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M
MAC
MAC stands for
Media Access Control. It is one of the media specific IEEE 802 standards
(802.3, 802.4, 802.5) that defines the protocol and frame formats for
Ethernet, Token Bus and Token Ring. It is the lower sublayer of the Data
Link Layer of the OSI model.
Mail Gateway
A mail
gateway is a machine that connects two or more electronic mail systems
(especially dissimilar mail systems on two different networks) and
transfers messages between them. Sometimes the mapping and translation can
be quite complex, and generally it requires a store-and-forward scheme
whereby the message is received from one system completely before it is
transmitted to the next system after suitable translations.
Mainframe
A mainframe is a
computer system on a grand scale. Mainframe computers are large and
powerful and attach to networks and high-speed peripheral devices such as
tapes, DASD, and printers. In the IBM world a mainframe is also referred to
as a host computer or central processing unit. An example of an IBM
mainframe would be the ESCON 9121. Other companies such as Amdahl and Hitachi also build
mainframe computers.
MAN
MAN stands for
metropolitan area network. MANs are evolving and will be developed by data
carriers to connect LANs in the same city. LANs, local area networks,
extend across a single site and consist of one or more subnets, which are
usually, but not necessarily, homogeneous. WANs are wide area networks,
often heterogeneous, which cover many sites, spanning large corporations
and sometimes continents.
Manchester Encoding
Manchester code is a commonly found Physical
Layer-encoding scheme found in local area networks such as Ethernet and
Token Ring. This self-clocking code provides a signal state change in every
bit transmitted. Differential Manchester
encoding is a similar technique and is also used in LANs. A binary zero is
represented by a transition at the start of the bit time. A binary one is
represented by no transition at the start of the bit time.
Master
In a communications
system that is hierarchical in arrangement, processes are often spoken of
as being either masters or slaves. The issue is one of control rather than
service. Master/slave relations often occur in cases where one node has
greater "intelligence" than the other does, that is, greater
computing capacity. Master/slave relations occur in the IBM SNA environment
because of its hierarchical nature. The protocol used in master/slave
relations is generally the same for communication in either direction, but
the master node is in complete control. An example is the relationship
between a mainframe and a 3270 cluster controller.
Mbps
The bit rate is
the number of bits transmitted per second (bps). For instance, commonly
used rates for modem speeds are 300bps, 1200bps, 9600bps, 14,400bps, and
19,200bps. Another way of writing 19,200bps is 19.2Kbps. Kbps stands for
kilobits per second which would mean 1000 bits transmitted per second times
19.2. 1.544 Mbps is the T1 rate and is 1,544,000bps.
Megahertz
Megahertz is most
commonly written MHz. One hertz is one cycle of a sine wave (electrical
wave) in one second. One million Hertz (1 MHz) would be one million cycles
per second.
Message
In data
communications a message is a sequence of characters used to convey information.
The term message is usually used in conjunction with the Transport Layer of
the OSI model referring to the complete unit of information being
transmitted from one node to another.
MHS
MHS stands for Message Handling System. It is the system of message user
agents, message transfer agents, message stores, and access units which
together provide OSI electronic mail. MHS is specified in the CCITT X.400
series of Recommendations.
MHz
MHz stands for
megahertz. One hertz is one cycle of a sine wave in one second. One million
Hertz (1 MHz) would be one million cycles per second. See
"Transmission Speeds" in the glossary for more information on
prefixes used in data transmission.
MIB
MIB stands for
Management Information Base. It is the standard for network management data
for TCP/IP and Internet systems. It specifies the data that must be kept by
a host or gateway in a network and the operations that may be performed on
each.
Microsegmentation
Microsegmentation
is a method used to increase the performance of an Ethernet based network.
It does this by reducing the amount of nodes connected to a single segment
through the use of a device such as a bridge or a switch. A heavily loaded
Ethernet segment can be divided into multiple segments. Each segment then
has access to the full Ethernet 10Mbps, so that bandwidth is effectively
increased on both sides of the bridge or switch.
Middleware
Middleware is
software normally used in client/server applications. Middleware is the
software that interfaces the application programs running on the client
with the protocols used to transmit application data to the server.
Middleware is positioned between front-end systems software and back-end
systems software.
MILNET
MILNET stands
MILitary NETwork. Originally part of the ARPANET, MILNET was partitioned in
1984 to make it possible for military installations to have reliable
network service, while the ARPANET continued to be used for research.
MIME
MIME stands for
multipurpose Internet mail extension. It is an Internet standard that
allows different types of data such as video, graphics and audio can be
sent with electronic mail messages.
Modem
Modem stands for
modulator/demodulator. A modem is a device used to transmit computer data
over the telephone network. It is a device that on the sending end converts
digital data to an analog signal for transmission over an analog network
(such as the telephone network). On the receiving end the modem takes the
analog signal and converts it back to digital.
Modulation
Modulation is the
process of modifying the form of a carrier wave (electrical signal) so that
it can carry intelligent information on some sort of communications medium.
Digital computer signals (baseband) are converted to analog signal for
transmission over analog facilities (such as the local loop). The opposite
process, converting analog signal back into their original digital state is
referred to as demodulation. There are three basic types of modulation:
frequency modulation, phase modulation, and amplitude modulation.
MOTIS
MOTIS stands for
Message-Oriented Text Interchange System, the Application Layer message
handling system developed jointly by the ISO and ITU-T.
MTA
MTA stands for
Message Transfer Agent, an OSI application process used to store and
forward messages in the X.400 message handling system. Equivalent to
Internet mail agent.
Multicast
Multicast is a
special form of broadcast where copies of the packet are delivered to only
a subset of all possible destinations.
Multi-Drop
Multi-drop refers
to a data communication configuration where multiple terminals, printers
and workstations are located on the same media and only one can communicate
with the "master" at a given time. It is a form of unbalanced
communication.
Multi-Mode Optical Fiber
There
are two basic types of fiber cables, multimode and singlemode. Light
propagates along the fiber core in one of two ways depending on the type of
material used. With multimode the diameter of the core is large enough that
the light reflects off the cladding as it travels down the core. With
single mode the diameter of the core is small enough that light does not
reflect and the signal is not altered as much as with multimode. Higher
data rates and longer distances can be achieved with singlemode fiber.
Multiplexer
Computer
equipment that allows multiple signals to travel over the same physical
media. There are different types of multiplexers such as time division
multiplexers and frequency division multiplexers.
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