Telephony Glossary

FXO and FXS Ports

Foreign eXchange Office (FXO) and Foreign eXchange Station (FXS) are typically RJ11 ports, the type you see on a PSTN analog line. FXO is a port connected to the analog telephone line provided by the telco. FXS is a port that typically connects to an analog phone. A modem provides an FXO port.

T1 and E1

T1 and E1 are digital circuits that can provide PRI functionality, point to point network connectivity or Internet connectivity with business quality in mind.


The power of Asterisk is in the dialplan, which consists of statements that define what happens in the phone system. What happens when someone calls extension 8000? What about an IVR menu? The dialplan defines all these features of a phone system. In Asterisk the dialplan is stored in /etc/asterisk/extensions.conf.


Voice over Internet Protocol (VoIP) is a term that is used to define a set of applications, codecs, hardware, et cetera, that transmits human voice in a digital form from one device to another. Asterisk is capable of both VoIP, using technologies such as SIP and IAX2, and analog devices provided by third party expansion boards, such as the Digium TDM400 analog board.


IAX and IAX2

Inter-Asterisk eXchange protocol allows Asterisk to communicate between Asterisk machines effectively. The IAX2 protocol can trunk many voice media into one packet, thus saving significant bandwidth and allowing an unusually large amount of voice calls over minimal bandwidth.



A softphone is a device that typically uses VoIP technologies to connect to a VoIP capable PBX. Softphones are generally defined by a software application running in a GUI environment that resembles a typical telephone. Softphones are available for Linux, Windows, Mac, and other intelligent devices, such as your iPhone.


A Private Branch eXchange (PBX) is system that serves a similar purpose in providing services as the phone company, such as hunt groups, call forwarding, et cetera, only it is private and therefore is maintained and used within a place of business. Simply put, the PBX is the central phone system that all of the phones in facility connect.

Hunt Group

The traditional definition of a hunt group is a circular rotation of a defined group of telephone devices and numbers by calling the group number. Hunt groups are possible in Asterisk, only, as with most features, Asterisk expands the idea and allows you to define more advanced hunt groups and ring groups.


Real-Time Transport Protocol (RTP) is the protocol that is used to transfer media directly between two devices on an IP network. As a comparison, SIP is the protocol that initiates the RTP communication between the devices. RFC-3550.

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