Main distribution frame (MDF) – A steel bar framework that is the major cross connect point for COs and TOCs. Used to interconnect loop cable pairs and line equipment terminals on a switching system. Synonym: Main Frame.

Main pbx – A small, local telephone office or a device with the same functionality directly connected to a tandem switch via an access trunk group.

Mainframe – A central processing unit.

Manual tie line - A private communication channel which requires the assistance of an attendant at both ends of the circuit in order to complete a call.

Mapping – In network operations, the logical association of one set of values, such as addresses on one network, with other quantities or values, such as devices on a second network (for example name-address mapping, internetwork-route mapping).

Mark – (1) Symbol or symbols that indicate the beginning or the end of a field, word, item of data or sets of data such as a file, a record or a block. (2) To place a lead or ground on a terminal so that a hunting circuit may find that position and connect other equipment to it. (3) An indication, often in software, which shows the class of service to which a line is entitled, such as residential, business or public coin phone.

Mean time to repair (MTTR) – The average time required to restore a customer’s service after a network failure. [In Sprint, this figure represents all failures due to either electronic components or fiber-optic cable malfunction POP-to-POP.]

Medium – [plural; media] (1) Material on which computer data is recorded, for example, magnetic tape, floppy diskette, winchester disk. (2) Any material substance that can be used for the propagation of signals from one point to another. (3) The communications forum chosen on marketing promotions and/or customer information.

Mega – (1) A Greek word meaning one million. (2) A prefix in the international system of measurements; for example: megabytes, that is one million bytes (106).

Megabyte – Literally 1,000,000 bytes; usually 1024 x 1024 bytes.

Megahertz (Mhz) – 1,000,000 (106) cycles per second.

Message – Complete transmission of data or text. Sometimes used interchangeably with block.

Message Processing System (MPS) – Sprint system which prepares calls for invoicing. [MPS feeds to all billing systems as well as providing informational input on all billable and nonbillable traffic to network systems.]

Message telephone system (MTS) – (1) Generic name for the switched long distance service offered by all interexchange carriers. (2) An offering for which a charge is made in accordance with a measured amount of usage referred to as message units. Also referred to as Measured Telephone Service or Direct Distance Dialing (DDD).

Message unit (MU) - Standard against which charges for messages based on time and distance are measured.

Microwave – Any electromagnetic wave in the radio frequency spectrum above 890 megahertz.

Millisecond – One-thousandth of a second.

Mode – (1) The most common or frequent value in a frequency distribution. (2) A particular form or variety of software.

Modem – [Contraction of Modulator-Demodulator.] A device which modulates and demodulates signals on a carrier frequency and allows the interface of digital terminals with analog carrier systems. [A modem is also known as a dataset.]

Modulation – Alterations in the characteristics of carrier waves, usually impressed on the amplitude and/or the frequency, for the purpose of transferring information.

Module – (1) A unit of something. (2) A unit of code performing a distinct function.

Monitoring device – Equipment which records data — originating number, length, location — on calls placed through a company telephone system.

Most economical route selection (MERS) – Method for automatic selection of the least costly facility for transmission of a call. Synonym: least cost routing

Mouse – Hand-held device which is moved around on a flat surface in order to position the cursor on a video display.

Multidrop line – Communications link with more than one terminal or station connected to it.

Multileaving - Transmission, usually via bisync facilities and protocols, of a variable number of datastreams between user devices and a computer.

Multiplex – (1) To interleave or simultaneously transmit two or more messages on a single channel. (2) A communications arrangement where multiple devices share a common transmission channel, though only one may transmit at a time.

Multiplexer – Hardware device that allows handling of multiple variable speed signals over a single, higher speed channel.

Multiplexing – Process or equipment that combines data from two or more individual circuits onto a higher speed circuit for transmission. [Two methods are used: (1) Frequency division where all channels are transmitted at the same time. (2) Time division where several messages timeshare a channel.]

Multipoint – (1) Line or channel connecting more than two different service points. (2) Circuit having points served by three or more switches.

Multiple Appearance Directory Number (MADN) – allows the same directory number to appear on more than one telephone.

Multitasking – System that can run two or more programs at the same time. [Apple's Multifinder, AT&T's UNIX and IBM's OS/2 are multitasking operating systems. By contrast, MS-DOS is a single-tasking operating system.]

Mux – [Abbreviation for multiplexing] The process or equipment for combining a number of paths into a single path or bit stream for transmission.