glossary of Common Telecommunications Terms, EECS 663

Glossary Of Common Telecommunications Terms

Acknowledgment (ACK)
Message returned from receiver to transmitter to indicate successful reception of transmission. Also communications control character used for acknowledgment.

Adaptive Differential Pulse Code Modulation (ADPCM)
A form for the digital coding voice signals, typically at 32 Kbps.

Add/Drop Multiplexer (ADM)
A multiplexer that allows for the insertion and removal of embedded data channels without the demultiplexing of the entire bit stream.

Address Resolution Protocol (ARP)
A protocol used to adjust addresses between different networks or domains.

Advanced Intelligent Network (AIN)
A system that allows the telecommunications providers a mechanism to quickly and economically create and/or modify services for their customers.

Alarm Indication Signal (AIS)

Algorithm for multiple access in which any station transmits when ready, determines whether collision has occurred, and then retransmits if necessary.

Alternate Route
A secondary path between the source and destination that will be used if the primary path is unavailable.

American National Standards Institute (ANSI)
Standards body in United States responsible for a number of telecommunications standards. ANSI was formed for the purpose of establishing voluntary industry standards.

Amplitude Modulation (AM)
The strength of the radio frequency carrier is made proportional to the information signal.

Automatic Number Identification (ANI)
The display for the number of the calling telephone.

Analog-to-Digital Conversion (A/D)
Conversion of analog signals, e.g., voice or video, into a digital format.

Application Layer
Highest layer of OSI Reference Model. Layer 7 of the OSI model. This layer determines the interface of the system with the user and provides useful application-oriented services.

American Standard Code for Information Interchange. The standard code, using a coded character set consisting of 7-bit coded characters (8 bits, including the parity check), used for information interchange among data processing systems, data communications systems, and associated equipment. The ASCII character set consists of control characters and graphic characters.

Asynchronous Transmission
Random start and stop of data which requires special start and stop information embedded in the data flow.

Asynchronous Balanced Mode (ABM)
HDLC mode defined for balanced point-to-point configurations with two combined stations. Either end can initiate transmission without waiting for a poll from the other end.

Asynchronous Response Mode (ARM)
HDLC mode for systems with one primary and one or more secondaries; if more than one secondary is present, all but one must be quiescent. With one primary and one active secondary, either active station may initiate transmission at any time without waiting for a poll or an F bit.

Asynchronous Transfer Mode (ATM)
Broadband ISDN mode similar to asynchronous time division multiplexing. ATM uses small fixed length packets called cells.

ATM Adaptation Layer (AAL)
A protocol layer that segments incoming data into ATM cells.

Automatic Repeat Request (ARQ)
A feature that automatically initiates a request for retransmission when an error in transmission is detected. An error correction technique. When the receiving DTE detects an error, it signals the transmitting DTE to resend the data.

Basic Rate ISDN
Two B channels at 64kbps and one 16 kbps signaling channel.

Baud Rate
Number of signal changes per second used to convey information. Often misused to denote bit rate of digital signal.

B Channel
Full duplex 64 kbps. ISDN channel used for user data.

Bell Communications Research (Bellcore)
The research and development organization for the RBOC's.

Bell Operating Companies (BOCs)
The 22 telephone companies that were members of the Bell system before divestiture. Before the divestiture of AT&T, the 22 Bell Operating Companies were AT&T subsidiaries that built, operated, and maintained the local and intrastate networks and provided most of the day-to-day service for customers. After divestiture, the BOCs retain their identity within seven regional companies (RBOCs) and are responsible for local service as defined by local access and transport areas (LATAs).

Bell System
The collection of companies headed by AT&T and consisting of the 22 Bell Operating Companies and the Western Electric Corporation. The Bell System was dismantled by divestiture on January 1, 1984.

Bit Error Rate (BER)
A measure of the quality of a digital transmission facility.

Bit Interleaved Parity (BIP)
A parity check method used in SONET.

Bit Stuffing
The insertion of extra bits into a data stream to avoid the appearance of unintended control sequences.

