ides a set of services that are available to the software layers provided with SLM. above it. As one moves up through the hierarchy, the software provides more advanced levels of operation. For example, at the bottom of the hierarchy is the layer which 32.15 DMS-Bus provides the basic tools required by the Support Operating System (SOS) and thus provides a set of services for all the layers above it. DMS-Bus is perhaps the key architectural innovation in DMS The top layer of the hierarchy contains the various features, such as SuperNode. It connects all system components in a uniform manner the Traffic Operator Position System (TOPS). TOPS is an integrated to give flexibility for future evolution. Because all sub-systems are operator call handling system that operates in conjunction with a connected directly or indirectly to ports on DMS-Bus they can DMS-100. communicate freely with one another by having DMS-Bus switch In addition to this functional layering, the software is also or- messages between its input and output ports. A sub-system wishing ganised into a hierarchy of modules in four layers: to communicate simply creates a message and sends it in the appropriate format to DMS-Bus which then routes it to the required 1. Area; a product line. destination. 2. Subsystem; a unit of software delivery. Assuming an average message length of 64 bytes, one fully 3. Module; a unit of loading. duplicated DMS-Bus module is capable of handling about 125K 4. Section; a unit of compilation. messages per second. The DMS-Bus comprises two Message Switches (MS), each Most of the control software created for the first generation having its own processor with associated memory and software. A system did not need to be modified for DMS SuperNode except to copy of the MS load forms part of the Office Image. The two MS remove processor dependencies and to introduce new features to operate in load sharing mode but either is capable of handling all the take advantage of the new processor. A proprietary Operating Sys- message traff