1、m 232b414 0005275 8 Page 1 Recommendation T/CS 68-04 (Montpellier 1984, modified in Odense 1986) PERFORMANCE AND AVAILABILITY DESIGN OBJECTIVES FOR DIGITAL TRANSIT EXCHANGES Recommendation proposed by Working Group T/WG 1 1 “Switching and Signalling” (CS) “The European Conference of Postal and Telec
2、ommunications Administrations, considering - that the achievement of long-term harmonisation of integrated digital networks and integrated services digital - the importance of specifying performance requirements appropriate to the integrated digital network and inte- networks represents a major targ
3、et in the current development of the European network, grated services digital network, recommends that the members of the CEPT, when introducing digital transit exchanges, take account of and give effect to the O following performance requirements specifications.” The reader is referred to CCITT Re
4、commendation 4.504 i to which this Recommendation is identical, except for the additions and modifications indicated below. The section references (indicated 8) are to 4.504 i. I, Jirst paragraph, last sentence: The reference is to CEE Recommendation T/CS 68-01 2. I, replace last paragraph with the
5、following two paragraphs and note: These design objectives may be used as guides in system design and by Administrations and RPOAs in evaluating specific designs and/or comparing the performance capabilities of different exchange designs. Degradation of service performance shall be said to exist at
6、an exchange if the specification in Section 4 for loss-of-service does not apply, and if a proportion of the trafic offered at reference trafic loading experiences a quality of service worse than that specified under normal conditions. In some cases, quality of service under normal conditions, and q
7、uality of service under degraded conditions, cannot be distinguished. So, maximum ratios of misprocessing considered in Section 2.4, Call processing performance objectives, are applicable for processing faults, due both to sporadic errors and permanent hardware failures. Note. This definition and st
8、ructure of the quality of service is used here on a provisional basis. 2.1.1, replace with: 2.1.1 Load on incoming interexchange circuits a) Reference Load A - 0.7 Erlang average occupancy on all incoming circuits with call attempts/hour/incoming O circuit = 20. This Figure assumes 25% ineffective c
9、all attempts. b) Reference Load B - 0.85 Erlang average occupancy on all incoming circuits with call attempts/hour/incoming circuit = 25. 9 2.2; replace with: 2.2 Inadequately handled call attempts Inadequately handled call attempts are all lost calls occurring in the exchange under consideration (b
10、locking as defined in CCITT Red Book 11/3), plus all call-attempts, in the handling of which, too many important delays associated with call-Set-up occur during the establishment of the connection. These are at least 3 times greater than the value specified for the “0.95 probability of not exceeding
11、” indicated in the Tables under Section 2.3. Note. The definition of inadequately handled call attempts is assumed to apply when at least one outlet is available in the appropriate direction. It is recommended that the probability of inadequately handling a call not exceed the values in Table I/T/CS
12、 68-04. Edition of September 15, 1986 H 232b414 0005276 T TICS 68-04 Page 2 I I Reference Load A I Reference Load B I I Probability I IO- I 10-2 I Table 1 (TICS 68-04) The requirement for multi-slot connections requires further study. The definition of objectives under failure conditions requires fu
13、rther study. 2.4.2, replace with: 2.4.2 64 kbit/s semi-permanent connections 2.4.2.1 Interruption rate A semi-permanent connection is said to be interrupted when the connection is released or an unacceptable 64 kbit/s path transmission quality across the exchange (worse than that for an alarm condit
14、ion) occurs for more than Y seconds. As a long-term average, the interruption rate of a semi-permanent connection shall not be greater than X interrup- tions per year. Note. X and Y require further study. 2.4.2.2 Release failure On a provisional basis, the same specification as for switched connecti
15、ons (see 5 2.4.1.2) applies. This item requires further study. 2.4.2.3 Other failure modes To be studied taking into consideration: - need to recognise an interruption; - probability of an interruption; - requirements for re-establishment of interrupted connection; - any other unique requirements. 2
16、.4.4, replace with: 2.4.4 n x 64 kbitls semi-permanent connections 2.4.4.1 Interruption rate A semi-permanent multi-slot connection is said to be interrupted, when the connection is released, or when an unacceptable transmission quality across the exchange (worse than that for an alarm condition) oc
17、curs for more than Y seconds on at least one 64 kbit/s path through the exchange, or when the time slot sequence integrity is no longer maintained. As a long-term average, the interruption rate of a semi-permanent connection shall not be greater than Z interruptions per year. Note. Y and Z require f
18、urther study. Y is the same value as in 2.4.2.1. 2.4.4.2 Release failure To be determined. 2.4.4.3 Other failure modes This item requires further study. 3, insert the following: 3 3.1 Causes of overload conditions EXCHANGE PERFORMANCE UNDER OVERLOAD CONDITIONS Two groups of causes can be distinguish
19、ed: a) overload by modified behaviour of subscribers (e.g. generating more call attempts because of emer- gency situations or altering their pattern of usage) in comparison with that assumed for the exchange trafic engineering, causing an increase in processing load; - on trunk lines, in higher (or
20、lower) level exchanges; - within the exchange itself, which causes a reduction of the available common resources resulting in non or incorrect handling of the call attempts with the consequent phenomenon of repeated call attempts. b) overload caused by equipment malfunction O Edition of September 15
21、, 1986 2326414 0005277 L TICS 68-04 Page 3 3.2 Definition of overload The following considerations should be taken into account when drafting that definition: One or more overload conditions of external origin should be defined by CCITT. In relation to Reference loads A and B, the parameters may inc
22、lude: number of call attempts, call-mix, duration and repetition frequency of the overload conditions in the busy hour. The overload conditions may also be defined indirectly as the periods of time during which, with no actions undertaken by the exchange, one or more of the performance objectives sp