Bit-oriented protocol
A communications protocol that uses only one special character, called the flag character, to mark the beginning and end of a message. All other combinations of bits are treated as valid data characters.

Blocking exist if a request for network resources can not be satisfied by the network.

A device for connecting like LAN's, e.g., Ethernets. Bridges operate at the data link layer.

Broadband ISDN (BISDN)
A second generation of ISDN. The key characteristic of broadband ISDN is that it provides transmission channels capable of supporting rates greater than the primary ISDN rate. Standards being developed for ISDN to handle applications such as video requiring high bandwidth.

Busy hour
The hour of the day when the traffic carried on a network is the highest.

Call Set-Up Time
The time requires to establish a dedicated end-to-end path in a circuit switching system.

Carrier sense multiple access (CSMA)
Medium access technique involving first sensing medium to see if other signals are already present, then transmitting if no signal is present. Several algorithms handle cases where signal is already present.

Carrier sense multiple access with collision detection (CSMA/CD)
A communications protocol frequently used on local area networks in which stations, upon detecting a collision of data caused by multiple simultaneous transmissions, wait a random period of time before retransmitting. Combination of CSMA with sensing to detect collisions during transmission. Standard medium access technique in some LANs, including Ethernet.

See Common Channel Signaling.

See Common Channel Signaling System #7.

Centa Call Seconds (CCS)
A measure of equipment or circuit utilization. One centa call second is 100 seconds of utilization.

Central Office (CO)
In the United States, the place where communications common carriers terminate customer lines and locate the equipment that interconnects those lines.

Central Office Switch
The equipment in a telephone company central office that allows any circuit to be connected to any other.

Channel Service Unit (CSU)
The interface to the T-1 line that terminates the local loop.

Circuit Switching
A method of communicating in which a dedicated communications path is established between two devices through one or more intermediate switching nodes. Unlike packet switching, information is sent as a continuous stream of bits. Data rate is guaranteed, and delay is essentially limited to propagation time. In circuit switching there is a temporary establishment of a connection between two pieces of equipment that permits the exclusive use until the connection is released. The connection is set up on demand and discontinued when the transmission is complete. An example is a dial-up telephone connection. This type of switching is used in ordinary telephone service, with dedicated path between source and destination set up for duration of call.

Code Division Multiple Access (CDMA)
A spread spectrum technique to share bandwidth among a set of users.

Result of multiple attempts to transmit at same time on multiple access medium. Usually all colliding transmissions wipe each other out and require retransmission.

Common Carrier
In the United States, companies that furnish long-distance telecommunication services to the public. Common carriers are subject to regulation by federal and state regulatory commissions.

Common Channel Interoffice Signaling (CCIS)
An AT&T system for sending signals between central offices.

Common Channel Signaling (CCS)
A method of signaling in which signaling information relating to a multiplicity of circuits, or function or for network management, is conveyed over a single channel by addressed messages. Signaling systems being used and installed in many telephone networks. CCS completely separates signaling information from user data by sending it over separate signaling network.

Common Channel Signaling System#7 (CCSS#7)
Version of CCS used in ISDN architecture.

A device that allows multiple end-point to share a common transmission facility.

Connectionless Transmission
Data transmission without prior establishment of a connection.

Constant Bit Rate (CBR)
A traffic type that generates a continuous flow of information, e.g., 64 Kbps voice or fixed rate video.

Connection-Oriented Transmission
Data transmission technique involving setting up connection before transmission and disconnecting it afterward.

Consultative Committee for International Telegraph and Telephone (CCITT)
International standards committee responsible for substantial portion of telecommunications network standards. CCITT is an international standards organization that is part of the International Telecommunications Union, which is an arm of the United Nations.

The condition that arises when two or more users simultaneously request access to the same network resource.

Crossbar Switch
A relay-operated device that makes a connection between one line in each of two sets of lines. The two sets are physically arranged along adjacent sides of a matrix of contacts or switch points.

