1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Series G Supplement 37 (1 0/98) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS ITU-T Recommendation G.763 digital circuit multiplication equipment (DCME) tutorial and dimensioning ITU-
2、T G-series Recommendations - supplement 37 (Previously CCIlT Recommendations) - _ STD-ITU-T RECMN SERIES G SUPP 37-ENGL 3998 I 4862593 Obb333b 87T I GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION G.400-G.449 WITH METALLIC LINE
3、S COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450-G.499 TES TING EQUIPM EN TS TRANSMISSION MEDIA CHARACTERISTICS G.600-G.699 DIGITAL TRANSMISSION SYSTEMS TERMINAL EQUI PMENTS G. 700-G .799 DIGITAL NETWORKS G.800-G.899 1 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900-G.999 ITU-T G-SERIES RECO
4、MMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS I INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS INTERNA TIONAL ANALOGUE CARRIER SYSTEM U GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER- TRANSMISSION SYSTEMS Il INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TEL
5、EPHONE SYSTEMS ON METALLIC LINES G.100-G.199 G.200-G.299 G.300-G.399 For further details, please refer to ITU-T List of Recommendations. STD=ITU-T RECMN SERIES G SUPP 37-ENGL 1998 = 48b2591 ObblL37 70b m SUPPLEMENT 37 TO ITU-T G-SERIES RECOMMENDATIONS ITU-T RECOMMENDATION G.763 DIGITAL CIRCUIT MULTI
6、PLICATION EQUIPMENT (DCME) TUTORIAL AND DIMENSIONING Summary This supplement gives tutorial information on DCME techniques and describes various DCME dimensioning methods for different route characteristics. Source Supplement 37 to ITU-T G-series Recommendations was prepared by ITU-T Study Group 15
7、(1997-2000) and was approved under the WTSC Resolution No. 5 procedure on the 13th of October 1998. G series - Supplement 37 (10/98) I STD-ITU-T RECMN SERIES G SUPP 37-ENGL L998 M 4b253L Obbll138 b42 FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the
8、 field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide b
9、asis. 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 turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the p
10、rocedure laid down in WTSC Resolution No. 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation the term recognized operating agency (ROA) includes any individual, compa
11、ny, corporation or governmental organization that operates a public correspondence service. The terms Administration, ROA and public correspondence are defined in the Constitution of the ITU (Geneva, 1992). INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or
12、implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation developmen
13、t process. As of the date of approval of this Recommendation, the ITU had received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore s
14、trongly urged to consult the TSB patent database. O ITU 1999 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. II G series - Supplemen
15、t 37 (10/98) STD=ITU-T RECMN SERIES . G SUPP 37-ENGL 1998 D 48b2591 ObbLL39 589 CONTENTS Page 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 DCME tutorial Use of Digital Circuit Multiplication System (DCMS) Transmission requirements . Location DCME gain (DCMG) . ISDN bearer services Res
16、toration of services . Control of transmission overload . Transmission link performance monitoring . References DCME dimensioning methods for different route characteristics Scope Route profiles . DCME operation 2.3.1 DSI gain for voice . 2.3.2 2.3.3 LRE gain for voice 2.3.4 LRE gain for data DCME D
17、imensioning . 2.4.1 Limitations 2.4.2 Example gain calculations using simplified techniques 2.4.3 Two pitfalls for the unwary . Conclusion DSI gain for data . G series . Supplement 37 (10198) 1 1 4 4 5 5 7 7 8 8 8 8 9 10 14 16 . 111 STDmITU-T RECMN SERIES G SUPP 37-ENGL 3998 48b259L ObbLlri0 2TO Sup
18、plement 37 to G-series Recommendations ITU-T RECOMMENDATION G.763 DIGITAL CIRCUIT MULTIPLICATION EQUIPMENT (DCME) TUTORIAL AND DIMENSIONING (Geneva, 1998) 1 DCME tutorial 1.1 DCMS provide the means to reduce the cost of transmission (e.g. long distance transmission) by making use of the combination
19、of Digital Speech Interpolation (DSI) and Low Rate Encoding (LRE) techniques. DSI is used to concentrate a number of input channels (generally referred to as trunk channels) onto a smaller number of output channels (generally referred to as bearer channels). It does this by connecting a trunk channe
