A

Access Services
Access services are specified sets of information transfer capabilities furnished to users at telecommunications network points-of-termination (POTs) to provide access to network transport services. Two examples are the following: subscriber access lines, the connection between a network POT (in this case more commonly known as a Network Interface NI) and a local exchange carrier switching system; and trunks between interexchange carrier points-of-presence and local exchange carrier switching systems (The POT at the POP is identified as the point of interface POI). End-to-end connections require originating and terminating access services.

Access Tandem (AT)
AT is a LEC switching system that performs concentration and distribution functions for inter-LATA traffic originating or terminating within a LATA.

ACD 
automatic call distribution

ACF 
access coordination fee

Address Signals
Address Signals convey destination information such as a called 4-digit extension number, central office code, and when required, area code and serving IXC carrier code. These signals may be generated by station equipment, or by a switching system.

AMI 
alternate mark inversion

Analog Carrier System
An analog carrier system is a transmission system that uses repeaters that compensate for analog medium impairments, and produce output signals that are linear-scaled versions of input signals. Analog carrier systems can carry speech, data, video and supervisory signals although they are best suited for speech signals.

Analog Signal
An analog signal is a continuous electrical signal that varies in direct correlation with an impressed phenomenon, stimulus, or event that bears intelligence. Sound waves and their electrical analogs are characterized by loudness (amplitude) and pitch. Analog signals can assume any of an infinite number of amplitude values or states within a specified range, in accordance with or analogous to, an impressed stimulus. Pitch refers to how many times per second the signal swings between high and low amplitudes, i.e., its frequency.

ANI 
automatic number identification

ANSI 
American National Standards Institute

ASB 
asynchronous balanced mode

ASI 
alternate space inversion

Asynchronous Transfer Mode (ATM)
ATM is a broad-bandwidth, low delay, packet-like (cell relay) switching and multiplexing technique. It is essentially connection oriented, although it is envisioned to support all services. ATM networks will accept or reject connections based on user's average and peak bandwidth requirements providing flexible and efficient service for LAN-to-LAN, compressed video and other applications that involve variable bit rate (VBR) traffic.

ATM 
asynchronous transfer mode (See Asynchronous Transfer Mode).

Automatic Call Distribution (ACD)
ACD is a means for efficiently directing and managing large numbers of incoming calls to specific departments/terminals within an organization.

AWG 
American Wire Gauge.

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B

B8ZS 
bipolar eight-zero substitution

Backbone Network
A backbone network is a transmission facility designed to interconnect often lower-speed distribution networks, channels, or clusters of dispersed terminals or devices.

Backbone Wiring
In a premises distribution system, backbone wiring is the cable connecting telecommunications closets and equipment rooms within a building, and/or between buildings in a campus. Backbone wiring is sometimes referred to as the riser subsystem.

Bandwidth
Bandwidth is a frequency range, usually specified by the number of hertz in a band or between upper and lower limiting frequencies. Alternatively, the frequency range that a device is capable of generating, handling, passing or allowing.

BFt 
terminal framing bit

BF 
framing bit

Binary digit
See Bit.

Binding Post
A binding post is a screw with a small nut mounted on a terminal box. Wires are joined together by wrapping them around a screw and then tightening the nut on them.

Bipolar Signals
Bipolar signals are signals in which positive and negative pulses, always alternating, represent one binary signal state only. The absence of pulses represents the other binary state. Bipolar has two forms, AMI and ASI. In alternate mark inversion (AMI) the pulses correspond to ls; in alternate space inversion (ASI) the pulses correspond to 0s.

BIPS 
billion instructions per second

BISDN 
broadband integrated services digital network

Bit
A bit is the most fundamental and widely used form of digital signals are binary signals, in which one amplitude condition represents a binary digit 1, and another amplitude condition represents a binary digit 0. Thus a binary digit, or bit is one of the members of a set of two in a numeration system that is based on two and only two possible different values or states.

Bit Error Rate (BER)
BER is the ratio of the number of bits received with errors to the total number of bits transmitted. BER and the average number of error-free seconds are the principal impairment measurements for digital channels.

Bit Rate
Bit rate is the capacity characteristic of digital signals as defined by the number of bits (or bytes) per second that a channel will support. For example, a transmission facility that can support information exchange at the rate of I megabit per second (1 Mbps or 1,000,000 bits per second) delivers the same quantity of information, i.e., throughput, as a 1 kilobit per second (kbps or 1,000 bits per second) facility, but, in only 1/1000 of the time.

