1、1u-T RECMN*V=Sb BIS 75 4862593 Ob08852 220 D INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU DATA COMMUNICATION OVER THE TELEPHONE NETWORK V.56 bis (08/95) NETWORK TRANSMISSION MODEL FOR EVALUATING MODEM PERFORMANCE OVER 2-WIRE VOICE GRADE CONNECTIONS ITU-
2、T Recommendation V.56 bis (Previously “CCIlT Recommendations”) ITU-T RECMNxV-5b BIS 75 W 4862571 Ob08853 167 FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operati
3、ng and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their
4、turn. produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). . IT-T Recommendation V.56 bis was prepared by ITU-T Study Group 14 (1993-1996) and was approved und
5、er the WTSC Resolution No. 1 procedure on the 29th of August 1995. NOTES 1. telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a 2. follows: The status of annexes and appendices attached
6、 to the Series V Recommendations should be interpreted as - - an annex to a Recommendation forms an integral part of the Recommendation; an appendix to a Recommendation does not form part of the Recommendation and only provides some complementary explanation or information specific to that Recommend
7、ation. O ITU 1996 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*V=Sb BIS 95 9 4862593 Ob08854 OT3 9 CONTENTS Page 1 Sc
8、ope 1 2 References 2 3 Definitions 5 4 Description of model 4.1 Overview . : 4.2 Echo control requirements 4.3 Round-trip delay criteria . 4.4 Modem impedance requirements 4.5 Model limitations 4.6 Digital impairment requirements Network Model Coverage (NMC) 4.7 5 Impairment level set-up 6 22 44 46
9、1.1 Model description . 46 47 56 57 58 Annex A . Attenuation and envelope delay distortion characteristics i Annex B . Minimizing test time Appendix I - Rare intracontinental network model . Appendix II - Basis for network models Bit- and block-error rate vs . network model coverage . Appendix IV -
10、Supplemental round trip delay model Appendix III - Error rate vs . network model coverage 111.1 Recommendation V.56 bis (08195) 1 SUMMARY This Recommendation provides a network model to be used in the evaluation and comparison of 2-wire voiceband modems. The model is intended to be representative of
11、 the transmission impairment conditions that will be encountered on both intracontinental and intercontinental connections i Recommendation V.56 bis (08/95) INTRODUCTION It has long been recognized that the test evaluation methodology used in Recommendation V.56 for evaluating modem performance does
12、 not provide a definitive answer for the user on how a particular modem will perform in the actual Network. This Recommendation has been developed with the understanding that the traditional technique for evaluating modem performance in the presence of Gaussian noise and other analogue transmission
13、parameters, did not effectively represent the vast majority of connections in the Network today that are digital transmission in nature. This Recommendation introduces the concept of using Network Model Coverage (NMC) as a tool for modem performance evaluation, and abandons the traditional method of
14、 Signal-to-Noise Ratio (SNR) (more recently referred to as 1 kHz Tone to Total Distortion ratio or TDD) waterfall performance curves. The concept of rating a modems performance statistically, based on the Likelihood Of Occurrence (LOO) of a given connection, is new to the industry. This new techniqu
15、e was first developed by CCITT SG XW (now ITU-T SG 14) for evaluating candidate modulation technologies for the then proposed CCIT V. fast study (now ITU-T Recommendation V.34), and is detailed in the test document referred to as the “V. fast test suite“ (CCITT SG XW D.229 - 1992). This methodology
16、was further refined by ANSI TIA TR-30.3 when creating ANSI TIA TSB-37A “Telephone Network Transmission Model for Evaluating Modem Performance“ approved in the United States in September 1994. The numerical values of the characteristic (impairment levels) of each type of connection, as well as the Li
17、kelihood Of Occurrence (LOO) of a certain type of connection, are based upon a model of the Network as is commonly found in the business environments in industrial countries. It has been assumed that the networks in these industrial countries are already primarily a digital Network. Additionally, it
18、 is further assumed that the Networks of all countries worldwide are evolving or will evolve towards a digital Network. Therefore, the characteristics and likelihood of occurrences proposed in this Recommendation are assumed to become more and more representative of the worldwide Network with each p