20、l to a bearer channel only for the period that the tnink channel is active, i.e. is carrying a burst of speech or voiceband data. Since in average conversations one direction of transmission is active only for 30% to 40% of the time, if the number of trunks is large the statistics of the speech and
21、silence distributions will permit a significantly smaller number of bearer channels (bearer channel pool) to be used. Control information must also be passed between the terminals to make sure that bearer and trunk channel assignments at each end remain synchronized. LRE uses digital filtering techn
22、iques to construct an estimate of the waveform at both the encoder and the decoder. Since the actual information rate of speech is much lower than the channel Nyquist rate, the link used between the LFE encoder and the decoder can operate at a rate which is dependent mainly on the quality of the mod
23、els and the permissible amount of transmission degradation. The ITU-T has standardized in Recommendations G.726 and G.727 a type of LRE known as ADPCM, the performance of which has been extensively characterized. DCME uses the ADPCM defined in Recommendation G. 72 6. Facsimile demodulationhemodulati
24、on uses recognition and decoding of some or all of the voiceband signals sent by the modem to enable the sub-multiplexing of the digital information from a number of trunk channels onto a reduced number of bearer channels with the object of enhancing both the quality and the efficiency of transmissi
25、on as compared to rate reduction of the signals using ADPCM. Use of Digital Circuit Multiplication System (DCMS) The simplest way to use DCMS is in the single destination mode as shown in Figure 1/G.763. This mode of operation is most economic for the largest routes. For smaller routes there are two
26、 options: - operation in multi-clique mode; - operation in multi-destination mode. Operation in multi-clique mode, see Figure 2G.763, divides the bearer channels into a number of blocks or cliques, each associated with a different route. There is normally a fixed boundary between cliques, and trunkh
27、earer channel assignments are generally carried in a control channel within the clique to which they refer. This limits the dynamic processing of received channels to those which are contained in the wanted clique; selection of the wanted clique channels can be done using a simple static digital swi
28、tch without reference to the assignment information. With a 2048 kbits bearer system in multi-clique DCMS the statistics of the DSI are unpromising with more than three routes. Recommendation G.763 provides for two cliques. G series - Supplement 37 (10/98) 1 STD-ITU-T RECMN SERIES G SUPP 37-ENGL 199
29、8 m 4Bb2591 Obbl141 133 m Operation in multi-destination mode, see Figure 3G.763, permits any bearer channel to be associated with any trunk channel of any of a number of different routes. There is no segregation of routes on the bearer, and therefore at the receive terminal it is impossible to sele
30、ct the wanted channels without reference to the assignment information. Multi-destination mode is economic for very small routes via satellite, but practical difficulties limit the number of routes which it is desirable to have on a single DCMS. 1.2 Location Location of DCME depends on its use. Equi
31、pment used in single destination mode or in multi-clique mode can in general be located at: - ISC; - earth station; - cable head, without significant restrictions. Equipment used in the multi-clique mode will typically be located at the ISC so that the advantages of DCMG can be extended over the nat
32、ional section. Equipment used in the multi-destination mode will typically be located at the earth station or cable head. The reason for this is that whereas in multi-clique mode the number of receive bearer channels at the DCME terminal is approximately equal to the number of transmit bearer channe
33、ls, in multi-destination mode the number of receive bearer channels at the DCME terminal is the number of transmit bearer channels multiplied by the number of destinations. It therefore may be uneconomic to provide sufficient transmission capacity between earth station and ISC to permit location of
34、multi-destination DCME at an ISC. 1.3 Transmission requirements DCMSs are usually required to carry any traffic which can be carried on ordinary General Switched Telephone Network (GSTN) connections. That includes voiceband data using V-series Recommendation GSTN modems, facsimile calls following Re
35、commendations T.4 and T.30 and using V.29 modems. In addition, in the ISDN 64 kbit/s unrestricted on-demand digital data and alternate speed64 kbitfs unrestricted bearer services must be carried. DCMSs are primarily designed to maximize the efficiency of speech transmission. Use with voiceband data,