Customer Premises Equipment (CPE)
Telecommunications hardware located at the users site.
Cyclic Redundancy Check (CRC)
An error detecting code in which the code is the remainder resulting from dividing the bits to be checked by a predetermined binary number.

Packet or short message transmitted through network without previously setting up a connection. Different datagrams are handled independently of each other. In packet switching, a self-contained packet, independent of other packets, that does not require acknowledgment and that carries information sufficient for routing from the originating data terminal equipment (DTE), without relying on earlier exchanges between the DTEs and the network.

Data Circuit Terminating Equipment (DCE)
The termination point for the network circuit. For example, a modem terminates the network circuit, the local loop, and is considered as the DCE. The users device in this example the remote terminal is considered the data terminal equipment (DTE).

Data Encryption Standard (DES)
A standard security technique for scrambling information.

Data Link Layer (DLL)
Layer 2 of OSI Reference Model and most other networking architecture's. Converts an unreliable transmission channel into a reliable one.

Data Link Connection Identifier (DCLI)
The address used in frame relay networks.

Data Terminal Equipment (DTE)
Typical the end-user devices, e.g., terminals or computers.

Digital-to-Analog Conversion (D/A)
The conversion of digital data to an analog signal.

Digital (Data) Service Unit (DSU)
Converts the terminal interface, e.g., RS-232, to line coding for transmission on the local loop.

Digital Speech Interpolation (DSI)
A voice compression technique based on the talk-spurt/silence property of speech.

D Channel
Full duplex 16 Kbps or 64 Kbps ISDN channel (in basic rate and primary rate interfaces, respectively) used to transmit control data or user data.

Distributed Queue Dual Bus (DQDB)
A protocol for metropolitan area networks, the bases for the IEEE 802.6 standard.

Digital service level-0, a 64kbps channel primarily used for digital voice.

Digital service level-1, a 1.544 Mbps channel in North America or a 2.048 Mbps channel in CCITT countries. Typically composed of multiple DS-0 channels. Typically a DS-1 is composed of 24 DS-0 signals.

Digital service level-3 a 44.736 Mbps digital signal, typically carries 28 DS-1's. Sometimes referred to as T3.

Dual Tone Multi-Frequency (DTMF)
The transmission of two tone (frequencies) that is used for dialing.

Electronic Document Interchange (EDI) -
The use of telecommunications to transmit documents electronically.

Electronic Mail (E-Mail)
The use of telecommunications for sending textual messages from one person to another. The capability to store the messages in an electronic mailbox is normally a part of the electronic mail system.

Electrical-Optical (E/O)
Electrical to Optical conversion.

The addition of control information by a protocol entity to data obtained from a protocol user.

End Office
A class 5 office of a local telephone exchange where a subscriberÕs loop terminates.

A measure of communications equipment or circuit usage. One Erlang is one hour of equipment usage or 36 CCS.

Erlang B
Blocking Formula The formula which characterizes the probability of blocking for a M/M/N/N system, i.e., a system that can hold N calls and has N servers. the systems has a Poisson Arrival process and a random exponentially distributed service or holding time.

A local area network that uses 1 persistent CSMA/CD protocol on a baseband bus.

Explicit Congestion Notification (ECN)
A mechanism used by the network to warn user equipment that the network is congested.

Extended Binary Coded Decimal Interchange Code (EBCDIC) -
The extended character set used in IBM hosts.

Facsimile Machine (FAX)
A machine that scans a sheet of paper and converts the light and dark areas to electrical signals that can be transmitted over telephone lines.

FDDI Fiber Distributed Data Interface
FDDI is a high-speed (100 Mbps) communications architecture standardized by ANSI, primarily for fiber optic links.

Fiber-Optic Link (FOL)

Federal Communications Commission (FCC)
A board of commissioners appointed by the president under the Communications Act of 1934; the commissioners regulate all interstate and foreign electrical telecommunications systems originating in the United States.

Fiber Distributed Data Interface (FDDI)
A 100 Mbps token ring LAN.

File Transfer Protocol (FTP)
A protocol for transferring files within TCP/IP networks.

