1、CCITT RECMN*E973L 92 4862593 0578644 830 INTERNATIONAL TELECOMMUNICATION UN ION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE TELEPHONE NETWORK AND ISDN QUALITY OF SERVICE, NETWORK MANAGEMENT AND TRAFFIC ENGINEERING E.731 (1 0/92) METHODS FOR DIMENSIONING RESOURCES OPERATING
2、 IN CIRCUIT SWITCHED MODE I Recommendation E.731 CCITT RECRN*E.731 92 II 4862593 0578645 777 m FOREWORD The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, o
3、pcrating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval
4、 of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988). Recommendation E.731 was prepared by Study Group II and was approved under the Resolution No. 2 procedure on the 30th of October 1992. CCIlT NOTES
5、 1) telecommunication administration and a recognized private operating agency. 2) In this Recommendation, the expression “Administration” is used for conciseness to indicate both a A list of abbreviations used in this Recommendation can be found in Annex A. o rarr 1993 All rights reserved. No part
6、of this publication may be rcprouced or utilized in any form or by any means, elecuonic or incchanical, including photocopying and microfilm. without permission in writing from the ITU. CCITT RECflN*E.731 92 4862591 0578646 603 Recommendation E.731 METHODS FOR DIMENSIONING RESOURCES OPERATING IN CIR
7、CUIT SWITCHED MODE (I 992) 1 Introduction This Recommendation provides methods for dimensioning resources operating in circuit switched mode in the ISDN. It proposes the use of traditional telephone dimensioning methods complemented with specific techniques that should be used for those ISDN issues
8、that are widely at variance with the traditional telephone traffic models. Techniques for the following ISDN issues are provided: - Multi-slot connection; - Service protection methods; - Attribute negotiation; - Service reservation; - Multipoint connections. 2 Evolution of network dimensioning metho
9、ds 2.1 Telephone trafic modelling All telephone network dimensioning procedures are based on mathematical models that approximate the statistical behaviour of telephone traffic in large populations. These models allow straightforward characterization of the traffic demand and network dimensioning by
10、 adopting simplifying assumptions concerning: - - call attempt inter-arrival times; - call holding times; - - stationarity of the traffic process during the reference period; disposition of blocked or delayed call attempts; dependency of call attempts and holding times from network state and other c
11、all attempts. For example, in the most commonly used Erlang loss formula, the following assumptions are used: - - - - blocked calls cleared; - stationary traffic process during the reference period; poissonian hval of call attempts; general distribution of call holding times; independence of call at
12、tempts and holding times on network state and other call attempts. Assumptions are not actual descriptions of individual caller behaviour, but give rise to statistical patterns that have been experimentally observed to closely approximate the aggregate traffic flows in real telephone networks. The h
13、istory of telephone network dimensioning can be viewed as primarily the extension of a few basic traffic models to a widening range of applications, including the development of techniques to replace situations not entirely consistent with the basic models by “equivalent” situations that permit cont
14、inued application of the same techniques. Recommendation E.731 (10/92) I CCITT RECRN*E.731 92 4862591 0578b47 54T The E.520-Series Recommendations provide telephone network dimensioning methods based on traditional traffic modelling. 2.2 Impact of ISDN circuit switched mode Narrow-band ISDNs will ev
15、olve from the PSTN and the basic network smcture for the circuit switched mode will be identical to that of the PSTN. However, many ISDNs will offer circuit switched services with modes of operation that are at variance with underlying assumptions of traditional telephone traffic models. For example
16、 multi-slot services require different amounts of network resources for different call attempts, an aspect that is not considered in traditional telephone traffic models. The technologies being introduced in conjunction with ISDN can result in new traffic patterns as well. For example, automatic rep
17、etition terminals coupled with fast network signalling may result in a sequence of closely spaced, correlated call attempts that cannot be represented by the independence assumptions in traditional telephone traffic models. 2.3 Evolution of ISDN dimensioning ISDN is an open, permissive concept with
18、a very large range of development paths. Many of the services and features presently identified might never materialize in quantity, and new services and features not yet described might dominate future ISDN evolution. In this situation new traffic models should not be introduced until actual experi
