been approved by a regulatory group.
exchange area—A geographical area with a uniform set of charges (tariffs), approved by a regulatory group for telephone services. Calls between any two points within an exchange area are local calls. See digital PBX and PBX.
F
fax (facsimile)—Refers to the bit-mapped rendition of a graphics- oriented document (fax) or to the electronic transmission of the image over telephone lines bit-mapped approximation of a graphical document and, therefore, cannot be accurately interpreted according to any character code.
firmware—Software that is stored in read-only memory (ROM). Unlike random access memory (RAM), ROM can retain its content without electrical power. Some ROM, known as EEPROM, can be reprogrammed electrically while it is in a device.
foreground—The application program currently running on and in control of the PC screen and keyboard. The area of the screen that occupies the active window. Compare with background.
frequency—A characteristic of an electrical or electronic signal which describes the periodic recurrence of cycles. Frequency is inversely proportional to the wavelength or pulse width of the signal (e.g., long wavelength signals have low frequencies and short wavelength signals yield high frequencies).
H
handshaking—A process that two modems go through at the time of call setup to establish synchronization over the data communications link. It is a synchronization and negotiation process accomplished by the exchange of predefined, mutually recognized control codes.
HDLC (High-Level Data Link Control)—An ISO standard, bit- oriented data communications protocol that provides nearly error- free data transfers.
hexadecimal—A base 16 numbering system in which one character represents a four-bit binary value. Hexadecimal notation uses the numbers 0–9 and the letters A–F, usually suffixed by an h (e.g., 4CF h).
I
IDN (Integrated Digital Network)—The culmination of transmission and switching functions using digital technology in a circuit- switched telecommunications network.
IEC (Inter-Exchange Carrier)—A telephone company that is allowed to provide long distance telephone service between LATAs, but not within any one LATA. Also called IXC.
IETF(Internet Engineering Task Force)—
A self-organized group of people who make technical and other contributions to the engineering and evolution of the Internet and its technologies. It is the principal body engaged in the development of new Internet standard specifications.
I-mux(inverse multiplexer)—A device that will dial up and combine several lower speed circuits into a virtual broadband circuit. It takes any multiple up to an aggregate bandwidth, giving users bandwidth on demand for videoconferencing, LAN-to-LAN connec- tions, etc. In ISDN devices, inverse multiplexing is performed by the BONDING protocol.
in-bandsignaling—A technique in which connection control information is sent over the same channel as assigned to the subscriber for end-to-end communication. Two common examples are DTMF line signaling and MF trunk signaling.
Internet—A computer network consisting of many millions of
university, government, and private users around the world.
Internet address—A unique 32-bit address for a specific TCP/IP host on a network. Normally printed in dotted decimal format (e.g., 129.128.44.227).
I/O address—A location 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.
IP address—A unique 32-bit address for a specific TCP/IP host on a network. Normally printed in dotted decimal format (e.g., 129.128.44.227).
IRQ (interrupt request)—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 that IRQs can be prioritized.
ISA (Industry Standards Architecture)—Pronounced ice-a. The classic 8- or 16-bit architecture introduced with IBM’s PC-AT computer. Due to speed constraints, IBM later introduced a restructured Micro Channel Architecture with a 32-bit bus and increased addressing capabilities.
ISDN (Integrated Services Digital Network)—International telecommunications standard for transmitting voice, video and data over a digital communications line.
ITU-T(formerly CCITT)—International Telecommunications Union–Telecommunications Sector; the United Nations organization that prepares standards for resolving communications issues and problems.
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 connec- tions; different from LAPB in its framing sequence. Transmission is in units called frames, and a frame may contain one or more X.25 packets.
link optimization—An ISDN feature that permits only user data to open the communications link and keeps administrative packets from opening the link. A cost saving feature that ensures that remote connections are not unnecessarily utilized.
local loop—A transmission path, typically twisted-pair wire, between an individual subscriber and the nearest public telecommunications network switching center. Also called the subscriber loop.
LT (loop termination)—A device, similar to an NT1, that terminates the local loop at the carrier’s central switching office. The LT provides the interface to the ISDN U reference point. Contrast ET (exchange termination).
M
MD5(MD5 Message-Digest Algorithm)—Takes a message if arbitrary length as input and produces it as output as a 128-bit fingerprint or message digest of the input. The MD5 algorithm is used for digital signature applications where a large file must be
