Glossary of Asterisk & Telecom Terms

FXO

Foreign eXchange Office

An FXO device can be an analog phone, answering machine, fax, or anything that handles a call from the telephone company like one. They should also operate the same way when connected to an FXS interface.

An FXO interface will accept calls from FXS or PSTN interfaces. All countries and regions have their own standards.

FXO is complimentary to FXS (and the PSTN).

FXS

Foreign eXchange Station

An FXS device has hardware to generate the ring signal to the FXO extension (usually an analog phone).

An FXS device will allow any FXO device to operate as if it were connected to the phone company. This makes your PBX the POTS+PSTN for the phone.

The FXS Interface connects to FXO devices (by an FXO interface, of course).

PSTN

Public Switched Telephone Network

i.e. the phone service we use for every ordinary phone call.

ADSI

Analog Display Service Interface

A complex set of standards for the telecom industry. Built off of FSK keying used by CallerID, ADSI is capable of remotely controlling a screenphone with softkeys. Effectively, session based applications can be used when the phone is online, or scripts can be preprogrammed into the phone for when no ADSI connection has been established (on or off hook). Originally, this technology was made by Telcos who thought they would use it to offer services to residential customers. They envisioned such features and buying airplane tickets from your screenphone. Adoption has been mixed. In order to protect their interests, all phones appear to be locked with a programming "code" to prevent users from using one phone provided by one company for a competing service. As such, it can be difficult to get a phone that you can program for yourself (if you have the software). Needless to say, getting codes for an existing phone is nearly impossible (it is incomprehensible to the support staff that anybody but a telco will be programming said phones). Rather, you need to order a phone specifically because you already have the code (eventually you can find a contact that will help you here).

ISDN

Integrated Services Digital Network

A set of communications standards to carry voice, digital network services. ISDN consists of a D channel that transmits data about the connections on a ISDN line, and any number of B channels that bear the data or voice traffic.

PRI

Primary Rate Interface

PRI is a standard network interface consisting of 1 D channel and 23 B channels on a T1, or 1 D channel and 30 B channels on a E1.

BRI

Basic Rate Interface

BRI is a standard network interface consisting of 1 D channel and 2 B channels.

VoIP

Voice over IP

VoIP encompasses many protocols. All the protocols do some form of signalling of call capabilities and transport of voice data from one point to another. Examples are SIP, H.323, IAX, and IAX2.

MSN

Multiple Subscriber Line

This is a telephone number associated with an ETS 300 BRI line. Providers of ETS 300 often give you three MSNs with a BRI, although additional MSNs can be purchased. An ISDN terminal will "ring" (provide an alerting signal) only when calls are made to the MSN (or MSNs) entered in that terminal. If a terminal has no MSNs entered it will "ring" whenever there is a call to any of the MSN.s on that BRI.

DID

Direct Inward Dialing

Direct Inward Dialing. The ability for an outside caller to dial to a PBX extension without going through an attendant or auto-attendant.

DTMF

Dual Tone Multi Frequency

The standard tone-pairs used on telephone terminals for dialing using in-band signaling. The standards define 16 tone-pairs (0-9, #, * and A-F) although most terminals support only 12 of them (0-9, * and #).