A (access) link
A link that interconnects an STP with either an SSP or an SCP. The SSP and SCP, collectively, are referred to as the signaling endpoints. A message sent to and from the SSPs or SCPs first goes to its home STP, which in turn processes or routes the message.

ABR (available bit rate)
One of the ATM service classes. ABR supports VBR data traffic with average and peak traffic parameters (for example, LAN interconnection and internetworking services, LAN emulation, critical data transfer that requires service guarantees). Remote procedure calls, distributed file services, and computer process swapping and paging are examples of applications that would be appropriate for ABR.

access charge
A cost assessed to interexchange carriers for access to the local exchange network.

access concentrator
A device that can be used to concentrate local subscriber lines and multiplex them over high-speed transport to another point in the network, creating a virtual POP.

access line
The connection from the customer to the local telephone company for access to the public switched telephone network, also known as the local loop; can also refer to the connection between the serving toll exchange and the serving office of the interexchange carrier used to access public switched network transport services.

ACK (acknowledge character)
A transmission control character transmitted by a station as an affirmative response to the station with which a connection has been set up. An acknowledge character may also be used as an accuracy control character.

adaptive equalization
Line equalization, sometimes known as impedance equalization, used for optimizing signal transmission to adapt to changing line characteristics.

adaptive routing
Routing that automatically adjusts to network changes such as traffic pattern changes or failures.

address
(1) A coded representation of the destination of data, as well as of its source. Multiple terminals on one communications line, for example, must each have a unique address. (2) A group of digits that makes up a telephone number. Also known as the called number. (3) In software, a location that can be specifically referred to in a program. (4) A name, label, or number that identifies a location in storage, a device in a network, or any other data source.

address signals
Signals that carry information that has to do with the number dialed, which essentially consists of country codes, city codes, area codes, prefixes, and the subscriber number.

ADM (add/drop multiplexer)
A device that facilitates easy dropping and adding of payload by converting one or more lower-level signals, such as T-1 or E-1 signals, to and from one of the optical carrier levels.

administrative domain
A collection of hosts, routers, and networks governed by a single administrative authority.

ADPCM (Adaptive Differential Pulse Code Modulation)
An encoding technique, standardized by the ITU-T, that allows analog voice signals to be carried on a 32Kbps digital channel. The voice input is samples at 8KHz with 4 bits used to describe the difference between adjacent samples.

ADSL (Asymmetrical Digital Subscriber Line)
A technology for supporting high bandwidth over conventional twisted-pair local loop lines that enables subscribers to access multimedia-based applications such as video-on-demand. ADSL-1 supports a downstream channel of 1.5Mbps (North America) or 2Mbps (Europe), with an upstream channel of 64Kbps. ADSL-2 supports a downstream channel of 6Mbps (North America) or 8Mbps (Europe) and an upstream channel of 640Kbps.

AES (Advanced Encryption Standard)
An encryption algorithm for securing sensitive but unclassified material by U.S. government agencies. It may eventually become the de facto encryption standard for commercial transactions in the private sector. Uses the Rijndael algorithm to specify three key lengths?128 bits, 192 bits, and 256 bits.

agent
Software that processes queries and sends responses on behalf of an application.

agnostic device
A device that supports multiple data protocols (for example, IP, Frame Relay, ATM, MPLS) and supports multiple traffic types, such as voice, data, and video.

AIN (advanced intelligent network)
The second generation of intelligent networks, pioneered by Bellcore (which is now Telcordia). A service-independent network architecture that enables carriers to create and uniformly support telecom services and features via a common architectural platform, with the objective of allowing for rapid creation of customizable telecommunication services.

AIP (application infrastructure provider)
A provider that manages the data center servers, databases, switches, and other gears on which the applications run.

A-law encoding
Encoding, according to ITU-T Recommendation G.711, that is used with European 30-channel PCM systems that comply with ITU-T Recommendation G.732. Employs nonuniform quantization to obtain the desired compression characteristic.

alerting signals
The ringing tones, the busy tones, and any specific busy alerts that are used to indicate network congestion or unavailability.

alternate mark inversion
A digital signaling method in which the signal carrying the binary value alternates between positive and negative polarities; zero and one values are represented by the signal amplitude at either polarity; no-value “spaces” are at zero amplitude. Also called bipolar.

alternate routing
The routing of a call or message over a substitute route when an established route has failed, is busy, or is otherwise unavailable for immediate use.

AM (amplitude modulation)
Varying of a carrier signal’s strength (amplitude) depending on whether the information being transmitted is a 1 or a 0 bit.

ambient noise
Communications interference that is present in a signal path at all times.

amplifier
A device that boosts an attenuated signal back up to its original power level so that it can continue to make its way across the network.

amplitude
A measure of the height of the wave, which indicates the strength of the signal.

AMPS (Advanced Mobile Phone System)
A standard for analog telephony that is deployed widely in the U.S.

analog
A signal that varies continuously (e.g., sound waves), along two parameters: amplitude (strength) and frequency (tone). The unit of measurement is the Hertz, or cycle per second.

analog loopback
A technique for testing transmission equipment and devices that isolates faults to the analog signal receiving or transmitting circuitry, where a device, such as a modem, echoes back a received (test) signal that is then compared with the original signal.

ANI (automatic number identification)
A feature, often associated with SS7, that passes a caller’s telephone number over the network to the receiver so that the caller can be identified. Also referred to as caller ID.