BOC (Bell-operating Company)
BOC is the common term for one of 24 local exchange carrier telephone companies that were part of the Bell System prior to divestiture. All but two of the BOCs (Southern New England Telephone in Connecticut and Cincinnati Bell in Ohio) are owned and managed by one of 7 regional Bell holding companies (RBHCs). Approximately 80% of America's local exchange users are served by the BOCs.

BOM
Bill of Materials

BOQ
Bill of Quantity

Bridge Tap
Bridge tap is an undetermined length of wire attached between the normal endpoints of a circuit that introduces unwanted impedance imbalances for data transmission.

Bridges
In IEEE 802 local area network (LAN) standards, bridges are devices that connect LANs, or LAN segments, at the data link layer. Bridges provide the means to extend the LAN environment in physical extent, number of stations, performance and reliability. Bridges perform three basic functions: frame (as opposed to packet) forwarding; learning of station addresses; and resolving of possible loops in the topology by participating in the spanning tree algorithm. Self learning bridges construct tables of network addresses by listening" to source address information contained in data signal frames. Other functions include the ability to filter traffic to keep traffic originating and terminating in one network segment from leaving that segment; restricting specified traffic to one segment that might otherwise be routed to other segments; and collecting and storing network management and control information obtained via traffic monitoring.

BRI 
basic rate interface

Broadband Integrated Services Digital Network (BISDN)
CCITT is developing a BISDN umbrella standard, incorporating underlying standards for integrated digital network switching, multiplexing and transmission facilities, that will be able to meet expanding voice, data, video and other requirements well into the future. In one of the first draft CCITT documents BISDN is simply defined as "a service requiring transmission channels capable of supporting rates greater than the primary rate". In the U.S. the primary rate for "narrowband" ISDN (as the current standard is sometimes referred to) is 1.544 Mbps.

Brouters
Brouters are devices that combine the functions of bridges and routers. See bridges, routers.

Bus
In digital systems, e.g., time division multiplexing equipment, a bus (also referred to as a highway) is defined as one or more conductors (or some medium) that connect a related group of devices.

Business Applications
Business applications are unique aggregations of telecommunications services that satisfy particular enterprise needs.

Byte
A byte, generally, is an 8-bit quantity of information, used mainly in referring to parallel data transfer, and data storage; also generally referred to in data communications as an octet or character.

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C

CAB
Carrier Access Business

Cable
A cable is a group of metallic conductors or optical fibers that are bound together, usually with a protective sheath, a strength member, and insulation between individual conductors/fibers and for the entire group.

CABS
Carrier Access Billing System

CALC 
customer access line charges

CAP 
competitive access provider

Carrier
A carrier is a local (intra-LATA) or long distance (inter-LATA) telecommunications service providing organization. A waveform, pulsed or continuous, that is modulated by another information bearing waveform.

Carrier System
A carrier system is a transmission system for transmitting one or more channels of information by processing and converting to a form suitable for the transmission medium used. Carrier systems are classified as either analog carrier systems or digital carrier systems.

CBR 
continuous bit rate

CCIS 
common-channel interoffice signaling

CCITT
CCITT is the International Consultative Committee for Telephone and Telegraph, a consultative committee to the International Telecommunications Union (ITU) which recommend international standards for telephone and telegraph services and facilities to aid international connectivity and interoperability.

CCSA 
common-control switching arrangement

CCS 
centi-call seconds or common-channel signaling

CDMA 
code division multiple access

Cell Relay
Call relay is the process of transferring data in the form of fixed length packets called cells. Cell relay is used in high-bandwidth, low-delay, packet-like switching and multiplexing techniques. The objective is to develop a single multiplexing/switching mechanism for dividing up usable capacity (bandwidth) in a l-manner that supports its allocation to both isochronous (e.g., voice and video traffic) as well as packet data communications services. Standards groups have debated the optimum cell size. Small cells favor low delay for isochronous applications but involve a higher header to user information overhead penalty than would be needed for most data applications. The current CCITT specification for BISDN is for a 53-byte cell which includes a 5-byte header and a 48-byte payload.

Centi Call Seconds (CCS)
CCS is a unit of the average traffic intensity of a facility during a period of time, a CCS is 100 call seconds of traffic during one hour. Therefore a single traffic source, e.g., one call that generates traffic 100% of the time produces 36 CCS of traffic per hour, i.e., 3600 seconds of traffic every 3600 seconds. An equivalent amount of traffic could also be generated by 10 sources that only generate traffic 10% of the time. That is, 10 sources of traffic generating 3.6 CCS each, contributes the same total traffic as a single 36 CCS traffic source. An alternative measure for traffic is erlangs where 1 erlang equals 36 CCS.