36、 especially at high rates, presents problems. These problems are mainly due to the difficulty for 32 kbitfs ADPCM of encoding voiceband data waveforms. 1.4 DCME gain (DCMG) The gain of DCME is the input trunk channel transmission multiplication ratio, which is achieved through application of DCME, i
37、ncluding LRE and DSI (for a specified speech quality at a certain level of bearer channel activity). The maximum available gain depends on: - number of trunk channels; - number of bearer channels; - trunk channel occupancy; - speech activity; - voiceband data traffic; - - type of signalling; - 64 kb
38、it/s traffic; ratio of half duplex to full duplex voiceband data; 2 G series - Supplement 37 (10/98) STD=ITU-T RECMN SERIES G SUPP 37-ENGt 1998 m 48b2571 ObbLL42 073 m - minimum acceptable speech quality; - dynamic load control threshold. Of these the factor which has the greatest significance is th
39、e percentage of 64 kbit/s digital data traffic. This is because a trunk channel carrying 64 kbits traffic requires two 32 kbit/s bearer channels to be removed from the pool of channels available to the DSI process. The peak percentage of voiceband data may vary between 5 and 30 per cent, depending o
40、n route. This is discussed in greater detail in clause 2. The type of signalling system used on the route can significantly affect the gain. Continuously compelled signalling systems hold channels active for undesirably long periods. In the case of ITU-T R2 digital signalling via a DCMS used on a sa
41、tellite, the channel might be active for 5 to 14 seconds. The measured speech activity depends on the characteristics of the activity detector. It is usual to assume 35 to 40 per cent. Channels with high ambient background noise can increase this activity factor. Outside of the route busy hour, the
42、occupancy of the tnink channels by traffic will be lower than in the route busy hour. The effect of this is to reduce the ensemble activity measured by the activity detector to about 27 per cent outside the route busy hour, whereas it will be close to the speech activity factor, i.e. about 40 per ce
43、nt during the route busy hour. The speech quality is governed by two main factors: the LRE encoding rate, and the amount of speech lost while a newly active trunk channel is awaiting connection to a bearer channel. If there are a great many newly active trunk channels in competition, the beginning o
44、f a burst of speech is more likely to be clipped or frozen out than if relatively few trunk channels are active. The speech quality of a DCME in a network with echo control devices may be affected by clipping introduced by echo control devices and by a possible noise contrast effect. In particular w
45、hen echo suppressors or echo cancellers are used on circuits where the near end generated noise is high with respect to the noise generated in the remainder of the link, suppression of the far end noise may be objectionable due to noise contrast. Possible means of eliminating this problem are use of
46、 echo control devices which insert idle line noise at the appropriate level during suppression periods, or insertion of idle line noise at the DCME during the relevant period when the echo control device is integrated in the DCME. Another approach is discussed in I.YG.763. When commissioning a new D
47、CMS, observations should be made of the type and characteristics of the traffic which will use it. It is unwise to rely solely on customer complaints to indicate when a system is poorly dimensioned. This is because interactions between the DCMS and echo control (see Note) may obscure the true proble
48、m, Furthermore the consequence of trying to concentrate too many trunk channels onto too few bearers may be simply to increase the calling rate and to reduce the call holding time. This may result in greatly reduced quality, especially where continuously compelled signalling systems are used, and le
49、vels of trunk channel activity occur far above what was envisaged in the original system dimensioning. NOTE - This highest speech quality is obtained when echo cancellers conforming to Recommendation G. 165 (1993) or G.168 (1998) are used for echo control. However echo suppressors conforming to Recommendations G.164 (1988) may be used. Two possible criteria for acceptable speech performance are an average of 3.7 bits per sample and less than 2.0% probability of clipping exceeding 50 ms, or alternatively that less than 0.5% of speech should be lost due to clipping if the 2-bi