First-in-first-out (FIFO)
Queueing in the order that calls or transactions arrive. Calls that arrive first get serviced first.

Flag (1)
Any of various types of indicators used for identification. (2) A bit sequence that signals the occurrence of some condition, such as the end of a word. (3) In high-level data link control (HDLC), the initial and final octets of a frame with the specific bit configuration of 01111110. A single flag may be used to denote the end of one frame and the start of another.

Flow Control
Regulation of traffic allowed into specific portions of a network to avoid excessive congestion. Flow Control is performed by a receiving entity to limit the amount or rate of data sent by a transmitting entity.

Forward Error Correction (FEC)
Approach to error control in which redundancy is included in transmitted messages to allow correction of errors at receiver without retransmission.

Data unit generated by data link layer, consisting of user data plus all pertinent headers and trailers.

Frame Relay (FR)
A telecommunications service/network that relies on simplified link-by-link protocols, high speed transmission (up to 45 Mbps), low link BER, virtual circuits, and end-to-end error recovery.

Frequency-Division Multiplexing (FDM)
Division of a transmission facility into two or more channels by splitting the frequency band transmitted by the facility into narrower bands, each of which is used to constitute a distinct channel.

Frequency Modulation (FM)
The frequency of the radio frequency carrier is made to be proportional to the information signal.

Full Duplex Channel
A full duplex channel is capable of transmission in both directions simultaneously.

A device that allows for the interconnection of dissimilar networks. Gateways provide protocol translation, address translation, and code translation between the connected networks. All seven layers of the OSI model may be needed to perform these functions.

Geosynchronous Orbit
The position at which communications satellites will remain in orbit above the same location on the earth, about 23,000 miles above the surface.

Giga (G)
One billion. For example, 1 gigahertz equals 1,000,000,000 hertz. One gigahertz also equals 1000 megahertz and 1,000,000 kilohertz.

Half Duplex Channel
A half duplex channel is capable of transmission in only one direction at a time.

High-Level Data Link Control (HDLC)
A data link layer protocol.

High Speed Network (HSN)
High-Speed Peripheral Parallel Interface (HiPPi) A computer channel interface clocked at 25 MHz, a 32 bit wide bus is used for a 800 Mbps channel while a 64 bit wide bus for a 1.6 Gbps rate. Primarily used to connect supercomputers.

Holding time
The duration of a switchboard call. Most often applied in traffic studies to the duration of a telephone call.

IEEE 802
Set of standards for LANs being developed by IEEE 802 Committee.

IEEE 802.2
Logical link control protocol developed by IEEE 802 Committee.

IEEE 802.3
CSMA/CD-MAC protocol developed by IEEE 802 Committee.

IEEE 802.4
Token bus MAC protocol developed by IEEE 802 Committee.

IEEE 802.5
Token ring MAC protocol developed by IEEE 802 Committee.

IEEE 802.6
Metropolitan Area Network MAC protocol developed by IEEE 802 Committee that uses the Distributed Queue Dual Bus (DQDB) protocol.

In-Band Signaling
The transmission of control information in the same 4kHz channel used for voice transmission.

Integrated Services Digital Network (ISDN)
Worldwide telecommunication service that will use digital transmission and switching technology to support voice and digital data communications. ISDN is being developed by telephone companies to serve all data communications needs of users.

Interexchange Carrier (IXC)
Long distance carriers. Contrast with local exchange carrier (LEC).

International Standards Organization (ISO)
An organization established to promote the development of standards to facilitate the international exchange of goods and services, and to develop mutual cooperation in areas of intellectual, scientific, technological, and economic activity. This international standards organization is responsible for telecommunications networking standards including OSI Reference Model.

International Telecommunications Union (ITU)
The specialized telecommunications agency of the United Nations, established to provide standardized communications procedures and practices, including frequency allocation and radio regulations, on a world-wide basis.

A conceptual entity that embodies one or more functions between an upper and a lower logical boundary.

