1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 6 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$!4!G0G0#/-5.)#!4)/./6%2G0G04(%G0G04%,%0(/.%G0G0.%47/2+34!.$!2$G0G0,)-)43G0G0 the degree of signaldistortion must be kept within certain limits, the ultimate objective being that the degree of distortion on r
2、eceivedsignals should be compatible with the margin of the receiving equipment.This distortion on received signals arises from the composition of:a) the sending distortion;b) the distortion introduced by the transmission channel.Hence, limits must be fixed for the degree of sending distortion and fo
3、r the degree of distortion due to thetransmission channel.The limits contemplated for the transmission channel are specified in Recommendation V.53; these limits,which are not yet final, are recalled below:Channel with modem V.21: 20-25%Channels with modem V.23:600 bauds - leased circuits: 20-30%120
4、0 bauds - leased circuits: 25-35%600 bauds - switched circuit: 25-30%1200 bauds - switched circuit: 30-35%(when this mode of operation is possible)These figures are expressed provisionally in maximum degrees of individual distortion and apply to the circuitincluding the modems. The limits for the de
5、gree of sending distortion must be fixed so that a reasonable margin is leftfor the receiving equipment, making allowance for the distortion introduced by the circuit.In view of the foregoing, the CCITT unanimously issues the recommendation that:1 with regard to the quality of transmission signals (
6、signals at point A - Figure 1/V.51), it is preferable, giventhe wide range of possible modulation rates, to adopt a single standard for each type of modem.2 when a Recommendation V.21 modem is used, the duration of a unit element should be at least 90% of theduration of the unit element at 200 bauds
7、 i.e. (1/200) (90/100) second, or 4.5 milliseconds.3 when a Recommendation V.23 modem is used, the duration of a unit element should be at least 95% of theduration of the unit element either at 1200 bauds (1/1200) (95/100) second, or 0.791 millisecond or at 600 bauds(1/600) (95/100) second, or 1.583
8、 millisecond.4 if a system sends signals of which the sending distortion is systematically well below the limits specifiedabove for the category concerned, the permissible margin for receivers of that system may be reduced.5 the values indicated above could be revised when a more accurate plan for transmission quality has beendrawn up.Note - The receive margin limits will be studied in liaison with the ISO.Reference1 CCITT Definition: Telegraph distortion, Volume I, Fascicle I.3 (Terms and Definitions).