23、ecified for Reference load B would not be met. Overload conditions may affect the whole exchange (general overload) or only parts of it (imbalanced/focus- sed overload). General overload may occur because the number of call attempts offered to the whole exchange reduces increasingly the processing c
24、apacity of the exchange. Imbalanced or focussed overload may occur when e.g. an abnormal number of outgoing calls are directed to a limited number of lines or trunks. In the absence of suitable control mechanisms, the onset of overload results in a rapid degradation of exchange performance, which ma
25、terialises as a sharp increase in process response times, leading therefore to reduced call-handling capability. Wherever, there is a mismatch between the processing capacity available and the call attempts offered to the exchange, due either to equipment malfunction or modified subscriber behaviour
26、, it is therefore necessary to reduce the demands made on the exchange. Overload can therefore be defined as the point at which post selection delays have become so great as to cause callers to clear, anticipating a call failure. At, or just before this point, which might exceed the Reference B leve
27、l of performance, a control mechanism is required. Actual behaviour and ideal behaviour of an exchange is represented by the general curve given in Fig- ures 1 and 2 (T/CS 68-04). This curve includes two regions which are referred to as quasi linear region and non linear region. Load A point belongs
28、 to the quasi linear region. Load B point may belong to the upper part of the quasi linear region or extend marginally in the non linear region. If no actions were taken by the exchange, under overload conditions the following consequences might take place : - increase of the call attempt inadequate
29、ly handled and of the delay probability for the call attempts - decrease of the call processing performance; - progressive decrease of the total call attempts handled by the exchange. In order to prevent and to limit these degradations, the exchange should provide the supervisions and automatic proc
30、edures (overload control) described below. The mean number of the call attempts handled in an overload condition should not be lower than that one corresponding to the system capacity in the trafic conditions assumed for the exchange engineering. For the call attempts handled in an overload conditio
31、n, the exchange should maintain a performance objective as close as possible to that specified for Reference load B in Sections 2.2, 2.3 and 2.4. The extent to which the above is possible, in relation to the number of calls offered and performance objective that should be maintained, depends on the
32、philosophy that might be considered for handling calls. One possibility would be for the exchange to handle a minimum number of calls, e.g. corresponding to load B, whatever the priority level of calls. Another situation could be based on a rejection of non priority calls leading ultimately to a num
33、ber of handled calls which may fall below a certain level and a level of performance which may exceed that for Reference load B. This requires further study. 3.3 Behaviour of an exchange handled ; 3.4 Grade of service Edition of September 15, 1986 2326434 0005278 3 M TICS 68-04 Page 4 Calls/s Proces
34、sed Calls/s Processed / Quasi linear I / region I Figure 1 (T/CS 68-04). Possible actual behaviour of an exchange with overload control. Figure 2 (T/CS 68-04). Ideal behaviour of exchange. 3.5 Overload indications Overload indications may be provided by: a continuous measurement of the occupancy of
35、the resources used for call handling during short observation time (e.g. few seconds); a supervision of the filling degree of the waiting input buffers to the various call handling processes, etc. Overload indications should be given to the maintenance staff. 3.6 Overload protection The control meth
36、od is dependent on the particular technical arrangement of the switching system and is not subject of international Recommendations. Edition of September 15, 1986 232b414 0005279 5 TICS 68-04 Page 5 When the exchange detects an overload condition and before the actions specified hereafter are undert
37、aken, the system should perform appropriate adaptation of processing priority of tasks to the new situation. Different parts of the exchange may be at the same time under different operational phases; however, the actions applied to each part of the exchange should be progressively eliminated in the
38、 reverse sequence in relation to the decrease of the overload condition. For any type of switching system architecture, the exchange should avoid to route further call attempts on a particular functional block in overload conditions from other functional blocks of the same exchange. In order to meet
39、 the specified performance requirements the call processing system may need to enter different operational phases as follows: Phase I : All seizure attempts are acknowledged and handled. The exchange can defer some or all the activities non essential to handling the trafic offered such as administra
40、tion maintenance processes (routine controls, automatic or operator handled tests on subscriber lines and circuits, etc.) and operational processes, (trafic measurements and other exchange measurements, call tracings etc.). Nevertheless the man-machine communications essential for priority operation
41、al tasks should always be preserved. Phase 2: All seizure attempts are acknowledged but a limited amount handled. Priority seizures are handled. When the preliminary actions of Phase 1 are not enough to absorb the overload condition, the exchange should adapt the trafic sources to its present call h
42、andling capacity. This is obtained by performing a discrimination of the call attempts to be handled on a priority basis. At least priority trafic should be handled. On the trunk side successive seizure of a number of idle circuits should be prevented. This can be done e.g. by blocking idle incoming
43、 circuits in the backward direction, or by sending an appropriate message to the distant exchange. Phase 3: Only a limited amount of seizure attempts are acknowledged or handled. No priority treatment is given. 4.1, add after the last sentence: See also Recommendation T/CS 10-13 3. 5, add after last
44、 sentence: Note: CO might not be a constant but depend on BHCA and trafic characteristics. REFERENCES i 2 3 CCITT Recommendation Q.504. Performance and availability design objectives for digital transit exchanges. T/CS 68-01 Recommendation. Introduction,$eld of application and basic functions for digital transit exchanges. T/CS 10-1 3 Recommendation. General maintenance concept and terms.
