5ESS Switching System

Each Switching Module (SM) handles several hundred to a few thousand telephone lines or several hundred trunks or combination thereof. Each has its own processors, also called Module Controllers, which perform most call handling processes, using their own memory boards. Originally the peripheral processors were to be Intel 8086, but those proved inadequate and the system was introduced with Motorola 68000 series processors. The name of the cabinet that houses this equipment was changed at the same time from Interface Module to Switching Module.

Peripheral units are on shelves in the SM. In most exchanges the majority are Line Units (LU) and Digital Line Trunk Units (DLTU). Each SM has Local Digital Service Units (LDSU) to provide various services to lines and trunks in the SM, including tone generation and detection. Global Digital Service Units (GDSU) provide less-frequently used services to the entire exchange. The Time Slot Interchanger (TSI) in the SM uses random-access memory to delay each speech sample to fit into a time slot which will carry its call through the exchange to another or, in some cases, the same SM.

T-carrier spans are terminated, originally one per card but in later models usually two, in Digital Line Trunk Units (DLTU) which concentrate their DS0 channels into the TSI. These may serve either interoffice trunks or, using Integrated Subscriber Loop Carrier, subscriber lines. Higher-capacity DS3 signals can also have their DS0 signals switched in Digital Network Unit SONET (DNUS) units, without demultiplexing them into DS1. Newer SM's have DNUS (DS3) and Optical OIU interfaces (OC12) with a large amount of capacity.

SMs have Dual Link Interface (DLI) cards to connect them by multi-mode optical fibers to the Communications Modules for time-divided switching to other SMs. These links may be short, for example within the same building, or may connect to SMs in remote locations. Calls among the lines and trunks of a particular SM needn't go through CM, and an SM located remotely can act as distributed switching, administered from the central AM. Each SM has two Module Controller/Time Slot Interchange (MCTSI) circuits for redundancy.

In contrast to Nortel's DMS-100 which uses individual line cards with a codec, most lines are on two-stage analog space-division concentrators or Line Units, which connect as many as 512 lines, as needed, to the 8 Channel cards that each contain 8 codecs, and to high-level service circuits for ringing and testing. Both stages of concentration are included on the same GDX (Gated Diode Access) board. Each GDX board serves 32 lines, 16 A links and 32 B links. Limited availability saves money with incompletely filled matrixes. The Line Unit can have up to 16 GDX boards connecting to the channel boards by shared B links, but in offices with heavier traffic for lines a lesser number of GDX boards are equipped.

ISDN lines are served by individual line cards in an ISLU (Integrated Services Line Unit).

The Administrative Module (AM) is a dual-processor mini main frame computer of the AT&T 3B series, running UNIX-RTR. AM contains the hard drives and tape drives used to load and backup the central and peripheral processor software and translations. Disk drives were originally several 300 megabyte SMD multi-platter units in a separate frame. Now they consist of several redundant multi-gigabyte SCSI drives that each reside on a card. Tape drives were originally half inch open reel at 6250 bits per inch, which were replaced in the early 1990s with 4 mm Digital Audio Tape cassettes.

The Administrative Module is built on the 3B21D platform and is used to load software to the many microprocessors throughout the switch and to provide high speed control functions. It provides messaging and interface to control terminals. The AM of a 5ESS consists of the 3B20x or 3B21D processor unit, including I/O, disks, and tape drive units. Once the 3B21D has loaded the software into the 5ESS and the switch is activated, packet switching takes place without further action by the 3B21D, except for billing functions requiring records to be transferred to disk for storage. Because the processor has duplex hardware, one active side, and one standby side, a failure of one side of the processor will not necessarily result in a loss of switching.

The Communications Module (CM) forms the central time switch of the exchange. 5ESS uses a time-space-time (TST) topology in which the Time-Slot-Interchangers (TSI) in the Switching Modules assign each phone call to a time slot for routing through the CM.

CMs perform time-divided switching and are provided in pairs; each module (cabinet) belonging to Office Network and Timing Complex (ONTC) 0 or 1, roughly corresponding to the switch planes of other designs. Each SM has four optical fiber links, two connecting to a CM belonging to ONTC 0 and two to ONTC 1. Each optical link consists of two multimode optical fibers with ST connectors to plug into transceivers plugged into backplane wiring at each end. CMs receive time-multiplexed signals on the receive fiber and send them to the appropriate destination SM on the send fiber.

The Very Compact Digital Exchange (VCDX) was developed with the 5ESS-2000, and marketed to mostly non-Bell telephone companies as an inexpensive, effective way to offer ISDN and other digital services in an analog switching center. This avoided the capital expense of retrofitting the entire analog switch into a digital one to serve all of the switch's lines when many wouldn't require it and would remain POTS lines.

An example would be the (former) GTE/Verizon Class-5 telephone switch, the GTD-5 EAX. Like the Western Electric 1ESS/1AESS, it served mostly medium to large wire centers.

The standalone VCDX was also capable of serving as a switch for very small wire centers (a CDX- Community dial office) of fewer than ~400 lines. However, for small wire centers, 400-4000 lines, that function was usually served by RSM's, a 5ESS "Remote SM", ORM's or Wired ORM's. The RSM is controlled by T1 lines connected to a DLTU unit. The first 2 T1's are the control of the RSM and are necessary for any Recent Changes to take place. RSM's can have up to 10 T1's. There can be multiple RSM's in an office. An ORM can be fed via direct fiber or via coax thus called Wired ORM's. An RSM or ORM can have many of the same peripheral units that are part of a full 5ESS switch. An RSM has a limited distance and can serve parts of a larger metro area or rural offices. An ORM or wired ORM can be anywhere technically, and preferred over the RSM once the ORM became available. Both the RSM and ORM is often used as a Class-5 wire center for small to medium towns hosted from a 5ESS located in a larger city. The Wired ORM is connected via coax from a MUX unit and fed to a TRCU which converts the coax to connection to the DLI, There was also a two-mile ORM that was used when an office was broken out or took an area from another office. The distance on this was 2 miles from a host office and fed direct via fiber. As with any SM, the size is dictated by the number of time slots needed for each peripheral unit. ORM's are linked with DS3, RSM's are linked with T1 lines. The VCDX was also used as a large private branch exchange (PBX). Small communities of less than 400 lines or so were also provided with SLC-96 units or Anymedia units.

The standalone VCDX has a single Switching Module, and no Communications Module. Its Sun Microsystems SPARC workstation runs the UNIX-based Solaris (operating system) that executes a 3B20/21D processor MERT OS emulation system, acting as the VCDX's Administrative Module. The VCDX uses the CO's normal telephone power sources (which are very large uninterruptible power supplies), and has connections to the CO Digital cross connect system for T1 access, etc.

The 5ESS has two different signaling architectures: Common Network Interface (CNI) Ring and Packet Switching Unit (PSU)-based SS7 Signaling.