19、ence indicates system behaviour to be sufficiently approximated. This Recommendation provides specific dimensioning techniques that should be used for services and features that are widely at variance with the traditional telephone traffic models and are anticipated to affect resource requirements i
20、n at least part of ISDN networks. The decisions of when and where to apply these techniques in evolving ISDN structures will be a major challenge for traffic engineering specialists in individual Administrations. 3 Multi-slot COM - virtual circuits (also called class limitation) where the number of
21、busy circuits occupied by each traffic component can not exceed its assigned limit. 4.2 Evaluation and dimensioning 4.2.1 Circuit reservation For the circuit reservation type of service protection, total capacity of circuit group and parameters for circuit reservation should also be determined so as
22、 to achieve the GOS target. For the no overflow case, efficient blocking calculation algorithms and method for optimizing values of trunk reservation parameters are available, 3, 4, lo, ill, 121. For circuit groups carrying ovefflow traffic and taking into account some reservation strategies, algori
23、thms for calculating blocking values for each component are given in 6. Methods for the calculations of optimal circuit reservation parameters are for further study. Recommendation E.731 (10192) 3 CCITT RECMN*E.731 92 4862591 0578647 312 4.2.2 Virtuai circuits For the virtual circuits type of servic
24、e protection, state probabilities still have the product form, if the arrival processes are independent Poisson processes. Methods for the calculations of minimal total circuit group capacity to achieve the GOS criteria and optimal virtual circuit parameters are available for the no overflow case 13
25、. The case of circuit groups carrying overflow traffic is for further study. 4.3 Performance characteristics Further study is required before detailed performance characteristics of service protection methods for ISDN traffic are recommended. 5 Attribute negotiation ISDN offers the possibility of ne
26、gotiating the service attributes between the user and the service or the network provider. Some Administrations may wish to introduce this facility; this section provides the means of evaluating the impact of the negotiation on the GOS and, in the cases that this impact is significant, the means of
27、revising the dimensioning. Only the negotiation of the connection characteristics of a call demand, i.e. of those service attributes which are significant from a traffic point of view, as defined and listed in Recommendation E.711, $ 2.2, will be considered. If the network cannot accept the service
28、as requested by the user, a negotiation may occur and, as a consequence, a call with some connection characteristics different from the ones initially requested may be served. Thus, negotiation allows a call, which without negotiation would be blocked, to be served. Consequently, the negotiation inc
29、reases the Uaffic carried by the network and, therefore, increases the blocking probabilities of all the calls. The impact of negotiation for services on demand is considered in this section, and for reserved services in $6. The values of some connection characteristics as, e.g. the information tran
30、sfer capability, may affect the choice of the circuit group to be used by the call, as indicated in 5 3.2 of Recommendation E.712. On the other hand, the value of other connection characteristics, as e.g. the information transfer rate, may only affect the required number of circuits of a given group
31、. 5.1 Connection characteristics affecting the choice of circuit group If a call cannot be carried by the circuit group required according to the connection characteristics requested by the user, the network may give the chance of carrying the call by another circuit group if the user accepts the im
32、plied change of connection characteristics. Thus, the network may work according to an overflow scheme, with a first choice circuit group and its subsequent choice circuit groups for each set of requested connection characteristics. The probabilities that the user accepts the changes of connection c
33、haracteristics implied by overflow to other circuit groups (or, by the contrary, give up the call if it cannot be carried by the previous choice circuit groups) has to be considered in the evaluation. A safe-side approximation when these Probabilities are unknown is to consider them equal to one. Ev
34、aluation of the impact on the blocking probabilities and, if necessary, revision of the dimensioning should be made using methods described in the E.520-Series Recommendations. 5.2 Connection characteristics affecting the required number of circuits According to Recommendation E.712, 5 3.3, the numb
