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Media Access Control
A protocol that defines the way workstations gain access to transmission media, most widely used in reference to LANs. For IEEE LANs, the MAC layer is the lower sublayer of the data link layer protocol.


An IEEE 802.1ah standard for layering the Ethernet network into customer and provider domains with complete isolation among their MAC addresses. It enables service providers to separate customer traffic from management traffic and also save on VLAN IDs.

Master Clock

The source of timing signals (or the signals themselves) that all network stations use for synchronization.

Media Converter

A media converter is a networking device that makes it possible to connect two dissimilar media types such as twisted pair with fiber optic cabling. They are important in interconnecting fiber optic cabling-based systems with existing copper-based, structured cabling systems. They are also used in metropolitan area network (MAN) access and data transport services to enterprise customers.


Metro Ethernet Forum
A non-profit organization chartered with the mission of accelerating the adoption of optical Ethernet as the technology of choice in future metro networks worldwide. Ethernet was chosen for its relative simplicity and popularity with end-users, as well as for lowering costs of Ethernet equipment.


Management Information Base
A directory listing the logical names of all information resources residing in a network and pertinent to the network’s management. A key element of SNMP management systems.


A device used to convert serial digital data from a transmitting DTE to a signal suitable for transmission over extended distances. It also reconverts the transmitted signal to serial digital data for acceptance by a receiving DTE.

Modem Eliminator

A device used to connect a local terminal and a computer port. The modem eliminator replaces the pair of modems ordinarily required.


Modular interfaces enable field changeable conversion.


The alteration of a carrier wave in relation to the value or samples of the data being transferred.


Multiple Spanning Tree Protocol
The Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2005, defines an extension to RSTP to further develop the usefulness of virtual LANs (VLANs). This "Per-VLAN" Multiple Spanning Tree Protocol configures a separate Spanning Tree for each VLAN group and blocks all but one of the possible alternate paths within each Spanning Tree.

If there is only one Virtual LAN (VLAN) in the network, single (traditional) STP works appropriately. If the network contains more than one VLAN, the logical network configured by single STP would work, but it is possible to make better use of the alternate paths available by using an alternate spanning tree for different VLANs or groups of VLANs.

MSTP allows formation of MST regions that can run multiple MST instances (MSTI). Multiple regions and other STP bridges are interconnected using one single common spanning tree (CST).


A communications configuration in which multiple devices share a common transmission facility (or multipoint line), although generally only one may transmit at a time. Usually used with some kind of polling mechanism to address each connected terminal with a unique address code.

Multimode Fiber

A fiber with a large core diameter. 50-200 microns compared with the wavelength of light. It therefore propagates more than one mode. With multimode fiber, light traverses multiple paths, some longer than others. This leads to dispersion, which reduces optical range at high bit rates.


A device allowing two or more signals to pass over and share a common transmission path simultaneously.