M
MAC	Media Access Control. Lower of the two sublayers of the data-link layer. Handles access to shared media, such as whether token passing or contention is to be used.
markup	In an XML environment, structural information that is stored in the same file as the content.
MARS	1. Modular access routers. 2. Multicast-address-resolution server. Server that supports IP multicast by serving a group of nodes (known as a cluster), each of which is configured with the ATM address of the MARS.
MCF message	Message Confirmation message. Message sent by a called fax machine in response to a PPS signal during the control-mode phase after content transmission.
MDCX message	Modify Connection Request message. Message sent by gateways in an MGCP call sequence to exchange SDP information.
MDL	Message Definition Language. High-level language for specifying protocols and protocol conversion operations on a virtual switch controller.
MDL message	Maintenance Data Link message.
MDN message	Message Disposition Notification message. Message returned to the originator of an e-mail message indicating that the e-mail message has been opened.
media gateway	Emerging industry-standard generic term for a gateway. Equipment that connects the PSTN or a PBX with the VoIP network and is controlled by an MGCP call agent. Sometimes abbreviated MG.
MF	Multifrequency or multiple frequency. Interoffice-address-signaling method in which 10 decimal digits and five auxiliary signals are each represented by a pair of the following frequencies: 700, 900, 1100, 1300, 1500, and 1700 Hz.
MGC	Media gateway controller. Provides call control capability to handle signaling traffic from a variety of sources. It also manages connections and resources of its media gateways. Can also be called a call agent.
MGCP	Media Gateway Control Protocol. Protocol that enables media gateway controllers and media gateways to communicate for call control on VoIP networks.
MIB	Management Information Base. Database of network-management information. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches. An object may be, for example, a counter or a protocol status. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands, usually through a network-management-system GUI. An object identifier uniquely designates any point in the tree, whether leaf object or branch point. An object identifier may be expressed as either a text string (for human use) or a numeric string (for machine use). Technically, the text form is the object descriptor and the numeric form is the object name. In practice, either is usually called an object identifier or OID. See also OID.
MIME	Multipurpose Internet Mail Extension. Standard for sending nontext data (or data that cannot be represented in plain ASCII code) in Internet mail, such as binary, nonEnglish-language text (such as Russian or Chinese), audio, or video data. Store-and-Forward Fax uses image/tiff MIME content type.
minimum timer	Configured minimum value for session interval accepted by SIP elements (proxy, UAC, UAS). This value helps minimize the processing load from numerous INVITE requests.
Min-SE	Minimum session expiration. Minimum value for session expiration.
MIX	Multiservice interchange. Feature that allows TDM connections between MIX-enabled ports.
MMoIP dial peer	Multimedia-Mail-over-IP dial peer. Dial peer used for Store-and-Forward Fax and VoiceXML Voice Store and Forward. Means by which you assign particular line characteristics (such as a destination phone number) to the connection between the router or access server and the SMTP mail server.
MOH	Music on hold. Feature that provides real-time or recorded sound to callers while they are on hold. Can be provided from an audio file or from a live feed from an external source.
monitor lamp, monitor mode	Line button on a phone that is used to display the line status (in-use or free) of an extension as it appears on another phone. The extension cannot be used on this phone to make or receive calls.
MRCP	Media Resource Control Protocol. Application-level protocol developed by Cisco and its media server partners Nuance Communications and SpeechWorks International. Used by client devices that process audio or video streams to control media resources on external media servers such as speech synthesizers and speech recognizers. The gateway that runs a voice application and the media servers that provide speech recognition and speech synthesis maintain a client/server relationship through an RTSP connection.
MSN	Microsoft Network.
MTA	Mail-transfer agent. Software that implements SMTP and provides storage for mail messages to be forwarded or delivered to a local user.
multicast	Process of transmitting PDUs from one source to many destinations. The actual mechanism (that is, IP multicast, multi-unicast, and so forth) for this process might be different for LAN technologies.
multipoint-unicast	Process of transferring PDUs where an endpoint sends more than one copy of a media stream to different endpoints. This can be necessary in networks which do not support multicast.
MWI	Message-waiting indication. Notification, such as a light on the phone, from a voice-mail system that a message has been left for the owner of the phone on which the notification was left.