End office

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In conventional wired telephony, an end office is the service provider location at which the local loop wiring from customer premises physically terminated, converted into digital signals if not already in digital form, and sent to a telephone circuit switch. That switch can "hairpin" calls back to local loops if the destination is served by the same end office. More commonly, the switch puts telephone calls onto multiplexed transmission facilities, either at a peer level (i.e., within the same local calling area) or to hierarchically higher switches that interconnect local calling areas, regions inside a country, and internationally.

While some end office switches, as for rural locations, may only be able to connect to hundreds or low thousands of line, it is relatively common for a switch to be able to terminate 10,000 local lines. There may be multiple switches in an end office building, but, for reasons of fault tolerance, more than 50,000 lines rarely terminate in the same physical end office.

With convergence of communications, the same basic functions of multiplexing, switching, and demultiplexing take place, but the switch is all digital. The subscrier local loops are digital and also carry television and data service.


Assuming copper local loops, they are mechanically connected to positions on the "outside" of a main distribution frame. Older frames used screw terminals, while more modern MDFs use "punch-down" insulation displacement tools to force the wire into a conducting slot. Each local loop has a pair of copper wires.

From the MDF, in a reasonably modern, non-VoIP end office, groups of 24 (North America and parts of Asia) or 32 circuits go from the "inside" of the MDF. Wired to the inside are electronic assemblies called channel banks'. The channel banks digitize the analog signals, and put them into digitally multiplexed subchannels of an intraoffice trunk. The intraoffice trunk may run directly to a switch, or it may go through an intermediate "electronic patch panel" called a digital access carrier system (DACS), which allows moving the different per-local loop signals into different trunks.

The DACS does not operate at a per-call basis, and it is incompatible with digital subscriber loops (DSL) for running data over the local loops. If DSL s in use, then there must be another device, called a DSL access multiplexer or DSLAM, that pulls off the DSL signals before they are lost in the channel bank.