Layered Network Architecture
Currently the basis of all telecommunication network architecture standards, with functions allocated to different layers and standardized interfaces between layers.

Local Access and Transport Area (LATA)
Within a LATA the LEC can carry all the traffic. The LEC is restricted from carrying traffic between LATA's, i.e., interLATA traffic can not be transmitted by the LEC.

Local Area Network (LAN)
Network spanning an area of not more than a few kilometers in diameter.

Local Exchange Carrier (LEC)
The BOCs and the independent telephone companies.

Local Loop
A channel connecting the subscriberÕs equipment to the line-terminating equipment in the central office exchange. A transmission path, generally twisted pair, between the individual subscriber and the nearest switching center of a public telecommunications network. Also referred to as a subscriber loop.

Logical Link Control (LLC)
Upper half of data link layer in IEEE 802 architecture.

M/M/1 Queue
A queue with infinite buffer space with a Poisson Arrival process and a random exponentially distributed service time (or message length).

Management Information Base (MIB)
A description of a network element that is used for network management purposes.

Manufacturing Automation Protocol (MAP)
Networking architecture, based on token bus protocol, developed by General Motors for automating manufacturing processes.

Medium Access Control (MAC)
Lower half of data link layer IEEE 802 architecture; handles medium access functions. Used to keep stations sharing common communications link from interfering with each other.

Message Switching
Switching technique involving transmission of messages from node to node, with storage at intermediate nodes until next portion of path available. (1) In a data network, the process of routing messages by receiving, storing, and forwarding complete messages. (2) The technique of receiving a complete message, storing it, and then forwarding it to its destination unaltered.

Metropolitan Area Network (MAN)
Network spanning area up to a few dozen kilometers in diameter. A network of limited geographic scope, generally defined as within a 50-mile radius. Standards for MANs are being defined by the IEEE.

Modem (Modulate/Demodulate)
A device the converts a binary information stream into an analog signal for transmission.

Multiplexer (MUX)
A device capable of interleaving the events of two or more activities or capable of distributing the events of an interleaved sequence to the respective activities.

(1) In data transmission, a function that permits two or more data sources to share a common transmission medium such that each data source has its own channel. (2) The division of a transmission facility into two or more channels either by splitting the frequency bank transmitted by the channel into narrower banks, each of which is used to constitute a distinct channel (frequency division multiplexing), or by allotting this common channel to several different information channels, one at a time (time division multiplexing).

Network Element (NE)

Network Layer
Layer 3 of OSI Reference Model. Responsible for routing data through a communication network.

Network Management Protocol (NMP)
A protocol used to control and manage network resources.

Network-Node Interface (NNI)
Point-to-Point interface between two switches in B-ISDN.

Any request for network resources can be satisfied independent of the condition (state) of the system.

Activated (in regard to a telephone set). By extension, a data set automatically answering on a public switched system is said to go off-hook. Contrast with on-hook.

Deactivated (in reference to a telephone set). A telephone not in use is on-hook. Contrast with off-hook.

Open Systems Interconnection (OSI) Reference Model
A model of communications between cooperating devices. It defines a 7-layer architecture of communication functions. The OSI reference model is a telecommunication networking architecture developed by International Organization for Standardization (ISO) and adopted as an international standard; also known as Reference Model for Open Systems Interconnection.

Operations, Administration, Maintenance, and Provisioning (OAM&P)
The day-to-day execution of functions within the telecommunications network needed to provide service to the customers.

Optical Carrier -N (OC-N)
Line rates used in SONET optical transmission systems. The base rate is 51.84 Mbps, the higher OC-N rates are N X 51.84 Mbps, e.g., OC-12 is at 622Mbps.

Optical-Electrical (O/E)
Optical to electrical conversion.

The information added to the users data to facilitate its transport through the network.

A sequence of binary digits (including data and call control signals) that is switched as a composite whole. The data, call control signals, and possible error control information are arranged in a specific format.

