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ProxyServer User Guide

MTPSR1-202ST

munications service that uses digital transmission and switching technology to support voice and

digital data communications. Frame relay was partially based on ISDN’s data link layer protocol

(LAPD). Frame relay can be used to transmit across ISDN services offering circuit-switched connec-

tion at 64 Kbps and higher speeds. Contrast Public Switched Telephone Network (PSTN).

ITU-TSS (formerly CCITT): International Telecommunications Union-Telecommunications Sector;

the United Nations organization that prepares standards (“Recommendations”) for resolving commu-

nications issues and problems.

No Entries.

Key Telephone System (KTS): Phone devices with multiple buttons that let you select incoming or

outgoing CO phone lines directly. Similar in operation to a PBX, except with a KTS you don’t have to

dial a “9” for a call outside the building.

Key Service Unit (KSU): A small device containing the switching electronics for a business key

telephone system (KTS).

Key Set: A telephone set with several buttons for call holding, line pickup, intercom, autodialing, etc.

Also called a touchtone phone (Ericsson) and a KTS (Key Telephone Set).

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 com-

monly 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

telephone 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 con-

nected 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 equiva-

lent 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.

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