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ProxyServer User Guide
ISDN (Integrated Services Digital Network): An International telecommunications standard for transmitting voice, video
and data over a digital communications line. ISDN is a world-wide telecommunications 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 connection at
64K bps 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 communications issues and problems.
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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).
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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 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 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 (i.e., not long distance) transmission
services. AKA "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. (AKA, "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).
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MTPSR3-200
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