MAC

Moves, Adds and Changes

Mailbox

A reference to the location where voicemail messages for a particular user are stored.

MAPI

Messaging Application Program Interface

Mb

Megabyte - 
A measure of computer processor storage and real and virtual memory, a megabyte (abbreviated Mb) is 2 to the 20th power bytes, or 1,048,576 bytes in decimal notation.

MPD


Meter Pulse Detection
 - A BT service for their analogue lines, to allow a subscriber to accurately establish the charge of a telephone call. To a subscriber, it was a Meter Pulse received by monitoring equipment down their telephone line. To BT, it was a Charge Unit recorded on the subscriber's meter in the local BT exchange. During a call, the first pulse would be transmitted the moment the call connected. Subsequent pulses would be transmitted at time intervals thereafter, the interval depending upon the telephone number dialled and the time of day.

Modem

Modulator-De Modulator

MPLS

Multi Protocol Label Switching
 - A data-carrying mechanism which emulates some properties of a circuit-switched network over a packet-switched network. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames.

MSN

Multiple Subscriber Numbering
 - An optional feature of ISDN2 lines allowing up to 10 telephone numbers to be assigned to a single line so that devices connected to that line can be called individually. Can be used to produce a limited version of DDI.

Multicast

A point-to-group message transmission within a network.

Music on Hold

An audio signal that is played to a caller on hold to reassure him that he has not been cut off. Typically music, it may be interspersed with advertising messages or could be simply a reassuring beep played every few seconds.