CENTRal EXchange (Centrex)
Centrex is a LEC-provided switching service for business customers that permits station-to-station dialing, listed directory number service, direct inward dialing and station number identification on outgoing calls. The switching functions are usually performed in a central office. Digital Centrex offers the advanced features of fourth generation PBXs, without the need to purchase or lease equipment and, in most cases, eliminates the need for floor space, electrical prime power and heating, ventilation and air conditioning.

Central Offices (CO)
A central office is a telephone company building in which network equipment such as switches are installed.

Channel
A channel is a single communications path in a transmission medium connecting two or more points in a network, each path being separated by some means; e.g., spatial or multiplex separation, such as frequency or time division multiplexing. "Channel" and "circuit" are often used interchangeably, however circuit can also describe a physical configuration of equipment that provides a network transmission capability for multiple channels. The characteristics of channels and circuits are determined by the network equipment and media used to support them.

Channel Service Unit (CSU)
Channel Service Units (CSUs) and Data Service Units (DSUs) are required to connect digital customer premises equipment (CPE) to carrier networks. A CSU is network channel terminating equipment (NCTE) attaching as CPE to telephone company's digital circuits, and protecting the network from harm. Other CSU functions include line conditioning and equalization, error control (e.g., bipolar signal violations), and the logical ability to respond to local and network loop back circuit testing commands. See Data Service Units (DSU).

Circuit Associated Signaling
Circuit Associated Signaling is a technique that uses the same facility path for voice and signaling traffic. Historically this approach was selected to avoid the costs of separate channels for signaling and because the amount of traffic generated by signaling is small compared to voice, minimizing the chance for mutual interference. Circuit associated signaling can be contrasted with some common-channel signaling systems that use completely separate packet switched networks for signaling traffic.

Circuit Switching
Circuit switching is a process that establishes connections on demand and permits the exclusive use of those connections until released. Packet and message switching, primarily used in data communications networks are alternative switching techniques.

City Wide Digital Centrex
A city wide digital centrex is a capability to serve multiple business locations within a single NXX (exchange code), using multiple LEC central office Centrex switches. Outside callers are unaware that multiple business locations are involved.

CMIP 
common management information protocol

Coaxial Cable
Coaxial Cable (also called coax cable or coax) consists of an insulated central conductor surrounded by a second cylindrical conductor that is clad with an insulating sheath. The outer conductor usually consists of copper tubing or copper braid.

COC 
central office connections

Codec
Codec is a contraction of coder and decoder; a device that encodes analog signals into digital signals, for transmission through a network in digital format, and decodes received digital signals back into analog signals.

Common Channel Signaling (CCS)
CCS is a signaling system developed for use between stored program control digital switching systems, in which all of the signaling information for one or more trunk groups is transmitted over a dedicated signaling channel, usually, but not always completely separate from the user traffic bearing facilities.

Common Management Information Protocol (CMIP)
CMIP is the OSI protocol for network management. A structure for formatting messages and transmitting information between reporting devices (agents) and data collection programs, developed by the International Standards Organization and designated ISO/IEC 9596.

Communications
Communications is the process of representing, transferring, interpreting or processing information (data) among persons, places, or machines. Communications implies a sender, a receiver, and a transmission medium over which the information travels. The meaning assigned to the data must be recoverable without degradation.

Conditioning
Conditioning is an adjustment made to the electrical characteristics of a cable pair to improve its performance for a specific use.

Conductor
In electrical circuits, a conductor is any material that readily permits a flow of electrons (electrical current) through itself. Analogously, optical fibers are sometimes said to conduct light waves and are also referred to as conductors.

Corporation for Open Systems (COS)
COS is a nonprofit organization composed of manufacturing, service, and user organizations in the computer communications area. Seeks to facilitate the development of the international, multi-vendor marketplace through the development, introduction, and verification of OSI and ISDN standards and by ensuring vendor equipment interoperability.

CO 
central office

CPU 
central processing unit

Cross Connect
1.in a premises distribution system, equipment used to terminate and administer communications circuits. In a wire cross connect, jumper wires or patch cords are used to make circuit connections, between horizontal and backbone wiring segments. 2. in transmission systems a patch panel for connecting circuits.