19、assing year. Depending on the country, the actual Network likelihoods of occurrences of particular Network combinations may vary significantly from this model. It is important to emphasize that the model is based on the available information on Network characteristics and not statistically valid sur
20、veys of the Network and that the Network is continually changing. Therefore, while the percentages of the Network Model Coverage (NMC) derived from tests using the model may provide a suitable basis for comparing modem performance, it should not be considered to be representative of actual Network c
21、overage. Differences in NMC percentages for different modems are more meaningful for modem comparisons in todays digital network than differences in the tolerance to noise obtained from traditional waterfall curve testing. This network model serves to remind the user that a large percentage of conne
22、ctions worldwide are very good lines and that most modems will perform suitably over a significant percentage of connections. The concept in the past was to create all severe stress conditions in which many modems would not even connect and yet the user knew that these same modems performed well mos
23、t of the time in the real network. As was the case in the V. fast test suite, IT-T SG 14 has developed a network model that contains three major sections, namely, local loop A, the 4-wire End-Office-to-End-Office (EO-EO) and local loop B. A LOO is assessed independently for each Test Loop Combinatio
24、n (TLC) and for each EO-EO setting. These are combined in such a way as to provide an overall NMC estimate for a particular modem as a percentage of the total network model for a specific data performance requirement. An important aspect of this network model is that there is a representative sample
25、 of local loop topologies and EO-EO facility combinations. The impedance characteristics, 1 kHz losses and frequency responses of the local loops are in Annex A, while the topology model of the loops is in Figure 2. The loss and frequency response characteristics of the EO-EO section are also in Ann
26、ex A while the network impairments associated with the EO-EO section are in Table 1. Recommendation V.56 bis (OW95) iii A LOO is assigned to each of the EO-EO impairment combinations in Table 1 and to each of the TLCs in Table 2. The scoring assessment of NMC using the complete model can be found in
27、 Table 3, while the scoring assessment for truncated versions of the model, representing decreasing percentages of the complete network model, are in Tables 4 through 6. The rationale for the network model, along with a description of the topology for EO-EO impairment combinations 1 through 16 (main
28、 body) and optional EO-EO impairment combinations 17 through 20 (Appendix I), is located in Appendix II. Test methods and procedures for bit- and block-error rate transmission performance has been included here in Appendix III. A supplemental test to determine the effects of low Round-Trip Delay (RT
29、D) on modem performance is contained in Appendix IV. iv Recommendation V.56 bis (08195) - ITU-T RECflN+V.Sb BIS 95 4862593 0608858 749 Recommendation V.56 bis NETWORK TRANSMISSION MODEL FOR EVALUATING MODEM PERFORMANCE OVER 2-WIRE VOICE GRADE CONNECTIONS - (Geneva, 1995) 1 Scope This Recommendation
30、contains the network model, test equipment configuration and calibration, and parameter values to be used in the evaluation and comparison of 2-wire modems. Means to reduce testing time are also specified in this Recommendation. This includes truncated network models and conditional testing procedur
31、es. The model is intended to be representative of the transmission impairment conditions encountered on intracontinental network connections within the continental United States, Europe and Asia and is represented in Table la. Additionally, Table la has been adjusted to account for the presence of t
32、he large Private Branch Exchange (PBX) systems commonly found in business environments. Finally, the model has been extended to include Intercontinental connections between the United States and both Europe and Asia and is represented by Table lb. The PBX systems represented are certainly not repres
33、entative of an exhaustive study of existing PBX systems as they are both too numerous and ever changing. However, it is felt that a substantial number of the larger PBX systems are represented by the transmission impairments in Tables la and lb. Each impairment combination has an associated “score“
34、that reflects the estimated LOO of a connection containing such impairments. The score provides a basis for estimating the percentage of connections (excluding those that may include trouble conditions or unusual conditions such as discussed in Appendix I) within a continental connection over which
