Remote Concentrator

In modern telephony a remote concentrator, Remote Concentrator Unit (RCU), or Remote Line Concentrator (RLC) is the lowest level in the telephone switch hierarchy.

Subscribers' analogue telephone/PSTN lines are terminated on concentrators. They have three main functions, namely:

• Digitize, that is, turn voice, and sometimes data, from analogue into a digital form.
• Connect off-hook lines through to the local exchange — the concentration function.
• Multiplex, interleaving many calls together on a single wire or optical fiber.

Only a few hundred telephone lines attach to each remote concentrator. In North America, concentrators are located in an SAI or other enclosure in each neighborhood. In Europe, the buildings that once contained local Strowger switch telephone exchanges are now usually empty except for a remote concentrator.

Only call packets from or destined to a phone serviced by the concentrator actually are processed by the concentrator — nonlocal phones' time slots just pass through the concentrator unchanged. If the concentrator malfunctions, a fail-safe relay connects the "in" wires to the "out" wires, and nonlocal phones detect no difference. The central switch periodically counts concentrators, and schedules maintenance, probably before users 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.

When a user picks up the phone, the concentrator produces the dial tone. When the user dials, it reads the tones. Once the user has completed 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.

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.

To arrange a connection, the switch just completes the circle between the user's phone and the remote phone. It interchanges the data from one to the other. In this limited sense, telephone "exchange" is exactly correct terminology.