CSDC 
circuit-switched digital capability

CSMA/CD
Carrier sense multiple access with collision detection; a local area network contention based access control protocol technique by which all devices attached to the network "listen" for transmissions in progress before attempting to transmit themselves and, if two or more begin transmission simultaneously, are able to detect the "collision". In that case each backs off (defers) for a variable period of time (determined by a preset algorithm) before again attempting to transmit. (Defined by the IEEE 802.3 standard).

CSR 
Centrex station rearrangement

CSU 
channel service unit

Customer Premises Equipment
Customer Premises Equipment is all telecommunications terminal equipment located on the customer premises. except coin operated telephones.

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D

D type Channel Bank
A D type channel bank is channel termination equipment used for combining (multiplexing) individual analog channel signals on a time division basis. D type channel banks provide interfaces for "n" analog signal inputs. Each analog input signal is directed to a codec for en coding to PCM samples. A part of a Tl carrier system.

DACS 
digital access & cross-connect system

DARPA 
Defense Advanced Research Projects Agency

Data Compression
Data compression is a technique that removes redundancy in transmitted bit patterns to reduce transmission rates by 20% to 200%. For example, a modem designed to send and receive data at 1200 bps without data compression may be capable of supporting 2400 bps with data compression, using the same network analog voice-grade channel.

Data Service Units (DSU)
DSUs are channel service units (CSI's) and data service units (DSUs) are required to connect digital customer premises equipment (CPE ) to carrier networks. A hardware device providing an interface between a digital line and a unit of data terminal equipment. DSUs provide transmit and receive control logic, synchronization and timing recovery across data circuits. DSUs may also convert ordinary binary signals generated by CPE to special bipolar signals. Bipolar signals are designed specifically to facilitate transmission at up to 1.544 Mbps rates over UTP cable, a media originally intended for 3 kHz, voice bandwidth signals. See Channel Service Units (CSU).

Data Terminal Equipment (DTE)
DTE is any device that can send data, receive data or perform both functions. (Note: sometimes DTE implies digital terminal equipment, a type of CPE used with digital Service--see CSU and DSU.)

DB Loss
Decibel (dB) is a unit of measure of signal strength, usually the relationship between a transmitted signal and a standard signal source. Loss is measured when the signal received across a communication circuit is less than what was transmitted by the signal generator.

DCE 
data circuit terminating equipment

DCP 
digital communications protocol

DCS 
digital cross-connect system

DDD 
direct distance dialing

DDN 
Defense Data Network

Digit L
Digital L is another amplitude condition that represents a binary digit 0. See binary digit or bit.

Digital Carrier Systems
Digital Carrier Systems are carrier systems for digital signals that uses regenerative versus linear repeaters and time division multiplexing.

Digital Cross Connect System (DCS)
DCS is a new generation of switching/multiplex equipment that permits per-channel DS0 (64 kbps) electronic cross-connection from one T1 transmission facility to another, directly from the constituent DSl signals. Commonly referred to as "DACS" (digital access and cross connect system), although this is a trademark of AT&T.

Digital Signal
A digital signal is a signal (electrical or otherwise) in which information is carried in a limited number of different (two or more) discrete states. The most fundamental and widely used form of the digital signal is binary, in which one amplitude condition represents a binary.

Digital Termination Service (DTS)
DTS is a service provided by some carriers permitting opera tors of private networks to use digital microwave equipment to gain access to carrier net works. The FCC has allocated a special microwave band for DTS.

Direct Inward Dialing (DID)
DID is PBX-to-central office trunks that allow incoming calls to a PBX to ring specific stations without attendant assistance. DID greatly reduces the number of required console attendants, compared with systems in which all calls must be extended by console attendants.

Direct Outward Dialing (DOD)
DOD is PBX-to-central office trunks that allow outgoing calls to be placed directly by PBX stations.

Dispersion
In dispersive media, dispersion are complex signals are distorted because the various frequency components which make up the signal have different propagation characteristics and paths. Due to the finite conductivity of copper, wire or cable media for guided wave transmission is fundamentally dispersive. Dispersion limits the upper bit rate that a medium can support by distorting the signal waveforms to the extent that transitions from one information state to another cannot be reliably detected by receiving equipment, (e.g., logical l logical 0 value changes).

