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Glossary of Terms

ACDAutomatic Call Distributor

A device or system that distributes incoming calls to a specific group of terminals that agents use. It is often part of a computer telephony integration (CTI) system.


A software library that contains the algorithms necessary to convert an analog signal to and from a digital one. 

FXOForeign Exchange Office

A device usually found on the customer end that is powered by the channel and can interface into the telephone company's network. Digium makes FXO modules that interface with PSTN lines using FXS signalling in either Loopstart(fxs_ls) or the more common Kewlstart(fxs_ks) modes. 

FXSForeign Exchange Station

A device usually located on the telephony company's property, a FXS device send power through a channel to a phone on the other end. Digium makes FXS modules that interface with PSTN phones using FXO signalling in either Loopstart(fxo_ls) or the more common Kewlstart(fxo_ks) modes. 


An uncompressed codec that samples a 64kbps channel at 8 bits per sample using pulse code modulation. The Two varients of G.711 are known formally as uLaw and aLaw.


The G.729 codec is an industry standard which allows for stuffing more calls in limited bandwidth to utilize IP voice in more cost effective ways. A typical call consumes 64Kbps of voice bandwidth. G.729 reduces the call to 8Kbps (normal IP overhead adds to this number). Many people are using Asterisk with G.729 to replace expensive gateways.

GSMA compressed speech codec that uses a rate of 13 kbps. 


A VOIP protocol that was deployed early and is widely adopted. 

IAXInter-Asterisk eXchange

A VOIP protocol designed to be much more NAT friendly. IAX currently only transports audio.

IVRInteractive Voice Response

An automated voice system that allows callers to navigate a phone system and be directed to the correct extension by pressing a series of numbers on a tuch-tone phone. (I.E. Push 1 for sales, push 2 for support, etc..)MGCPMedia Gateway Control Protocol
A VOIP Protocol that has both signaling and control and was designed to reduce complexity between media gateways.

PRIPrimary Rate Interface

A PRI is a truly digital circuit, containing 24 ISDN channels. One of these channels is a dchannel and used for signaling. The rest are bchannels and used to transport audio.

PSTNPublic Switched Telephone Network

This refers to the analog telephone network that's very common in residential networks. PSTN lines are usually much cheaper then T1 lines, but only provide one channel. Digium's FXO and FXS modules interface with this network. 


RENRinger Equivalency Number

A number which represents the ringer loading effect on a line. A ringer equivalency number of 1 represents the loading effect of a single traditional telephone set ringing circuit. Most modern telephones probably will have a REN lower than 1. The total REN expresses the total loading effect of the equipment on the ringing current generator (FXS). The REN of a Digium FXS board is 5 (representing "extension," i.e., parallel-connected telephones). The actual number of devices on the line may be greater than the REN limit, if their respective individual RENs are less than 1.

TDMTime Division Multiplexing

A processes of splitting one medium into two or more channels by using timed segments to transmit information.TranscodeThe process of converting a channel with one type of encoding to a different type of encoding in real time.

 VoIPVoice Over Internet Protocol

A general method for transporting voice through the internet. VOIP commonly refers to the general method and not a specific protocol. SIP is currently the most widely used VOIP protocol. 

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