ITU-T R 57-1988 STANDARD LIMITS OF TRANSMISSION QUALITY FOR PLANNING CODE - INDEPENDENT INTERNATIONAL POINT-TO-POINT TELEGRAPH COMMUNICATIONS AND SWITCHED NETWORKS USING 50-BAUD ST.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$G0G0,)-)43G0G0/ amended at Arnhem, 1953,New Delhi, 1960, and Melbourne, 1988; see also Recommendation R.58)The CCITT,considering(a) that Administrations must agree on

2、the composition of the international section and the national sectionsbefore setting up an international point-to-point telegraph circuit;(b) that for the interconnection of switched public or private national networks a plan for distributing telegraphdistortion between national networks and interna

3、tional circuits connecting the international terminal exchanges isrequired;(c) that for this purpose, provisional standards, based on the results of practical experience and on studies ofthe composition of telegraph distortion, should be laid down for Administrations;(d) that on well-maintained chan

4、nels, with modulation at the standard rate of 50 bauds, the values inTable 1/R.57 should not normally be exceeded on the trunk sections (see Recommendations R.53 and R.75). Thesevalues are valid whether the channels are amplitude or frequency modulated,TABLE 1/R.57Number of channels in tandemwithin

5、the trunk circuit(excluding the localsection at each end)The limit of bias distortion onreversals at the modulation rateemployed for adjustment shallbe equivalent to the followingvalues at 50 baudsLimit of the degreeof isochronousdistorsion onstandardized textLimit of the degreeof inherent start-sto

6、pdistorsion, in serviceon standardized text123454%7%10%12%10%18%24%28%8%13%17%21%25%unanimously declares the following view1 In planning international point-to-point and switched telegraph communications, Administrations should usethe following standard limits valid for start-stop equipment and for

7、50-baud channels conforming to CCITTRecommendations and set up by amplitude-modulation or frequency-modulation.Note Although the figures in Recommendation R.57 are for planning purposes, they do not correspond toconventional degrees of distortion but to routine measurements.2 Fascicle VII.1 - Rec. R

8、.57a) Limit of the degree of gross start-stop distortion, measured by a start-stop distortion measuring setat the beginning of the trunk section of the circuit (i.e. at the point where the circuit enters the long-distance line telegraph equipment) and including the effect of the emission distortion

9、of thetransmitting apparatus 12%b) Limit of the degree of isochronous distortion on standardized text in the trunk section of the connection:When one voice-frequency telegraph (VFT) channel is used for the communication 10%When two VFT channels are used for the communication 18%When three VFT channe

10、ls are used for the communication 24%When four VFT channels are used for the communication 28%orc) Limit of degree of inherent start-stop distortion on standardized text of the trunk section of the connection:When one voice-frequency (VF) channel is used for the communication 8%When two VF channels

11、are used for the communication 13%When three VF channels are used for the communication 17%When four VF channels are used for the communication 21%When five VF channels are used for the communication 25%Note The limits for the degrees of isochronous and start-stop distortions indicated under b) and

12、c) above donot establish a law of correspondence between the degree of isochronous distortion and the degree of start-stopdistortion; this law of correspondence depends on the composition of the distortion (relative magnitudes ofcharacteristic and fortuitous distortion).d) Limit of the degree of the

13、 gross start-stop distortion, measured by a start-stop distortion measuring set,which can be present in signals at the input of the extension circuit of the connection 30%Note The (physical) extension circuit (tail) is the permanent connection extending a telegraph station tonearby centre, giving ac

14、cess to the long-distance network.2 These standards take no account of the possibility of including regenerative repeaters in circuits.3 These standards presuppose that the distortion introduced by the local section of the circuit is negligible, andthat, should that not be so, Administrations should

15、 agree amongst themselves on the degree of distortion admissible inthe various sections of the communication, and on the number of VFT channels that can be used.4 Administrations should use them, in order to agree on the maximum number of VFT channels that maycompose the international section of a circuit and in order to determine the characteristics of their national networksdue to be connected to the networks of other countries, on the understanding that the isochronous distortion in service,originated by the trunk section, may not in any circumstances exceed 28%.