Packet Switching
Switching technique involving breaking long messages up into shorter packets transmitted individually through network, with storage at intermediate nodes until next portion of route available. A method of transmitting messages through a communications network, in which long messages are subdivided into short packets. Each packet is passed from source to destination through intermediate nodes. At each node, the entire message is received, stored briefly, and then passed on to the next node.

Permanent Virtual Circuit (PVC)
A virtual circuit that is an assigned connection over a packet switched network and is not switchable by the users.

Personal Communications Network (PCN)
The network that supports Personal Communications Services.

Personal Communications Services (PCS)
A family of end-to-end telecommunications services that provide: 1) Reception/Initiation of voice or data calls via a personal number that is associated with the customer rather than a physical location, i.e., a plug in the wall, or specific terminal device. 2) Access to wire line services using a wireless terminal device. 3) The quality and security comparable to that of the wireline network.

Physical Layer
Layer 1 of the OSI model. Concerned with the electrical, mechanical, and timing aspects of signal transmission over a medium.

The inclusion of an acknowledgment of a previously-received protocol data unit in an outgoing protocol data unit.

Plain Old Telephone Service (POTS)
Basic telephone service with no special features.

Possion Arrival Process
A common model for the random arrivals to a queue, or telecommunication server where the number of arrivals, n, in a time interval t follow a Poisson probability mass function. The random interarrival time follows an exponential probability density function.

Postal, Telegraph, and Telephone (PTT)
A government organization that operates a nationalized public telecommunications network.

Presentation Layer
Layer 6 of the OSI model. The presentation layer is responsible for code conversion, data encryption, and data compression.

Primary Rate ISDN
Twenty three B channels an one D channel for control signaling.

Private Automatic Branch Exchange (PABX)
A private automatic telephone exchange that provides for the transmission of calls to and from the public telephone network. See also private branch exchange (PBX).

Private Branch Exchange (PBX)
A private telephone exchange connected to the public telephone network on the userÕs premises. A PBX provides a circuit switching facility for telephones on extension lines within the building and access to the public telephone network.

A formal statement of the procedures that are adopted to ensure communication between two or more functions within the same layer of a hierarchy of functions. (1) A specification for the format and relative timing of information exchanged between communicating parties. (2) The set of rules governing the operation of functional units of a communications system that must be followed if communications are to be achieved. A protocol is a set of mutually agreed upon rules of procedure stating how two or more parties are to interact to exchange information.

Protocol Data Unit (PDU)
Type of data unit used by a particular protocol. Often preceded by letter to indicate protocol layer--for example, NPDU and TPDU for network and transport layers. A PDU is the information that is delivered as a unit between peer entities of a network and may contain control information, address information, or data.

Public Switched Telephone Network (PSTN)
The dial up public telecommunications network.

Pulse Code Modulation (PCM)
A process in which a signal is sampled, and the magnitude of each sample with respect to a fixed reference is quantized and converted by coding to a digital signal.

Quality of Service (QoS)

Queued Packet Synchronous exchange (QPSX)
The old name for DQDB.

Random Access
Unscheduled access to communications medium in which stations transmit when ready (possibly after sensing medium), and later resolve any conflicts that arise.

Regional Bell Operating Company (RBOC)
One of the seven corporations formed when divestiture occurred and that comprise the 22 Bell Operating Companies.

Registered Jack (RJ)
The connector of user network interface, the standard telephone jack is the RJ-11.

The signal made by a telephone to indicate an incoming call.
Segmentation and Reassembly (SAR)
The protocol that divides packets into cells and combines cells into packets.

Session Layer
Layer 5 of OSI Reference Model. Manages a logical connection (session) between two communicating processes or applications.

Service Access Point (SAP)
The logical address of a session within a physical terminal.

The exchange of information specifically concerned with the establishment and control of connections, and with management, in a telecommunication network.

Simple Network Management Protocol (SNMP)
A management protocol based on TCP/IP and MID's.

Sliding Window Flow Control
Type of flow control in which transmitter is given permit to transmit ÒwindowÓ of packets or other data units, and not allowed to transmit more until it receives another permit.

