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GL-16 GLOSSARY
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listed directory number
(LDN)
The listed number in a public directory for a communications system. An
incoming call to an LDN is usually answered by an attendant.
local area network (LAN) A networking arrangement specifically designed for a limited geographical
area. Generally, a LAN is limited in range to a maximum of 6.2 miles and
provides high-speed carrier service with low error rates. Common
configurations include star (including circuit switched), ring, and bus. See
also wide area network.
local exchange company
(LEC)
A company franchised to provide public intra-LATA (local access and
transport area) telephone service to subscribers within a defined
geographical area. Also called a local exchange carrier or local telephone
company.
logical channel (or
circuit)
A message slot on a digital communications link. A logical channel is one
of the logically independent elements of a data stream that is multiplexed
onto a single communications carrier.
manual dialing Use of the dialing pad on a telephone to place a call by dialing each digit
individually.
MEGACOM AT&T’s tariffed digital WATS offering for outward calling. See also 800
Service.
MEGACOM 800 AT&T’s tariffed digital 800-service offering for inward calling.
message-oriented
signaling (MOS)
See DMI with message-oriented signaling.
modem A device that converts digital data signals to analog signals for
transmission over telephone circuits. The analog signals are converted
back to the original digital data signals by another modem at the other end
of the circuit.
modem pooling A capability that provides shared conversion resources (modems and data
modules) for cost-effective access to analog facilities by data terminals.
When needed, modem pooling inserts a conversion resource into the path
of a data call. Modem pooling serves both outgoing and incoming calls.
modular asynchronous
data unit (MADU)
A rack-mounted equipment unit housing up to eight plug-in asynchronous
data units (ADUs).
modular processor data
module (MPDM)
A processor data module (PDM) that can be configured to provide several
kinds of interfaces (RS-232C, RS-449, and V.35) to customer-provided
data terminal equipment (DTE). See also processor data module.
modular trunk data
module (MTDM)
A trunk data module (TDM) that can be configured to provide several
kinds of interfaces (RS-232C, RS-449, and V.35) to customer-provided
data terminal equipment (DTE).