Introduction

Quality of service in circuit-switched public data networks has been considered in five basic areas as follows:

i) call processing delays (Recommendation X.130);

ii) failures due to congestion (blocking) (Recommendation X.131);

iii) failures due to malfunction;

iv) loss of service; and

v) transmission performance (including throughput).

This Recommendation specifies the objectives for i) above. Each of the other areas of circuit-switching quality of service identified above will be the subject of a separate Recommendation in the X-series.

In telecommunication networks it is necessary, for economic reasons, to limit the resources provided for carrying the offered traffic. This limitation may affect the quality of service to the user of circuit-switched services in two different ways: by call processing delays and by blocking. Both of these aspects, that are consequences of the finite traffic handling capacity of the network, constitute the grade of service. Grade of service together with malfunction, loss of service and transmission performance constitute the quality of service.

In this Recommendation the values for the network delay are quoted for two types of connection according to Recommendation X.92 as follows:

Type 1: Typical terrestrial international connection of moderate length with no satellite circuits either in the national or international portions (International portion: 1000 km).

Type 2: Long distance international connection with a satellite circuit in one national portion and two satellite circuits in the international portion (International portion: 160 000 km).

Where appropriate, values are also specified separately for the following network portions:

- originating national network,

- international portion,

- destination national network.

The boundaries for these portions are shown in Figure 1/X.130.

For the present, the values apply also to other normal routing options within the international portion.

Following the allocation of a delay allowance to the international portion of an international transit connection, it will be necessary to further apportion the allowance to individual transit networks and/or their component parts within the international portion. The means by which useful and realistic constraints can be applied, consistent with maintaining the maximum possible freedom for each involved Administration in the design and implementation of its own network, is for further study.

The values for call processing delays established in this Recommendation are to be considered as design objectives in network planning together with the forecast traffic for the planned period. The actual delay performance that will be obtained will depend on the accuracy of the traffic estimations. Normally, the actual delay performance will not coincide with the one used as a basis for planning. Furthermore, if the network is planned for the traffic forecast at the end of the period considered, the actual delay performance of the network may be better than the design value, worsening gradually to the end of the planning period as traffic increases.

The non-coincidence of busy hours in originating and destination national networks as well as in the international network will improve the overall delay performance with respect to the sum of the nominal delays of the constituent parts of the connection.

Delays are specified under conditions of normal busy hour load and are expressed where appropriate in terms of mean and 95% probability values. The term "mean" is taken to be the expected value of delay in the statistical sense. The "95% probability" value is taken as the limit within which 95% of the delays fall. Delays at higher loadings are for further study.

Call processing delays are defined for a basic call which does not include any optional user facilities, e.g. those defined in Recommendation X.21.

Where appropriate, separate limits are quoted for common channel signalling and channel associated signalling between DSEs. For common channel signalling, the values given in this Recommendation are also applicable to lower signalling rates (less than 4800 bit/s), when the associated mode of operation is used.

The quality of service implications of regional or national satellite systems using demand assignment for resource allocation require further study.