Modification of ALOHA medium access technique in which time is divided into slots of duration equal to packet duration and each transmission is required to start at the beginning of a slot.

Synchronous Optical Network is a standard for optical transmission. It is a physical layer framing structure at rates from about 51.84 Mbps through over 9.9 Gbps.

Step-by-step Switch
A switch that moves in synch with a pulse device, such as a rotary telephone dial. Each digit dialed moves successive selector switches to carry the connection forward until the desired line is reached.

Stop-and-Wait Protocols
Protocols in which the sender sends one frame and then waits for an acknowledgment before sending the next frame.

Store-and-Forward System
An application in which input is transmitted, usually to a computer, stored, and then later delivered to the recipient.

Strowger Switch
A step-by-step switch named after its inventor, Almon B. Strowger. See also step-by-step switch.

Subscriber Line Interface Circuit (SLIC)
A SLIC is the interface between the local loop and the switch. SLIC functions include, -48 V dc battery, ringing voltage supply, overload protection, and loop supervision.

A measure of a network's ability to withstand failures in links and nodes.

Switched Multimedia Data Service (SMDS)
A Bellcore data MAN service offered by telecommunications carriers.

Switched Virtual Circuit (SVC)
A temporary logical connection in a packet switched network.

Synchronous Transmission
Data bit and characters are transmitted at a fixed rate with the transmitter and receiver synchronized at the bit and character level.

Synchronous Transport Signal -N (STS-N)
The electrical equivalent of the OC-N.

T-1 Carrier System
Digital telephone carrier system developed by Bell Labs in United States as first purely digital carrier system.

The standards committee responsible for the transmission issues in the US., corresponds to the ETS1 in Europe and the Telecommunications Technology Committee in Japan.

Technical Office Protocol (TOP)
A set of standard protocols aimed at office automation.

Using telecommunications to work from home or other locations instead of on the businessÕs premises.

A measure of system efficiency.

Time Assignment Speech Interpolation (TASI)
Also known as Digital Speech Interpolation (DSI) A technique of multiplexing telephone calls by taking advantage of the pauses in normal speech and assigning the channel to another call during the pause.

Time Division Multiplexing (TDM)
Communication channel sharing technique carrying different signals in different time slots. The division of a transmission facility into multiple channels by allotting the facility to different channels, one at a time.

Time Shared Space Division Switch
A crosspoint array where each input is a TDM bus and which can reconfigure the connection on each time slot. Known as the S stage in a multi-stage switch architecture.

Time-Slot Interchange (TSI)
The interchange of time slots within a time-division multiplexed frame. Known as the T stage in a multi-stage switch architecture.

Transport Layer
Layer 4 of the OSI model. Provides reliable, sequenced transfer of data between endpoints.

Transmission Control Protocol/Internet Protocol (TCP/IP)

A telephone channel between two central offices or switching devices that is used in providing a telephone connection between subscribers.

User-Network Interface (UNI)
The demarcation between the network and the customer premises.

Virtual Circuit
Transmission path set up, end to end, by connection protocol before transmission. A packet-switching mechanism in which a logical connection (virtual circuit) is established between two stations at the start of transmission. All packets follow the same route, need not carry a complete address, and arrive in sequence.

Virtual Circuit Identifier (VCI)
An address associated with a specific VC that is part of the packet header.

Valued Added Network (VAN)
Typically a packet switched network with the added access to data bases and other services.

Variable Bit Rate (VBR)
VBR traffic is typically from bursty traffic sources and usually requires packetized bandwidth an demand for effective transport.

Voice Grade Channel
A communications channel suitable for the transmission of voice.

Virtual Terminal
A concept that allows an application program to send or receive data to or from a generic terminal. Other software transforms the input and output to correspond to the actual characteristics of the real terminal being used.

Wavelength Division Multiplexing (WDM)
Two or more optical frequencies(colors) are use on the same fiber.

Wide Area Network (WAN)
A network that covers a large geographic area, requiring the crossing of public right-of ways and the use of circuits provided by a common carrier.