User guide

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Glossary
LAPB: Link Access Procedure Balanced; based on the X.25 Layer 2 specification. A full-duplex point-to-point bit-synchronous protocol
commonly used as a data link control protocol to interface X.25 DTEs. LAPB is the link initialization procedure that establishes and maintains
communications between the DTE and the DCE.
LAPD: Link Access Protocol for the D-Channel; based on the ISDN Q.921 specification. A full-duplex point-to-point bit-synchronous link-
level protocol for ISDN connections; different from LAPB in its framing sequence. Transmission is in units called "frames", and a frame may
contain one or more X.25 packets.
Line Coding: The representation of 1s and 0s on a T1 line. The two methods of line coding commonly used, B8ZS and AMI, differ in the
restrictions placed on user data. T1 line coding ensures that sufficient timing information is sent with the digital signal to ensure recovery
of all the bits at the far end. Timing information on the T1 line is included in the form of 1s in the data stream; a long string of 0s in the data
stream could cause problems recovering the data.
Line Termination (LT): The electronics at the ISDN network side of the user/network interface that complements the NT1 at the user
side. The LT and the NT1 together provide the high-speed digital line signals required for BRI access.
Listed Directory Number (LDN): The main number assigned by the telco; the number listed in the phone directory and also provided by
Directory Assistance. Some devices can have more than one LDN, such as ISDN devices that have one LDN for voice and another LDN
for data.
Local Area Network (LAN): 1) A computer network located on a user's premises within a limited geographical area. Communication
within a local area network is not subject to external regulations; however, communication across the LAN boundary may be subject to
some form of regulation. 2) A LAN does not use store and forward techniques. 3) A network in which a set of devices are connected to
one another for a communication and that can be connected to a larger network.
Local Access and Transport Area (LATA): A post-divestiture geographical area generally equivalent to a Standard Metropolitan
Statistical Area. At divestiture, the territory served by the Bell system was divided into approximately 161 LATAs. The Bell Operating
Companies (BOCs) provide Intra-LATA services.
Local Exchange Carrier (LEC): The local phone company which provides local (not long distance) transmission services. Also known
as "telco". LECs provide T1 or FT1 access to LDCs (unless the T1 circuit is completely intra-LATA). Inter-LATA T1 circuits are made up of
a combination of Access and Long Haul facilities.
Local Management Interface (LMI): A specification for frame relay equipment that defines status information exchange.
Local Loop: A transmission path, typically twisted-pair wire, between an individual subscriber and the nearest public telecommunications
network switching center. The wires provide ISDN service, but require an NT1 at the user end and an LT at the network end. (also known
as "loop" or "subscriber loop".)
Logical Link Control (LLC2): In a local area network, the protocol that governs the exchange of transmission frames between data
stations independently of how the transmission medium is shared. The LLC2 protocol was developed by the IEEE 802 commitee and is
common to all LAN standards.
Logical Unit (LU): A type of network accessible unit that enables end users to gain access to network resources and communicate with
each other.
Long Haul: The T1 element that connects to the Access portion of the long distance company's (LDC's) central office. The LDC is
commonly called the point of presence (POP). Each LDC has a number of POPs, located throughout the country. The LDC is also called an
IEC (Inter Exchange Carrier).
Long Haul Communications: The type of phone call reaching outside of a local exchange (LE).
M
Management Information Base (MIB): A database of network management information used by the Common Management Information
Protocol (CMIP) and the Simple Network Management Protocol (SNMP).
Megacom: An AT&T service with a normal WATS line (typically T1) between the customer premise and the AT&T serving class 4 CO are
the customer's responsibility.
MegaLink: BellSouth's leased T1 service.
Message: Associated with such terms as packet, frame, and segment. 1) In information theory, an ordered series of characters intended
to convey information. 2) An assembly of characters and sometimes control codes that is transferred as an entry from an originator to one
or more recipients.
Modem: A communications device that enables a computer to transmit information over a phone line. It converts the computer's digital
signals into analog signals to send over a phone line and converts them back to digital signals at the receiving end. Modems can be internal
and fit into an expansion slot, or external and connect to a serial port.
Multiplexer (Mux): 1) A device that takes several input signals and combines them into a single output signal in such a manner that each
of the input signals can be recovered. 2) A device capable of interleaving the events of two or more activities or capable of distributing the
events of an interleaved sequence to the respective activities. 3) Putting multiple signals on a single channel.
Multiprotocol: A device that can interoperate with devices utilizing different network protocols.