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Remote concentrator

In modern telephony a remote concentrator is the lowest level in the telephone switch hierarchy. Up to several hundred telephones attach to a remote concentrator. In the U.S., you and your neighbors share a concentrator in a little box near your houses. In Europe, the building that once contained your local Strowger telephone exchange is usually empty except for a remote concentrator -- the switching mostly occurs elsewhere.

The wire coming into the concentrator is to the local phones, and the wire leaving it is from the local phones. Nonlocal phones' time slots just pass through the concentrator unchanged. If the concentrator breaks, a fail-safe relay shorts the in wires to the out wires, and nonlocal phones detect no difference. (The central switch periodically counts concentrators, by the way, and schedules maintenance, probably before you notice the failure.) Concentrators for several hundred customers can be threaded on this loop like pearls.

The interface between remote concentrators and their parent telephone switches has been standardised by ETSI as the V5 protocol.

Note: please check that the V5 description matches the interface description above

When you pick up the phone, the concentrator produces dial tone. When you dial, it reads the tones. When you're done dialing, the concentrator's microcomputer sends the dialing data to the central switch, which allocates a time slot for the dialing phone on the wire pairs that pass through the concentrator and through the switch.

The trick is that after the central switch tells the concentrator which time slot to use, the concentrator "opens" a time-slot on the loop to a local phone. The allocated time slot on the wiring into the concentrator is used to send data from the remote telephone's microphone to the local telephone's speaker. The allocated time slot on the wiring out of the concentrator (with the same time slot number) carries data from the local microphone to the remote speaker.

So, to arrange a connection, the switch just completes the circle between your phone and the remote phone. It interchanges the data from one to the other. Telephone "exchange" is exactly correct terminology.