ISDN Demonstrator User’s Manual
40-400-00011, Rev. B
38
ITU (Internal Telecommunications Union)
The principal international standards organization chartered by the United Nations
and supported by treaty agreements among member nations. The ITU is the parent
organization for the ITU-T (Formerly CCITT) and the ITU-R (formerly CCIR). In
addition, the ITU has two other permanent organs: the International Frequency
Registration Board (IFRB), and General Secretariat; founded in 1865
LAPB (Link Access Procedure Balanced)
Link Access Procedure B. The Layer 2 protocol defined for X.25.
LAPD (Link Access Procedure D)
Layer 2 protocol defined in CCITT recommendations I.440/441/442 cross referenced
as Q.920/921/922. Reliably transfers blocks of information across a single Layer 1
link. Unlike LAP and LAPB, supports multiplexing or different connections at Layer 2.
Layer 1
The Physical Layer in the OSI Reference model. Includes transmission of signals and
the activation and deactivation of physical connections.
Layer 2
The Data Link layer in the OSI Reference model. Includes synchronization and some
control over the influence of errors within the physical layer.
Layer 3
The Network layer in the OSI Reference model. Includes routing and switching
functions.
Layer 4
The Transport layer in the OSI Reference model. Uses Layers 1 to 3 to provide an
end-to-end service with the required characteristics for the higher layer functions.
Layer 5
The Session layer in the OSI Reference model. Allows presentation entities to
organize and synchronize their dialogue and to manager their data exchange.
Layer 6
The Presentation layer in the OSI Reference model. Includes data formatting and
code conversion.
Layer 7
The Application layer in the OSI Reference model. Provides the means by which the
user programs access the OSI environment and may contain part of these user
programs.
LEC (Local Exchange Carrier)
Local Exchange Company. A telephone company affiliate of a Regional Bell
Operating Company or an independent telephone company such as Southern New
England Telephone (SNET), Cincinnati Bell, GTE (Contel), United, Centel, Rochester
Telephone, Wamego Telephone, or hundreds of others.
Line
A communication path between a switch and one end user. Analog lines are termed
tip and ring and carry one telephone call. ISDN lines are called Digital Subscriber
Lines and can carry several calls.
