ITU-T R 121-1988 STANDARD LIMITS OF TRANSMISSION QUALITY FOR START-STOP USER CLASSES OF SERVICE 1 AND 2 ON ANISOCHRONOUS DATA NETWORKS《在不等时数据网上起止式用户第1和第2类业务的传输质量的标准限值》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.34!.$!2$G0G0,)-)43G0G0/(b) that, on the other hand, to enable national switched networks to be interconnected, it is necessary to have adistribution plan of the telegraph dist

2、ortion between national networks and the international junction circuitsconnecting the international gateway switching centres;(c) that it is difficult to lay down standards applicable both to small and to large national networks;(d) that it should be possible to fix limit values for large countries

3、 and they should apply to the great majorityof user locations taking part in the international service,unanimously declares the view1 The following standards of transmission quality should be observed for the interconnection of nationalanisochronous data networks set up by means of transmission chan

4、nels and start-stop terminal equipment in accordancewith CCITT Recommendations to provide service for user classes of service 1 and 2 to Recommendation X.1 1 (up toand including 300 bit/s).1.1 The degree of gross start-stop distortion in service (i.e. including the effect of distortion due to the se

5、ndingterminal equipment and the switching centres) at the point of exit of the national network should provisionally notexceed 22%.Note The international gateway exchange of a country is considered as forming part of the national networkof that country.1.2 The degree of inherent start-stop distortio

6、n of the international junction circuit should provisionally not exceed13%.Note 1 In establishing the provisional 13% limit for the degree of start-stop distortion in the internationaljunction circuit account has been taken of the fact that in a global connection, the international junction circuit

7、mightconsist of 2 channels in tandem. If the international junction circuit is established on a single channel, an 8%provisional limit would be applicable to that circuit.Note 2 No limit for distortion on the entry to an international gateway centre at the receiving end has beenindicated in this Rec

8、ommendation. The values mentioned in 1.1 and 1.2 above are adequate for planning purposes.2 The provisional limit values mentioned above are applicable to large countries that are directly interconnectedwithout switching in a transit country. Where national networks are unable to satisfy 1.1 above,

9、signal regenerationwill be required.3 Small countries (defined as countries in which all user terminal equipment can be reached via not more thanone carrier channel in the national network) will have to try to obtain values less than the maximum 22% distortionmentioned in 1.1 above.2 Fascicle VII.1 - Rec. R.1214 The provisional standard limits mentioned under 1 above can also apply to private switched telegraph andanisochronous data networks.Reference1 CCITT Recommendation International user classes of service in public data networks, Rec. X.1.