In modern
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...
a remote concentrator, remote concentrator unit (RCU), or remote line concentrator (RLC) is a
concentrator at the lowest level in the
telephone switch
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
hierarchy.
Subscribers' analogue telephone/
PSTN
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists ...
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.
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Only a few hundred telephone lines attach to each remote concentrator. In North America concentrators are located in a serving area interface
The serving area interface or service area interface (SAI) is an outdoor enclosure or metal box that allows access to telecommunications wiring.
Alternate names
*Access point (AP)
*Cabinet (cab)
*B-box (breakout box)
*Cross box
*Cross-connect b ...
(SAI) or other enclosure
Enclosure or inclosure is a term, used in English landownership, that refers to the appropriation of "waste" or "common land", enclosing it, and by doing so depriving commoners of their traditional rights of access and usage. Agreements to enc ...
in each neighborhood. In Europe the buildings which once contained local Strowger switch
The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Based on its ...
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
The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of Information and communications technology, information and communications. ETSI supports the de ...
as the V5 interface.
Operation
When a user picks up their phone, the concentrator produces the dial tone
A dial tone (dialling tone in the UK) is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is ...
. 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.
Technology implications
Having a concentrator near a subscriber's telephone results in very low signal degradation before the analog signal is digitized. This provides reliably good voice quality.
Concentrators are often placed alongside a digital subscriber line access multiplexer
A digital subscriber line access multiplexer (DSLAM, often pronounced ''DEE-slam'') is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital co ...
(DSLAM). This can provide access to asymmetric digital subscriber line
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide ...
(ADSL) Internet service for subscribers who are beyond the normal signaling limit on a copper wire loop. For example, a fiber optic cable might run up to without a repeater from the telephone exchange to a concentrator site, and local subscriber wire local loop
In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the co ...
s can extend an additional 4 km beyond the concentrator and its DSLAM. With repeaters in the fiber optic cable the distance from the telephone exchange can be extended much farther.
See also
*Remote digital terminal
In telecommunications
Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication techn ...
* Distributed switching
* Pair gain
*Access network
An access network is a type of telecommunications telecommunications network, network which connects subscribers to their immediate telecommunications service provider, service provider. It is contrasted with the core network, which connects l ...
References
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Local loop