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VoIP Glossary & terms

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Glossary for VoIP associated terms.

MOSMean Opinion Score the Mean Opinion Score (MOS) provides a numerical indication of the perceived quality of voice transmission after compression and/or transmission and is expressed as a number in the range 1 to 5, where 1 is lowest perceived audio quality and 5 is the highest perceived audio quality measurement.

MTA Multimedia Terminal Adapter A device that connects a traditional telephone to a cable line, converting analog voice to digital signals.

NAT Network Address Translation  An Internet standard allowing a local network to use one public IP address to connect to the Internet and a set of local IP addresses to identify each PC or device in the local network.  NAT translations are a challenge for VoIP and result in one-way audio in some cases

PBX  Public Branch Exchange  A private telephone switching system that allows outside phone lines from a telecommunications provider to connect to extensions within the office or building.  They usually have multiple features including call forwarding, rollover, paging and voice mail.

POTS Plain Old Telephone System  The familiar single phone line, single phone number system that has been in existence for many years.

PRIPrimary Rate Interface  The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel using a T-1 line and can have up to 1.544 Mbps service.   Typically, it is a dynamic circuit that delivers both voice and data, giving preference for voice.  When a channel is not carrying voice it is automatically allocated for data.

PSTN Public Switched Telephone Service  The combination of local, long-distance and international carriers that make up the worldwide telephone network.

Rate Center The geographic area used by local exchange carriers to set rate boundaries for issuing phone numbers and for billing.  Rate centers are important when considering porting of numbers through LNP to and from VoIP service providers.

RJ-11  The typical four or 6 wire connector used to connect telephone equipment.                                          


RJ-45   An 8 wire connector used to connect Ethernet connections in computers, routers and other Internet devices.  This connector is slightly larger than a (RJ-11) telephone connector.

Router  A router is a device connected to at least two networks that determines the next network point to forward a packet to.  The decision of which way to send each information packet is based on it's current understanding of the networks that it is connected to.

RTP   Real Time Transport Protocol  An Internet protocol that functions for end-to-end network connections for applications that use audio or video.+

Satellite Phone  A satellite phone is a particular type of mobile phone that connects to earth orbiting satellites instead of the typical terrestrial cell towers that most cell phones connect to. The features on most systems are similar to traditional mobile telephones and transmit voice, short messaging service and allow low-bandwidth internet access are supported. Depending on the particular system the coverage may include the entire Earth, or only specific regional areas.

Silence Suppression and Comfort Noise Generation  Silence Suppression is a means of increasing the number of calls supported by reducing the bandwidth for a single call.  If speech is not present, the silence suppression and comfort noise generation is activated. This is accomplished by removing and not transmitting the natural silence that occurs in normal two-way connection.
Comfort Noise Generation provides an artificially-generated background white noise to reassure callers that their calls are still connected during silent periods rather than thinking the call dropped.

SIP Session Initiation Protocol  is a signaling protocol for Internet conferencing, telephony, and instant messaging.  It is a request-response protocol, dealing with requests from clients and responses from servers. initiating an interactive user session.

SIP Session Initiation Protocol Trunking is the use of VoIP to facilitate the connection of typically a PBX to the Internet, where the Internet replaces the conventional telephone trunk, allowing a business to communicate with traditional PSTN telephone subscribers worldwide by connecting to an ITSP (Internet Telephony Service Provider) SIP trunking can save money and offer services to an IP-PBX.

Soft Phone   IP telephony software that allows end users to send and receive calls over the computer or hand held PC device (PDA) over the Internet.  Typically used in conjunction with a headset and microphone there are many free softphones that are available.

STUN  Simple transversal of UDP through NATs  is a protocol for assisting devices behind a NAT firewall or router with their packet routing.

Switch  A switch is a device that keeps a record of the MAC addresses of all devices connected to it and then channels incoming data from any of multiple input ports to the specific output port that will take the data toward its intended destination.


T38 A recognized standard for sending fax transmissions over an IP network in real time mode. Messages are sent as UDP or TCP/IP packets.

UDPUser Datagram Protocol is a communications protocol that does not provide sequencing of the packets. The application must be able to make sure that the entire message has arrived and is in the right order.

VADVoice Activation Detection is a software application, also known as silence suppression that allows the detection of the absence of audio for a specified amount of time to use techniques to conserve bandwidth, as many conversations can actually include one half silence. Voice activation detection can also be used to forward idle noise or comfort noise to a remote IP telephone or IAD giving the illusion of a constant transmission stream during silence, so the listener would not think the line had gone dead.

VoIPVoice over Internet Protocol The transmission of voice over the Internet as digital packets rather than the traditional circuit-committed protocols of the PSTN.  VoIP uses real-time protocol (RTP) to help ensure that the packets get delivered in a timely way.  For a full explanation about the benefits and features of Voice over IP read what is VoIP.