ITU-T I.355

The purpose of this Recommendation is to define parameters and objectives describing availability performance of the following 64 kbit/s ISDN connection types: Packet-Switched Connection Type (PSCT); Circuit-Switched Connection Type (CSCT); Dedicated-Circuit Connection Type (DCCT). Connection types are categorized according to their characteristics into different portion types, with Measurement Points (MPs) delimiting each portion. Parameters defined are Connection Portion Availability (A) and Mean Time Between Connection Portion Outages (MO). Availability performance is described by specifying worst-case objectives for these parameters for each portion type. These objectives are intended to assist in network design and planning by limiting the aggregate effect of network impairments, including blocking, equipment failures and transmission errors, on ISDN network availability. In order to define the availability function for the different connection types, the approach taken is to specify a set of performance decision items and associated outage criteria. A two-state availability model is used whereby the availability state of a portion over a specific measurement interval is determined by comparing observed performance with the particular outage criteria. Guidance is provided on practical measurement of the parameters, including statistical techniques for sampling. Examples are also included illustrating how representative end-to-end availability performance can be calculated using the observed performance of the individual portions.