Company normally requires that the CO digital switching equipment shall operate with a very high degree of reliability. The effective performance of the telephone exchange is depended on its product specific properties as well as on the properties regarding the operational, environmental and maintenance conditions under which it is used. The objective of this section is to assure that systems meet satisfactory reliability performance standards without imposing undue constraints on the design of the system.
Reliability involves redundancy of hardware so that failures of individual components do not cause widespread failure, together with software and operating systems that enable the system to take proper action to overcome faults.
Reliability of Digital Switching Equipments
The design of the system shall include reliability features, including redundancy as appropriate in all functional subunits. Equipment of vital importance shall be duplicated. It shall be possible to estimate the annual failure rate of the total volume of components of a local exchange by means of a formula that incorporates the number of subscribers, total number of trunks and other parameters.
Since the reliability performance of a switching system is a function of the specific system architecture, the manufacturer shall explain how his reliability predictions were analyzed and calculated.
A block diagram representing the specific system architecture with all blocks named, shall illustrate the failure relationships between them, the failure probabilities for each block, and a representation of overall system reliability.
Maximum acceptable values are given in the following table:
Details of faults that could cause degradation in service to one-half or more of all served subscribers shall be provided. Switches shall be designed to include safeguards and checks against possible incorrect actions by the maintenance staff, particularly during system reconfiguration. The details of these safeguards and checks shall be clearly explained.
5.40.2 Mean Time between Failure (MTBF)
The expected maximum monthly circuit board replacement rate shall be one (1) circuit module per 1000 subscribers per month. Hardware maturity shall occur within the first twelve (12) months of service or six (6) months after final acceptance of the system.
The hardware’s expected Mean Time Between Failure (MTBF) shall not exceed 1.6/n months (with “n” equal to the number of subscriber lines in the CO digital exchange divided by 1000).
5.40.3 Mean Time to Repair (MTTR)
The manufacturer shall indicate the mean time to repair a fault, based on the availability of spares at the rate he proposes and the location of the fault in relation to:
a. The location where the spare parts are available.
b. The location of the personnel necessary to identify and repair the fault.