1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU$)4!,G0G0.%47/2+3$)4!,G0G042!.3-)33)/.G0G0-/$%,3)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation G.801 was published in Fascicle III.5 of the Blue Book. This file is an ext
2、ract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration”
3、 is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, with
4、out permission in writing from the ITU.Fascicle III.5 - Rec. G.801 1Recommendation G.801Fascicle III.5 - Rec. G.801DIGITAL TRANSMISSION MODELS(Malaga-Torremolinos, 1984)The CCITTconsidering(a) that digital networks support a wide variety of connections for which digital transmission impairments ando
5、ther performance parameters need to be controlled;(b) that, if proper control is not exercised, then under certain circumstances, digital transmission impairmentscause unacceptable service degradations;(c) that various network performance objectives need to be allocated to the elements of a digital
6、network;(d) that equipment design objectives need to be formulated for individual digital elements;(e) that networks need to be configured to a level of transmission quality consistent with the needs of differentservices (voice and non-voice) and in particular of services in the ISDN;(f) that Admini
7、strations need to examine the effect on transmission quality of possible changes of impairmentallocation in national networks;(g) that there is a need to test national rules for prima facie compliance with any impairment criteria whichmay be recommended by the CCITT for national and international sy
8、stems,(h) that guidelines need to be formulated governing the use of certain digital elements (e.g. satellite links,transcoders, digital pads, circuit multiplication devices, etc.),recommendsthat in the study of digital transmission impairments and other performance parameters, the following network
9、models and associated guidelines should be applied.1 IntroductionDigital transmission network models are hypothetical entities of a defined length and composition for use in thestudy of digital transmission impairments (e.g. bit errors, jitter and wander, transmission delay, availability, slip, etc.
10、).The diversity of possible network situations requires that individual models can only represent a small portion of typicalreal entities. However, a limited number of such models (e.g. 2 or 3) together may be sufficiently representative toprovide a useful tool upon which studies may be based.The ne
11、twork models, where applicable, take account of the following features:a) physically reflect the length of the overall connection with some indication of frequency of occurrence,2 Fascicle III.5 - Rec. G.801b) identify boundaries between switching and transmission elements,c) give no indication of t
12、he means of implementing transmission between switching elements (e.g. metallic,optical, radio media, satellite etc.),d) describe in detail the user/network access arrangement in the local portion (i.e. customer to localexchange),e) take account of all possible usages or be independent of them,f) re
13、flect the use of additional digital processing elements required in particular network configurations (e.g.A- converters, digital pads, transcoders, etc.).This Recommendation makes no statement in respect of the electrical and physical environment in which thenetwork models operate. These aspects ar
14、e currently the subject of study. In the application of these network models tothe study of specific digital impairment (e.g. errors) arbitrary judgements may need to be made concerning thesignificance, in particular, of the electrical environment.2 Hypothetical reference connection (HRX)A digital H
15、RX is a model in which studies relating to overall performance may be conducted, therebyfacilitating the formulation of standards and objectives. In order to initiate studies directed at the performance of anISDN, an all digital 64 kbit/s connection is considered. Since the overall network performan
16、ce objectives for anyperformance parameter need to be consistent with user requirements, such objectives, in the main, should relate to anetwork model which is representative of the very long connection. The HRX shown in Figure 1/G.801 serves thispurpose. It does not represent the rare worst case co
17、nnection; although it does aim to encompass the vast majority ofconnections for each relation. Moreover, the difficulty of identifying every conceivable practical implementation of aconnection and the undesirability of producing too many options naturally requires that this “standard HRX“ may needto
18、 be appropriately modified in composition to suit the particular task in hand. A situation can be envisaged where manysimilar HRXs exist to serve specific functions, but in all cases they are derivatives of the “standard HRX“. The potentialproliferation of HRXs prevent their inclusion in this Recomm
19、endation. Any departure from the “standard HRX“ mayneed to be shown in the Recommendation appropriate to that impairment or performance parameter. For example, seeRecommendation G.821. They are not intended to be used for the design of transmission systems.The diversity in composition is particularl
20、y apparent when a distinction is made between average size and largecountries and, therefore no one HRX can possibly accommodate such variations. In the process of apportionment thedemarcation between national and international portions is unimportant as in most instances the intrinsic quality ofcir
21、cuit comprising both portions is the same. In contrast, however, the overall length is regarded as being critical and itschoice is “country size“ independent. Accordingly, the level of impairment actually experienced over a real connection isconsidered satisfactory if compatible with that stipulated
