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MultiModemZPX User Guide
request attention from a host. The host typically invokes an
interrupt service routine to handle the condition which caused
the request.
I/O Addresses: Locations within the I/O address space of your
computer used by a device, such as an expansion card, a serial
port, or an internal modem. The address is used for
communication between software and a device.
IRQ Level (Interrupt Request Level): The notification a
processor receives when another portion of the computer’s
hardware requires its attention. IRQs are numbered so that the
device issuing the IRQ can be identified, and so IRQs can be
prioritized.
Isochronous data: Information delivered in a continuous
stream at a steady rate.
ISA (Industry Standards Architecture) (pronounced “ice a”):
The classic 8 or 16-bit architecture introduced with IBM’s PC-AT
computer.
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 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 communications issues and problems.
J
No Entries.
K
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” to 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 phone set with several buttons for call holding, line
pickup, intercom, autodialing, etc. Also called a touchtone
phone (Ericsson) and a KTS (Key Telephone Set).
L
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 (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).
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).