Editing Remote concentrator (section)

{{refimprove|date=December 2007}}
In modern [[telephony]] a '''remote concentrator''', '''remote concentrator unit''' ('''RCU'''), or '''remote line concentrator''' ('''RLC''') is a [[concentrator]] at the lowest level in the [[telephone switch]] hierarchy.

Subscribers' analogue telephone/[[Public switched telephone network|PSTN]] lines are terminated on concentrators. They have three main functions:
* Digitize: convert voice (and sometimes data) from analogue to a digital form.
* Connect off-hook lines to the local exchange—the concentration function.
* Multiplex, interleaving many calls together on a single wire or optical fiber.<ref>[https://web.archive.org/web/20101101172342/http://www.btplc.com/21CN/Thetechnologyofthenetwork/Glossary/Glossary.htm BT 21CN technology Glossary] via Web Archive</ref>

Only a few hundred [[telephone lines]] attach to each remote concentrator. In North America concentrators are located in a [[serving area interface]] (SAI) or other [[Enclosure (electrical)|enclosure]] in each neighborhood. In Europe the buildings which once contained local [[Strowger switch]] telephone exchanges are now usually empty except for a remote concentrator.{{Citation needed|date=September 2015}}
[[File:VZ SAI workers Secaucus jeh.JPG|thumb|This [[Verizon Communications]] SAI in New Jersey may contain a concentrator.]]

Only call [[Network packet|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 interface]].