m - Milli, one-thousandth.

M - Mega, one million (e.g. Mbps or Mbit/s, one million bits per second).

MAC (Media Access Control) - A media-specific access control protocol within IEEE 802 specifications; currently includes variations for Token Ring, token bus, and CSMA/CD; the lower sublayer of the IEEE's link layer (OSI), which complements the Logical Link Control (LLC).

MAC Address - Hardware address that uniquely identifies each node on a network. 

Make-Before-Break Contacts - Contacts which make with new contacts before they break with the contacts with which they are associated in the rest position of a relay or key.

Make Contacts - The contacts of a relay which will be open when the relay is de-energized and closed when the relay is energized.

MAN (Metropolitan Area Network) - A network designed to carry data over an area larger than a campus, such as an entire city and its outlying area.

Manual By-Pass - The ability to manually switch a standby power system to operate from AC current only for maintenance purposes.

Master Office - A central office which acts as parent office for smaller offices and through which all calls to the smaller offices are routed. Also called Host Office.

MAU (Media Attachment Unit) - Device used in Ethernet and IEEE 802.3 networks that provides the interface between the AUI port of a station and the common medium of the Ethernet. The MAU, which can be built into a station or can be a separate device, performs physical layer functions including conversion of the digital data from the Ethernet interface, collision detection, and injection of bits onto the network.

maxCR (Maximum Cell Rate) - The maximum cell rate for an entire path regardless of the number of channels on the path.

maxCTD (Maximum Cell Transfer Delay) - Specifies end-to-end cell transfer delay.

Maximum Burst Tolerance - the largest burst of data that a network device is guaranteed to handle without discarding cells or packets. Bursts of data larger than the maximum burst size may be subject to discard.

MBS (Maximum Burst Size) - In the signaling message, the Burst Tolerance (BT) is conveyed through the MBS which is coded as a number of cells. The BT together with the SCR and the GCRA determine the MBS that may be transmitted at the peak rate and still be in conformance with the GCRA.

MCC (Maintenance Control Center) - The area of the central office used by the plant personnel to survey system operation, as well as traffic and alarm monitoring. The center will group reporting devices, data sets and visual displays in one space. This is the area from which a digital system would be monitored and controlled within the building. Power alarm indications which impact on the switching system are often extended into this area.

MCDV (Maximum Cell Delay Variance) - This is the maximum two-point CDV objective across a link or node for the specified service category.

MCLR (Maximum Cell Loss Ratio) - This is the maximum ratio of the number of cells that do not make it across the link or node to the total number of cells arriving at the link or node.

MCR (Minimum Cell Rate) - Parameter defined by the ATM Forum for ATM traffic management. MCR is defined only for ABR transmissions, and specifies the minimum value for the ACR.

MCTD (Maximum Cell Transfer Delay) - This is the sum of the fixed delay component across the link or node and MCDV. 

MDF (Main Distribution frame) - A distribution frame on one part of which terminate the permanent outside lines entering the central office building and on another part of which terminate the subscriber line multiple cabling, trunk multiple cabling, etc.

Membership Scope - The level of routing hierarchy within which advertisement of a given address is constrained.

Message Alert (also called "call-in-absence" indicator) - A light or other indicator announcing that a phone call came in, an especially important feature if the cellular subscriber has voice mail.

Metasignaling - An ATM Layer Management (LM) process that manages different types of signaling and possibly semipermanent virtual channels (VCs), including the assignment, removal, and checking of VCs.

Metasignaling VCs - The standardized VCs that convey metasignaling information across a User-to-Network Interface (UNI). 

Meter Assembly - Display of the discharge current and the battery string voltage. 

MHz - Abbreviation for Megahertz, one million Hertz (one million cycles per second).

MIB (Management Information Base) - The set of parameters that an SNMP management station can query or set in the SNMP agent of a networked device (e.g., router).

MIC (Media Interface Connector) - The optical fiber connector that joins the fiber to the FDDI controller.

MicroChannel - A proprietary 16- or 32-bit bus developed by IBM for its PS/2 computers' internal expansion cards; also offered by others. 

Microprocessor - An electronic circuit, usually on a single chip, which performs arithmetic, logic and control operations, customarily with the assistance of internal memory.

Microsecond - One-millionth of a second.

Microwave - A term loosely applied to those radio frequency wavelengths which are sufficiently short to exhibit some of the properties of light. Commonly used for frequencies from about 1 GHz to 30 GHz.

Microwave Frequency - Any of the frequencies suitable for microwave communication. Most common are the 2 GHz, 4 GHz and 6 GHz bands.

Microwave Radio - Radio communications using frequencies between approximately 1 GHz and 1000 GHz.

MID (Message Identifier) - The message identifier is used to associate ATM cells that carry segments from the same higher layer packet.

Millisecond - One-thousandth of a second.

MJ - A major alarm classification, not necessarily a power alarm.