ANSI (American National Standards Institute)
A standards-forming body affiliated with the ISO that develops U.S. standards for transmission codes, protocols, media, and high-level languages, among other things.

ANSI X12
A U.S. standard for electronic data interchange.

answer signal
A supervisory signal (usually in the form of a closed loop) from the called telephone to the exchange and back to the calling telephone (usually in the form of a reverse battery) when the called number answers.

API (application programming interface)
A set of routines that an application program uses to request and carry out low-level services performed by the operating system.

AppleTalk
Apple Computer’s set of specifications for connecting computers and other devices to share information over LANs. It describes network hardware, software, and protocols and lets an assortment of Mac and non-Mac devices communicate over a variety of transceivers and communications media.

application
(1) Software with which the user interacts. (2) The use to which a system is put; for example, e-mail, videoconferencing, high-speed data access, and network management.

application layer
Layer 7 of the OSI model, which enables users to transfer files, send mail, and perform other functions that involve interaction with the network components and services.

application-layer multicasting
A technique which ensures that just one stream goes across the backbone whenever possible.

application program
A software program that contains no I/O coding (except in the form of macro instructions that transfer control to the supervisory programs) and is usually unique to one type of application.

architecture
The physical interrelationship between the components of a computer or a network.

area code
A three-digit code designating a toll center that is not in the NPA of the calling party.

area code restriction
The capability of switching equipment to selectively identify three-digit area codes and to either permit or deny passage of long-distance calls to those specific area codes.

ARQ (automatic repeat request)
An error-control technique that requires retransmission of a data block that contains detected errors. A special form, called “go-back-n,” allows multiple blocks to be acknowledged with a single response. “Stop and wait” requires an acknowledgment after each block.

artificial intelligence
The capability of a computer to perform functions that are associated with human logic such as reasoning, learning, and self-improvement.

AS (autonomous system)
A collection of TCP/IP gateways and networks that fall under one administrative entity and cooperate closely to propagate network reachability (and routing) information among themselves, using an interior gateway protocol.

ASCII (American Standard Code for Information Interchange)
The code developed by ANSI for information interchange among data processing systems, data communication systems, and associated equipment. The ASCII character set consists of 7-bit coded characters (8 bits including the parity bit), providing 128 possible characters. The ASCII character set consists of 34 control codes and 94 text characters, including the letters of the alphabet in both upper- and lowercase, the 10 digits, and a number of special characters.

ASP (application service provider)
A supplier that makes applications available on a subscription basis.

aspect ratio
The horizontal:vertical size ratio used for television. Traditional television has an aspect ratio of 4:3, and new DTV standards include a 16:9 aspect ratio, which more closely resembles human vision.

asynchronous
(1) Occurring without a regular or predictable time relationship to a specified event. (2) In data communication, a method of transmission in which the bits representing a character are preceded by a start bit and followed by a stop bit, which are used to separate the characters and to synchronize the receiving with the transmitting station. It does not use a regular time relationship between the sending and receiving devices.

asynchronous transmission
A transmission in which each information character, or sometimes each word or small block, is individually synchronized, usually by the use of start and stop elements. Also called start-stop transmission.

ATM (Asynchronous Transfer Mode)
An international packet-switching standard that uses a cell-based approach, in which each packet of information features a uniform cell size of 53 bytes. ATM is a high-bandwidth, fast packet-switching and multiplexing technique that allows the seamless end-to-end transmission of voice, data, image, and video traffic. It’s a high-capacity, low-latency switching fabric that’s adaptable for multiservice and multirate connections and offers an architected approach to Quality of Service.

ATM adaptation layer
The ATM layer that is responsible for the adaptation of the information of the higher layer to the ATM cells. It is composed of two layers, the Segmentation and Reassembly sublayer and Convergence sublayer. ATM Adaptation Layer 1 supports CBR voice and video network services. ATM Adaptation Layer 2 supports VBR voice and video network services. ATM Adaptation Layer 3 supports VBR connection oriented data services. ATM Adaptation Layer 4 supports VBR connectionless oriented data services. ATM Adaptation Layer 5 supports connectionless variable bit rate data (e.g., IP or signaling) over ATM.

ATM layer
The layer that performs four main functions: multiplexing and demultiplexing of cells of different connections; translation at ATM switches and cross-connects; cell header extraction or addition before or after cell is delivered to or from the adaptation layer; and flow control.

ATM physical layer
A layer composed of two sublayers: Physical Medium, which supports pure medium-dependent functions, and Transmission Convergence, which converts the ATM cell stream into bits to be transported over the physical medium.

ATM reassembly
Restructuring of data units from information contained in cells.

ATM segmentation
Parsing of the information units of the higher layers into ATM cells.

ATSC (Advanced Television Systems Committee)
An organization that establishes voluntary technical standards for advanced television systems, including DTV.

attenuation
A decrease in the power of a received signal due to loss through lines, equipment, or other transmission devices. Usually measured in decibels.

audible ringing tone
A tone received by the calling telephone indicating that the called telephone is being rung (formerly called ringback tone).

audio frequencies
Frequencies that correspond to those that can be heard by the human ear (usually 30Hz to 20,000Hz).

AUI (attachment unit interface)
The connector used to attach a device to an Ethernet transceiver.

authentication
Any technique that enables the receiver to automatically identify and reject messages that have been altered deliberately or by channel errors. Also can be used to provide positive identification of the sender of a message.