The 4ESS Switch is often considered as a switching network and its controlling processor. The major functional equipment areas include: • 1B processor, the 4ESS primary controlling processor • 3B computer and associated attached processor system, which provides disk storage and additional functions • CNI ring, a group of peripheral processors serially interconnected with each other in a dual ring configuration, and a 3B20D processor that functions in a distributed input/output processing architecture. • Terminal equipment provides the interface between metallic trunk facilities in the toll network and the 4ESS Switch. Two-wire and 4-wire voice-frequency trunks terminating at metallic terminal facilities and equipment which provides an interface between digital carrier equipment and the 4ESS switching network. • The switching network contains the equipment which actually switches pulse code modulated data from one trunk or service circuit to another. The switching network also contains timing equipment that generates the precise timing signals required to switch traffic. The switching network is also used to connect test equipment, announcements, and various tones to trunks when required.
Processor The processor acts as the
CPU for the switch. The processor includes a central control, call stores, and program stores. In addition it had access to additional units through the auxiliary unit bus (AUB) and peripheral unit bus (PUB). A master control console (MCC) provides office technicians access to the switch through the processor peripheral interface (PPI). Early versions used the same 1A processor as the contemporaneous improved
1AESS switch. All existing switches have been subsequently upgraded to use the 1B processor.
File store and CNI ring The file store provides long term storage (
disk storage) of the processor programs (program store) and office data (call store). It was first implemented using disk technology but was replaced by the 4E attached processor system (4EAPS). The 4EAPS is a
3B computer running 4EAPS application software on the
DMERT operating system. The 4EAPS interfaces to the 4ESS processor via the attached processor interface (API) units. The "1A file store" became partitions on the 3B computer disks. At first the 4EAPS just provided "file store" but soon it also provided access to the
common-network interface ring (CNI ring) to provide
common-channel signaling (CCS). The 4EAPS originally used the
3B20D computer. These were all converted to the
3B21D around 1995.
Peripheral units The peripheral units include units that interface to the central control over the peripheral unit bus. This includes the common channel interface signaling (CCIS) terminal, signal processors,
time-slot interchanges (TSI) and time multiplexed switches (TMS). It also includes equipment not directly on the PUB including terminating equipment used to connect the switch to the transport network and the TSIs and TMSs, which actually perform the "time-space-time" switching function. Timing is provided by a high speed, high accuracy network clock. ==History==