35、a modem should be expected to provide satisfactory performance. For modems that adjust their transmission rate dependent upon the channel impairments, the model provides a basis for estimating the percentage for each rate at which operation is possible. It is important to emphasize that the model is
36、 based on the available information on network characteristics, not a statistically valid survey of the network and that the network is continually changing. Therefore, while the percentages of NMC derived from tests using the model may provide a suitable basis for comparing modems, it should only b
37、e considered as an indication of potential coverage of the real network. A satisfactory V.56 bis test result should be considered as an indication, not a formal assurance of success on any particular connection of the real network complying with relevant M- and G-Series Recommendations. This model a
38、ssumes that network echo cancellers, suppressors, and speech enhancement devices have been disabled. The model does not take into account the effects on performance of digital link slips (“Tl slips“), or automatic balancing of End-Office hybrids. Furthermore, loop current is not specified, nor are c
39、onditions for evaluating the compatibility of a modem with the range of loop currents that may be encountered. The derivation of the model impairment combinations is discussed in Appendix II. This Recommendation also contains network conditions (Appendix I) covering very unusual situations that an i
40、ndividual user may encounter on all or most intercontinental connections. While these conditions may represent a very small percentage of connections, they are important in providing a basis for explaining the limited performance that may be available to individual users. This Recommendation also co
41、ntains an intercontinental network model (Appendix II). The model does not include scores for the different impairment combinations such as provided in the intercontinental network model. However, the impairment combinations that represent unusual conditions are distinguished from those that represe
42、nt conditions that have a significant LOO. Recommendation V.56 bis (OW95) 1 2 References The following Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were
43、valid. All Recommendations and other referenced Standards are subject to revision; all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent editions of the Recommendations and other references listed below. A list of currently valid IT-T Re
44、commendations is regularly published. - CCITT Recommendation V.56 (1988), Comparative tests of modems for use over telephone type circuits. - CCITT Recommendation 0.42 (1992), Equipment to measure non-linear distortion using the to employ ADPCM properly in a mixed voicehoiceband- data application, c
45、areful choice of which adaptive techniques to combine, or alternatively careful choice of a method for switching between separately optimized ADPCM algorithms, is required. Standard versions of ADPCM are specified in Recommendation G.726. Non-standard versions of ADPCM (CCITT Contributions COM XVIII
46、-101 and 102: 1984-1988) are also encountered in practice. 3.2 of a 1 kHz reference signal, expressed in dB. attenuation distortion (AD): Differences in signal loss as a function of frequency compared to the signal loss 3.3 multiple loops to a cluster of subscribers remote from the office. digital l
47、oop carrier system (DLC): A system that provides access to a local office via a digital carrier link for 3.4 envelope delay distortion (EDD): Differences in narrow-band signal envelope propagation time through a connection as a function of frequency compared to the propagation time of a 1.8 kHz refe
48、rence signal, expressed in microseconds. A linear network (channel) has the property that a sinusoidal input signal produces a sinusoidal output signal of the same frequency. The output signal will, in general, not have the same amplitude and phase as the input signal; the amplitude and phase differ
49、ences will be functions of frequency. Envelope delay is the derivative of the difference between the input phase and the output phase, in radians, with respect to radian frequency. 3.5 is franchised to provide service to the customer for all local calls or connections in a specific geographic area. exchange carrier (EC): Refers to the local telephone company (telecommunications common carrier). The EC 2 Recommendation V.56 bis (08/95) ITU-T RECflN*V-Sb BIS 95 U 4hb259L QbO8dbO 3T7 3.6 result of the hybrid not achieving complete rejection. far-end (talker) echo: Talker
