Communications Systems Glossary

M-ary frequency-shift keying (M-ary FSK).  In binary frequency-shift keying (binary FSK or BFSK) carrier is shifted between two frequencies. A convenient way of trading bandwidth for signalling speed is to increase the spread between the lowest and highest frequency (increasing the bandwidth) but to increase the signalling speed by extending the number of frequencies to a number M. Such a system is called an M-ary frequency-shift keying system.

Man-machine interface (MMI).  The man-machine interface (MMI) refers to the interface presented by a machine to a human operator.

Manchester encoding.  Manchester encoding is a digital line-coding technique in which a transition is guaranteed in the middle of each bit period, which assists the receiver in retaining synchronisation.

Media access control protocol (MAC).  The media access control protocol (MAC protocol) is a sub-layer of the data link of the OSI reference model.

Media / medium.  In data communications, the medium is the physical carrier of data signals.

Medium earth orbit (MEO).  A medium earth orbit (MEO) is a circular satellite orbit of heights between 10,000 km and 15,000 km above the earth. Also called intermediate circular orbit (ICO).

Medium frequency (MF).  The medium-frequency (MF) band is a radio-frequency (RF) band in the range 300-3,000 kHz. The lower portion of the MF band is useful for communications systems requiring reasonably stable transmission over moderately long distances.

Mesh topology.  A mesh topology is a network topology in which each communicating node is directly connected to all other nodes.

Message switching.  Message switching is a store-and-forward concept where a message with an appropriate destination address is sent into the network and is stored at each intermediate switching point (network node) where its integrity is checked before it is sent onto the next stage of its journey.

Microcom network protocols (MNP).  Microcom network protocols are a set of commonly used proprietary modem protocols supporting data compression and error checking.

Microwave.  The term "microwave" refers to electromagnetic waves in the frequency range of about 2-40 GHz.

Millimetric wave.  The term "millimetric wave" refers to electromagnetic waves in the frequency range 30-300 GHz.

Modem.  Modem is a contraction of modulator/demodulator, which transforms a digital bit stream into an analogue signal (in the modulator), and vice versa (in the demodulator) so that the digital signal can be passed over an analogue channel.

Modulation.  The shifting or translation of a signal from one frequency band to another is accomplished by the process of modulation.

Monopole.  A monopole is a simple antenna structure where the two lines of a transmission line are opened up at the end to form an antenna with two arms in a T shape and one of the arms of the T is removed, where the arm of the T is one-quarter of a wavelength long.

Multicast address.  A multicast address is an address that designates a group of entities within a domain.

Multipath.  Multipath describes propagation that arrives at a receiver having travelled over different paths, and consequently arrives at slightly times, with potentially slightly different phases.

Multiple access.  Multiple access describe the ability for nodes to remotely access a transmission medium such as a satellite transponder.

Multiplexer (mux).  A multiplexer is a hardware device that performs multiplexing.

Multiplexing.  Multiplexing schemes combine messages from several information sources to be transmitted as a complete group over a single transmission facility with provision at the receiver(s) for separation (demultiplexing) back to the individual messages.

Multipoint connection.  A multipoint connection is a connection in which more than two terminals share one communications link.

Multistation access unit (MSAU).  A multistation access unit (MSAU) is a network hub in Token Ring local area networks (LANs).

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