No. 5 ESS is a stored program controlled digital switching system using distributed processing to provide smooth growth across the entire central office line range. The No. 5 ESS maintenance hardware and software is designed to meet all Bell System reliability and maintenance requirements. New approaches for maintenance software design have been required to accomplish this in a distributed processing environment. The responsibility for carrying out maintenance functions is distributed so that major functional units are almost totally responsible for their own maintenance. This allows system evolution to take place with minimum impact on the overall software structure and also allows loose coupling of system elements. Loose coupling means that relatively slow control channels can be used for intrasystem communication. All of these features facilitate the remoting of major functional units away from No. 5 ESS. Switch maintenance, terminal maintenance, system integrity, and human-machine interface software are present in each of No. 5 ESS's distributed processors. Software in these processors is decomposed into small, easily understood components with well-defined interfaces. Operating systems are used to switch messages between software processes and to provide common services for the maintenance subsystems. Most of the software is written in the C high-level language. This simplifies software development by allowing the same coding language to be used for all system processors and provides future code portability when new generations of processors are introduced into the system. The No. 5 ESS maintenance software architecture and design approach helps ensure a high-quality end product. It should also minimize future effort needed to introduce new hardware, services, and features into the system.

Range extension allows the No. 2 electronic switching system (No. 2 ESS) to serve subscriber loops out to 2500 Ω as part of the Bell system's unigauge plan. Range-extender circuits provide both audio and dc gain for loops greater than 1300 Ω. These circuits have been designed specifically to be low cost, as small as possible, and to use minimum power. Network concentration allows 70 percent fewer range extenders to be provided than customer lines served. All trunk, service, and test circuits that had to be modified to work range extended are provided on a standard basis in every No. 2 ESS office. This means that range extension can be provided in a No. 2 ESS office at very low cost; essentially the cost of the range-extender circuits.

The choice of fault detection methods to be used in control and data transfer circuits is strongly dependent on system organization. This paper presents a unified parity check scheme for both.