35、er of circuits required by a call depends on the information transfer rate and on the communication configuration (point-to-point or multipoint). The negotiation of the information transfer rate is considered here, while the negotiation of the communication configuration is considered in 7. 4 Recomm
36、endation E.731 (10192) CCITT RECHNaE.731 92 YBb2591 0578650 03Li Recommendation E.731 (10/92) 5 Consider a .circuit group of C circuits serving M independent uaffic components, each component characterized by an arrival rate, hm, a holding time, hm, and a number of circuits required per call, dm. Co
37、mplete sharing of capacity of the total circuit group is allowed. (The complete partitioning of the circuit group into sub- groups, each sub-group serving a traffic component, is covered by 0 5.1 .) In case of no overflow traffic or negotiation, the state probabilities have the product form i and ma
38、y be evaluated by the recurrent formula: M m= 1 where Q(n) is the probability of having n circuits busy. The blocking probability for the components rn, Bm, is given by: Consider now that there is negotiation. If a call of the component rn is offered and there is only k free circuits, with k dm, the
39、 network gives the chance of carrying the call with only k circuits. Define 7Cm.k as the probability that the user accepts this chance. (If %m,k is unknown, it could be considered equal to 1 as a safe-side approximation). The state probabilities, Q(n), with this type of negotiation can be estimated
40、from those without negotiation, Q(n), by the following approximate formula: forn C being: m=l k=l If the change of the requested dm circuits by only k circuits affects the holding time of the call which becomes h,k instead of hm, hm must be substituted by hm,k in the previous formula. The probabilit
41、y, Bm, that a call of the component rn cannot obtain the initially requested dm circuits is: CCITT RECMNmE.731 92 4862593 O578651 T70 6 Service reservation Some services (videoconference, data communications, etc.) can be provided, exclusively or not, on an advanced reservation basis: users pre-book
42、 the necessary network resources for a planned call in a preliminary dialogue with a reservation centre. Both the user and the network provider may have reasons for using telecommunications services with advanced reservation: the former achieves in this way guaranteed service for scheduled calls, th
43、e latter can more easily offer high bit rate services (6, 24, 30 x 64 kbitls), that, in many cases, could not be offered on a demand basis. The user request may be characterized by the following relevant parameters 3: - - the required bandwidth (4. the starting instant (r) and duration (D) of the re
44、served time; If the system cannot accept the reservation as requested, a negotiation could be made about the values of these parameters. The required resources may be either only dedicated to reservation services or also shared by on demand services. 6.1 Dedicated resources and no negotiation The no
45、tice interval, defined as the time interval between the reservation request instant and the desired starting instant, is an essential characteristic of the reservation. There are not yet specifications on GOS requirements for reservation services in the E.720-Series Recommendations. It can be assume
46、d that the specifications will be related to the parameters B and either R(T) or R(T) defined as follows: B: probability of blocking of a cai1 whose reservation had been accepted; R(T): probability of rejection of a reservation request made with a notice interval equal to T; R(z): Probability of rej
47、ection of a reservation request made with a notice interval greater than of equal to z. In the case of resources only dedicated to reservation services, B is zero. The investigation on the effect of the notice interval has led to the conclusion 14 that the system performance is independent of the me
48、an of the notice interval (of the whole population of requests). It means that, e.g. two notice interval distributions Gl(x) and G2(x), such that Gdx) = G;! (n - c), c being a constant lead to reservation rejection probabilities RI(T) and R(T) respectively such the RI(T) = R2 (z - c). System perform
49、ance however depends on the variance of the notice interval. If the notice interval is constant, the system behaves as with on demand services. Dimensioning methods for on demand services should be used. The blocking probabilities obtained using the on demand service models should be interpreted as reservation rejection probabilities, R(c), c being the constant value of the notice interval. For non-zero variance of notice, a call can be blocked by (previously reserved) calls commencing after its own requested start time. This phenomenon of “retro-blocking” 141
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