Media Access Control. MAC is the lower sub-layer of the Data Link Layer that concerns with how data is taken from or inserted into the physical medium. For example in Ethernet, CSMA/CD mechanism is used to control access to the shared network medium.
Also see LLC and Data Link Layer.

A 6-byte (48-bit) hexadecimal number that is assigned and physically hard-coded to a network device by its manufacturer. MAC address is unique for each network device and is used to identify it on the network.
Every network adapter (NIC) has a MAC address. You can check yours from a written label or by using IPCONFIG /ALL instruction in Command Prompt. An example of a valid MAC address: 62-C1-E9-6A-23-22.
MAC address is also known as physical address.

Metropolitan Area Network. A city-wide network that provides connectivity for high rise buildings, government offices, schools, residential areas, hospitals, transportation hubs, or even open public places. Modern cities build MANs to provide high speed Internet access to city-dwellers and guests.
Also see PAN, LAN, WAN.

Medium Access Unit. The IEEE term for transceiver that is used in Thicknet (10Base5) to connect coax from each computer to the cable trunk (bus).
Also see 10BaseT.

Megabyte. 1 MB = 1,048,576 bytes. Also see byte.

MultiBand OFDM Alliance. An organization formed by semiconductor, personal computing, consumer electronics and mobile devices vendors that promotes global standard for ultrawideband wireless communication using multi-band OFDM technology. Now it is part of WiMedia Alliance.

Megabit per second. 1 Mbps equals a million bits per second. Also see bps.

Media Dependent Interface.
Also see auto-MDI/MDIX.

Media Dependent Interface Crossover.
Also see auto-MDI/MDIX .

MEdia GAteway COntroller. IP telephony signaling protocol that is an evolution of MGCP.

Mesh topology is a network topology in which each node can have more than one route for connecting it to the other nodes in a network. Mesh topology is the most reliable because if a route is offline, data packet can go through alternate route to reach its destination node.
See picture. Also see bus, star, and ring topologies.

Medium Frequency. Radio frequencies (RF) in the range of 300 kHz - 3 MHz, corresponding to wavelength from 1 km to 100 m. A.k.a. medium wave. During the day medium waves follow earth's curvature (i.e. groundwave), at night medium waves are reflected and refracted by the ionosphere (i.e. skywave). MF is commonly used for AM radio broadcast.

Media Gateway Control Protocol. IP telephony signaling protocol defined by that IETF that resembles PSTN telephony system.
Also see MEGACO.

Part of electromagnetic spectrum that is longer than infrared but shorter than short-wave radio. It ranges from 1 GHz to 300 GHz, corresponding to wavelength from 30 cm to 1mm. It spans UHF, SHF, and EHF bands. However, the frequency range of microwave is not strictly defined in handbooks or standards.
Microwave has been very long used in radar and satellite communications. But the most familiar application is in microwave oven. In cellular networks, microwave is used to link base stations to the mobile switching system. And due to its bandwidth potential,  microwave is the choice for broadband wireless access technologies such as WiMAX.
Also see electromagnetic and radio wave.

Multiple-Input Multiple-Output. Refers to a technique that allows a wireless device (transceiver) to have multiple antennas that can transmit and receive multiple signals simultaneously to boost performance.
MIMO is the basis for the development of the next generation Wi-Fi (802.11n) and the mobile version of WiMAX (802.16e).

Multichannel Multipoint Distribution System. A microwave based access network that operates at 2 GHz bands and has larger coverage than LMDS of up to 35 miles.
Also see LMDS.

Modulator Demodulator. Modem is the device you need to connect your computer to the Internet. You may need dial up, DSL, or cable modem depending on the type of Internet access you subscribe to.
As its name implies, a modem modulates the carrier (wave) by the baseband signal (information) at the transmitter and demodulates the modulated carrier at the receiver to take the information.
A modem usually incorporates other functions, such as analog-to-digital or digital-to-analog conversion, compression or decompression, encryption or decryption, and error correction.
Also see DSL, cable modem, V.90, V.92.

The carrying of baseband signal (information) by a carrier for efficient delivery across a transmission medium. If a carrier is modulated by the baseband signal, the carrier wave shape changes in accordance with the variation in a characteristic (i.e. amplitude, frequency, or phase) of the baseband signal.
Contrast with demodulation.

A printed circuit board (PCB) in a computer that contains a processor chip and other main electronic components. It also provides expansion slots for connections to peripherals.

Multi Protocol Label Switching. A protocol that allows IP packet routing based on priority or QoS level using short-length label. Because routing decision is determined by label examination instead of IP packet header, MPLS makes routing process faster.
MPLS is intended for multi-service IP network that requires traffic management. With MPLS, voice traffic which is delay-sensitive is prioritized over other traffic such as Web browsing, e-mail, and file transfer.

Multi Station Access Unit. The central connection point in a Token Ring network. Often mentioned as MAU.

Maximum Transmission Unit. The maximum frame size that can be transmitted across a network.

The transmission of data (packet) from a transmitter (server) to multiple receivers (workstations) at the same time.
Compare with broadcast and unicast.

Refers to fiber optic in which light travels in multiple modes (reflective paths) and has larger core diameter.
See picture. Also see fiber optic. Compare with single-mode.

A technique for accessing and sharing a communication medium by multiple stations.
Also see CDMA, CSMA/CA, CSMA/CD, FDMA, TDMA.

A technique for aggregating signals from multiple subscribers into a single communication channel to maximize the channel use up to its maximum capacity.
Also see FDM, TDM, WDM.

Mobile Virtual Network Operator. A mobile (cellular) communication service provider who doesn't own cellular network facilities, instead leases the network from other licensed carriers (operators). A typical case is a fixed network operator leases network capacity from mobile operator to provide fixed mobile convergence service to its existing subscribers to reduce churn rate.