Editing 5ESS Switching System (section)

===Switching Module===
Each Switching Module (SM) handles several hundred to a few thousand telephone lines or several hundred [[Trunking#Telecommunications|trunk]]s or combination thereof. Each has its own [[CPU|processors]], also called Module Controllers, which perform most [[Signaling (telecommunications)|call handling processes]], using their own [[computer memory|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.{{citation needed|date = June 2019}}

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 [[Digital Signal 3|DS3]] signals can also have their DS0 signals switched in Digital Network Unit [[SONET]] (DNUS) units, without demultiplexing them into [[Digital Signal 1|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 fiber]]s 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 card]]s with a [[codec]], most lines are on two-stage [[analog electronics|analog]] space-division concentrators or ''Line Units'', which connect as many as 512 lines, as needed, to the 8 ''Channel card''s 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).