MMF (Multimode Fiberoptic Cable) - Fiberoptic cable in which the signal or light propagates in multiple modes or paths. Since these paths may have varying lengths, a transmitted pulse of light may be received at different times and smeared to the point that pulses may interfere with surrounding pulses. This may cause the signal to be difficult or impossible to receive. This pulse dispersion sometimes limits the distance over which a MMF link can operate.

MN - (1) A minor alarm classification, not necessarily a power alarm. (2) A lead designation connecting between a self-contained alarm circuit and the office alarm frame signifying a minor audible alarm. 

Mobile Telephone - Radio telephone service provided to vehicles from a broadcast point located within range of the moving vehicle. The broadcast point in turn can be connected to the public network so that calls can be completed to or from any stationary telephone.

Modem - Acronym for modulator-demodulator. (1) A device that modulates and demodulates signals, primarily converting digital signals into quasi-analog signals for transmission and for reconverting the quasi-analog signals into digital signals; (2) in frequency division carrier systems, devices that change the frequency of a signal, voice band on one side, and the carrier frequency with either erect or inverted sidebands on the other side.

Modular - (1) Having dimensions which are integral multiples of a unit of length called a module; (2) in switching equipment, designed and manufactured in functional modules so that individual modules may be later replaced by newer versions.

Monitor - A device for recording or controlling an activity or process.

Mother Board - The heart of the alarm interface/isolation bay design consisting of diode sets, relays, terminal strips and lamps which are housed in a framework intended to be located in the power board line.

MPC (MPOA Clients) - Edge devices and host terminals running MPOA software.

MPEG (Motion Picture Experts Group) - An ISO Standards group dealing with video and audio compression techniques and mechanisms for multiplexing and synchronizing various media.

MPOA (Multiprotocol over ATM) - An effort taking place in the ATM Forum to standardize protocols for the purpose of running multiple network layer protocols over ATM.

MPOA Client - A device which implements the client side of one or more of the MPOA protocols, (i.e., is a SCP client and/or an RDP client. An MPOA Client is either an Edge Device Functional Group (EDFG) or a Host Behavior Functional Group (HBFG).

MPOA Server - An MPOA Server is any one of an ICFG or RSFG.

MPOA Service Area - The collection of server functions and their clients. A collection of physical devices consisting of an MPOA server plus the set of clients served by that server.

MPOA Target - A set of protocol address, path attributes, (e.g., internetwork layer QoS, other information derivable from received packet) describing the intended destination and its path attributes that MPOA devices may use as lookup keys.

MPS - MPOA Servers ATM Routers running MPOA software. 

MSA (Metropolitan Statistical Area) - The FCC divided the United States into MSA markets and Rural Service Area (RSA) markets. There are 306 MSAs in the United States, all of which now have cellular service.

MSB (Most Significant Bit) - The highest order bit in the binary representation of a numerical value. 

MTBF (Mean Time Between Failure) - The average time a manufacturer estimates before a failure occurs in a component, a printed circuit board or a complete system. 

MTBO (Mean Time Between Outages) - The mean time between equipment failures or significant outages which essentially render transmission useless.

MTSO (Mobile Telephone Switching Office) - The central switch that controls the entire operation of a cellular system. It is a sophisticated computer that monitors all cellular calls, keeps track of the location of all cellular-equipped vehicles traveling in the system, arranges handoffs, keeps track of billing information, etc.

MTSR (Mean Time To Service Restoral) - The mean time needed to restore service following system failures that result in a service outage.

MTTR (Mean Time To Repair) - The vendor's estimated average time required to do routine repairs on equipment.

MTU (Maximum Transmission Unit) - The largest unit of data that can be sent over a type of physical medium.

Mu-Law - The PCM coding and companding standard used in Japan and North America.

Multicasting - The ability to broadcast messages to one node or a select group of nodes.

Multi-homed - A device that has both an ATM and another network connection, typically Ethernet.

multimode fiber - Type of fiber optic cable with a larger center core than single mode fiber. Multimode fiber is generally used over relatively short distances.

Multiplexing - A function within a layer that interleaves the information from multiple connections into one connection (see demultiplexing).

Multipoint Access - User access in which more than one terminal equipment (TE) is supported by a single network termination.

Multipoint-to-Point Connection - A Point-to-Multipoint Connection may have zero bandwidth from the Root Node to the Leaf Nodes, and non-zero return bandwidth from the Leaf Nodes to the Root Node. Such a connection is also known as a Multipoint-to-Point Connection.

Multipoint-to-Multipoint Connection - A collection of associated ATM VC or VP links, and their associated endpoint nodes, with the following properties:
1. All N nodes in the connection, called Endpoints, serve as a Root Node in a Point-to-Multipoint connection to all of the (N-1) remaining endpoints.
2. Each of the endpoints can send information directly to any other endpoint, but the receiving endpoint cannot distinguish which of the endpoints is sending information without additional (e.g., higher layer) information.

Multitasking - The simultaneous performing of two or more tasks by a computer.

MUX (Multiplexer) - Electronic equipment which allows two or more signals to pass over one communication circuit.