange circuits indicated by X shall be properly terminatedi
24、n the data terminal equipment and in the data circuit-terminating equipment in accordance with the appropriateRecommendation for electrical characteristics (see 8).6.3 Response times of circuits 106, 121 and 122Circuit 106OFF to ON 65-100 ms (see Note 1)(Provisional)25-45 ms (see Note 2)(Provisional
25、)ON to OFF 2 msCircuit 121OFF to ON 80 ms to 160 msON to OFF 2 msCircuit 122OFF to ON 80 msON to OFF 15 to 80 msNote 1 - These times shall be used when infrequent operation of circuit 105 is required, e.g. as in many cases ofpoint-to-point usage. Further study is required to verify the range quoted.
26、Note 2 - These times shall be used when frequent operation of circuit 105 is required, e.g. in many cases ofmultipoint usage. Further study is required with a view to reducing these times.6.4 Threshold of circuit 122- greater than -34 dBm: circuit 122 ON- less than -39 dBm: circuit 122 OFFThe condit
27、ion of circuit 122 for levels between -34 dBm and -39 dBm is not specified except that the signaldetector shall exhibit a hysteresis action such that the level at which the OFF to ON transition occurs is at least 2 dBgreater than that for the ON to OFF transition.6.5 Fault condition of interchange c
28、ircuits(See Recommendation V.28, 7 for association of the receiver failure detection types.)6.5.1 The DTE should interpret a fault condition on circuit 107 as an OFF condition using failure detection type 1.6.5.2 The DCE should interpret a fault condition on circuits 105 and 108 as an OFF condition
29、using failuredetection type 1.6.5.3 All other circuits not referred to above may use failure detection types 0 or 1.Fascicle VIII.1 - Rec. V.26 57 Timing arrangementsClocks should be included in the modem to provide the data terminal equipment with transmitter signal elementtiming, circuit 114 and r
30、eceiver 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 113.8 Electrical characteristics of interchange circuitsUse
31、 of electrical characteristics conforming to Recommendation V.28 is recommended together with theconnector pin assignment plan specified by ISO 2110 2.Note - Manufacturers may wish to note that the long-term objective is to replace electrical characteristicsspecified in Recommendation V.28, and that
32、 Study Group XVII has agreed that the work shall proceed to develop amore efficient, all-balanced, interface for the Series V application which minimizes the number of interchange circuits.9 The following information is provided to assist equipment manufacturers:The data modem should have no adjustment for send level or receive sensitivity under the control of theoperator.Reference1 CCITT Recommendation Characteristics of special quality international leased circuits, with specialbandwidth conditioning, Vol. IV, Rec. M.1020.