22、 for the longest HRX, taking due account of differences betweenthe construction of the hypothetical and real connections. For a large proportion of real connections configured usingequipment designs recommended by CCITT, the actual performance is likely to be significantly better. Those CCITTcomplia
23、nt connections which exceed the longest HRX in either length or complexity may not have controlled levels ofperformance; however, their impairment levels are unlikely to exceed those of the longest HRX by more than a factor of2 and the design margins provided with individual items of equipment may w
24、ell bring the impairment to within CCITTend-to-end performance specifications.In formulating the above HRX no account was taken of the following aspects:- maritime applications,- semi-automatic connections (i.e. auto-manual),- standby routing in case of failure.Two other HRXs have been included to f
25、acilitate studies over shorter connections with a view to establishingthe typical performance levels likely to be achieved over frequently realized international circuits. These are given inFigures 2/G.801 and 3/G.801.Fascicle III.5 - Rec. G.80134 Fascicle III.5 - Rec. G.8013 Hypothetical reference
26、digital link (HRDL)To facilitate the study of digital transmission impairments (e.g. bit errors, jitter and wander, slip, transmissiondelay) it is necessary to define network models comprising a combination of different types of transmission elements(e.g. transmission systems, multiplexers, demultip
27、lexers, digital pads, transcoders). Such a model is defined as aHypothetical Reference Digital Link (HRDL). The exact length and composition in respect of the number, type anddisposition of equipments will depend on the digital impairment under study. For example, in the analysis of jitteraccumulati
28、on in a network both transmission systems and muldexes would need to be included to take account of thedifferent jitter characteristics exhibited by such equipment types. In addition the HRDL can be regarded as a constituentelement of an HRX thus permitting the apportionment of overall performance o
29、bjectives to a shorter model. A length of2500 km is considered as a suitable distance for a HRDL.The formulation of such models is the subject of further study.In CCIR Recommendations the term Hypothetical Reference Digital Path (HRDP) is sometimes used. This isequivalent to a Hypothetical Reference
30、 Digital Link (see Definition 3005 in Recommendation G.701).4 Hypothetical reference digital section (HRDS)To accommodate the performance specification of transmission systems (i.e. digital line and radio systems) it isnecessary to introduce a Hypothetical Reference Digital Section (HRDS). Such a mo
31、del is defined in Figure 4/G.801 foreach level in the digital hierarchies defined in Recommendation G.702. The input and output ports are the recommendedinterfaces as given in Recommendation G.703 for hierarchical bit rates. The lengths have been chosen to berepresentative of digital sections likely
32、 to be encountered in real operational networks, and are sufficiently long toFascicle III.5 - Rec. G.801 5permit a realistic performance specification for digital radio systems. The model is homogeneous in that it does notinclude other digital equipments such as multiplexers/demultiplexers. This ent
33、ity can form a constituent element of aHRDL.It is possible to relate the two following types of performance requirement to an HRDS:- the Network Performance Objectives (NPO) which are the objectives to be realized in a real network;- the Equipment Design Objectives (EDO) which provide guidance to th
34、e designer of systems using specifictransmission media and transmission techniques.Note 1 - The Equipment Design Objectives which normally appear in the appropriate transmission andswitching system recommendations are formulated to ensure compatibility with the corresponding network performanceobjec
35、tives.Note 2 - An explanation of a Network Performance Objective and an Equipment Design Objective is given inRecommendation G.102.Note 3 - The formulation of a homogeneous entity of a realistic length permits specification and commissioningacceptance testing under real operational conditions.In a s
36、imilar manner CCIR and CMTT have formulated media and application orientated models for use in theirstudies. The following recommendations describe the relevant models.- Recommendation 502-2 (Draft). Hypothetical Reference Circuit for Sound Programme Transmission.(Terrestrial systems and systems in
37、the fixed-satellite service).- Recommendation 521-1. Hypothetical Reference Digital Path for systems using digital transmission in thefixed-satellite service.- Recommendation 556. Hypothetical Reference Digital Path for radio-relay systems for telephony.ANNEX A(to Recommendation G.801)The applicatio
38、n of Hypothetical Reference Modelsin the formulation of equipment design objectivesAn important use of hypothetical reference models is to facilitate the apportionment of network performance6 Fascicle III.5 - Rec. G.801objectives to constituent elements, prior to the derivation of equipment design o
39、bjectives. To satisfactorily achieve thisobjective a diagrammatic representation of the approach adopted by CCITT in the formulation of equipment designobjectives is shown in Figure A-1/G.801.The approach recognizes that it may be necessary to derive from the “standard HRX“ a more appropriate HRXwhi
40、ch better takes into account both the usage and the specific network performance parameter under study. Theadoption of this approach will facilitate the formulation of rules governing the use of certain digital elements such assatellite links, transcoders, digital pads, etc.National Administrations are advised to develop their own representative network models reflecting the featuresof their evolving national digital network in order to validate prima facie compliance with international standards.
