1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ amended at Melbourne, 1988)The CCITT,considering(a) that digital hierarchy bit rates are those bit rates which are or will be used as the basis for higher digitalhierarchy levels if such
2、levels exist (see Recommendation G.701, definition 4003);(b) that the specification of hierarchical bit rates is necessary to prevent the proliferation of interface standardsused in digital networks;(c) that international interconnections of digital network components are preferably carried out at h
3、ierarchicalbit rates;(d) that when determining hierarchical bit rates a number of factors relating to services, transmission mediaand networking need to be taken into account, for example:- characteristics of and suitable coding method for analogue source signals;- bit rates of digital source signal
4、s;- use of available transmission media;- compatibility with analogue multiplex systems;- modularity and flexibility in assembling and routing groups of source signals,recommendsthat the following bit rates should be used as hierarchical bit rates in digital networks:Figure 1/G.702 and Figure 2/G.70
5、2 show the recommended hierarchical bit rates only, including references torelated Recommendations on network interfaces, multiplex equipments, digital sections/systems.Recommendations on the following may also exist, but are not identified in Figure 1/G.702 and Figure 2/G.702:- multiplex equipments
6、 operating between non-adjacent levels of the digital hierarchy;- network interfaces, multiplex equipments and digital sections/systems at non-hierarchical bit rates.Annex A to this Recommendation provides guidelines on usable bits available for services.Digital Hierarchy LevelHierarchical bit rates
7、 (kbit/s) for networks with the digital hierarchybased on a first level bit rate of1544 kbit/s 2048 kbit/s126415446312642 0488 4483432 06497 72844 736 34 368139 2642 Fascicle III.4 - Rec. G.702 Fascicle III.4 - Rec. G.702 3 4 Fascicle III.4 - Rec. G.702ANNEX A(to Recommendation G.702)Usable bit rate
8、s available for servicesIn the case of access to the ISDN for broadband services, bit rates up to the primary hierarchy level arespecified in the I.200 series of Recommendations.In general, with regard to bit rates available for transport of service signals, the following guidelines apply:A.1 For th
9、e hierarchies based on the 1544 kbit/s primary rate, the principle has been established that some bits inthe frame should be reserved, in particular to perform quality control of the digital paths when several digital sections intandem are involved. Table A-1/G.702 provides the necessary information
10、.TABLE A-1/G.702Bit rates available for services and frame structures required at hierarchical interfacesa)The use of the F-bit for purposes additional to framing is under study.b)In networks with asynchronous operation.c)In networks with synchronous operation.d)In some applications, the C-bits (Cj1
11、Cj2and Cj3) may also need to be reserved. In this case, the bit rate available for servicesbecomes 44 209 kbit/s.A.2 In case of networks using 2048 kbit/s based hierarchy there is in principle no basic restriction on the use of fullcapacity of the digital path. However, it is recognized that compati
12、bility with recommended frame structures at thevarious levels of the 2 Mbit/s hierarchy (e.g. the use of the same frame alignment pattern) could be a preferred solutionsince it offers the following advantages:- use of the same framing devices for switched and non-switched applications;- end-to-end q
13、uality control performed in a unique way by the network when the maintenance entity thatterminates the service (e.g. the encoding device) does not belong to the network;- possibility of performing additional network management functions that could be required, depending onthe applications.The prefer
14、ence for compatibility with recommended frame structures could be reconsidered for the applicationswhere significant restrictions on the efficient use of the digital path capability can be identified.Hierarchy bit rate(kbit/s)Frame structure as perRecommendationsReserved bitsBit rate available for services(kbit/s)1 544 G. 704 F a)1 5366 312b)Not applicable None 6 3126 312 c)G.704F bits and bits in time slots97 and 986 14444 736 G.752Mjd)F0, F11,F1244 407 d)