DLCI 
data link connection identifier

DOD 
direct outward dialing

DQDS 
distributed queue dual bus

DS"N" Digital Signal Hierarchy
A time division multiplexed hierarchy of standard digital signals used in telecommunications systems. DS1 level in the hierarchy corresponds to a 1.544 Mbps TDM signal which comprises 24 DS0 signals. DS0 refers to individual digital signals at channel rates of 64 kbps. Four DS1 signals digitally multiplexed produce a DS2 level signal, containing 96 DS0 channels, and requires a transmission medium that supports 6.312 Mbps. A DS3 level signal results from the digital multiplexing of 7 DS2 signals, supports 672 DS0 signals and requires a 44.736 Mbps transmission medium. Finally a DS4 level signal supports 6 DS3 level signals, 4032 DS0 signals and requires a 274.176 Mbps transmission medium. The DS hierarchy accounts for non synchronism in the multiplexing plan, hence the term "asynchronous digital hierarchy" and the use of overhead bits. Note that bit rates at higher levels are not integer multiples of 64 kbps.

DSP 
digital signal processor

DSS/BLF 
direct station selection/busy lamp field

DSS 
digital subscriber service

DSU 
data service unit

DTMF 
dual tone multiple frequency

DTS 
digital termination service

Dual-Tone Multiple Frequency (DTMF)
the generic name for the tone signaling scheme used to signal from telephones to switching equipment, in which 10 decimal digits and two auxiliary characters are represented by selecting two frequencies of the following group: 697, 770, 852, 941,1209,1336,1447 Hz.

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E

E&M Leads Signaling
E&M Leads Signaling is an interface, used for connections between switches and transmission systems and between transmission systems themselves. Signaling information is transferred across the interface via 2-state voltage conditions on two leads, each with a ground return, separate from the leads used for message information. The message and signaling information are combined and separated by means appropriate to the transmission facility.

E-mail electronic mail
electronic mail

ECSA 
Exchange Carriers Standards Assoc

EKTS 
electronic key telephone system

Electrical Signal
An electrical signal is a signal consisting of an electrical current (i.e., a flow of electrons) that varies with time or space in accordance with specified parameters.

Electronic Mail
Electronic mail is a generic term for non-interactive communication of text, data, image or voice messages between a sender and designated recipients using telecommunications.

Electronic Switched Network (ESN) Service
ESN service is a private network service that provides user organizations with a uniform numbering plan and numerous call-routing features. The electronic tandem switching functions are furnished by either PBX or Centrex switching equipment.

EMI 
electromagnetic interference

End Office (EO)
EO is a LEC (BOC or an ITC) switching system within a LATA where local loops to customer stations are terminated for purposes of interconnection with each other and with trunks. CO (central office) and EO are often used interchangeably.

Entrance Facilities
1. in a premises distribution system, the point of interconnection be tween the building wiring system and external telecommunications facilities (LEC networks, other buildings, etc.). Bellcore defines the interface with LEC networks as end-user points of termination (POT). 2. has a further specific meaning in interstate access, entrance facilities for interstate access (ENFIA).

EO 
end office

EPSCS 
enhanced private switched communications service

Equipment Room
In a premises distribution system, an equipment room is a special purpose room(s), with access to the backbone wiring, for housing elecommunications, data processing, security, and alarm equipment.

Erlang
An erlang is an international dimensions unit of the average traffic intensity of a facility during a period of time; one erlang of traffic is equivalent to a single user who uses a single resource 100% of the time. See centicall seconds.

ESF 
extended superframe

ESN 
electronic switched network

Exchange Carrier (or local exchange carrier LEC)
An exchange carrier is any company, BOC or independent which provides intra-LATA telecommunications within its franchised area.

Extended Superframe Format (ESF)
ESF is an extension of the superframe format of T1 carrier systems from 12 to 24 frames and the use of framing bits for error checking, a facilities data link (FDL) as well as frame synchronization. See superframe format.

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F

Facilities Based Private Switched Network Services
Facilities Based Private Switched Network Services is a private network for which LECs and IXCs dedicate physical switching and transmission facilities for the exclusive use of a particular customer.

Fast Packet
Fast Packet is a term referring to a number of broadband switching and networking paradigms. Implicit is the assumption of an operating environment that includes reliable, digital, broadband, nearly error free transmission system.

FCC Federal Communications Commission
The FCC is a board of commissioners empowered by the U.S. Congress to regulate all interstate and international communications, as well as use of the radio frequency media. www.fcc.gov

FDDI 
fiber-distributed data interface

FDL 
facility data link

FDM 
frequency division multiplexing

FEP 
front-end processor

FIPS 
Federal Information Processing Standard

Foreign Exchange (FX)
FX is a service that provides a circuit(s) between a user station, a PBX, or a Centrex switch, and a central office other than the one that normally serves the caller.

Frame
In time division multiplexing systems, Frame is a sequence of time slots each containing a sample from one of the channels carried by the system. The frame is repeated at regular intervals, (normally the sampling rate used in analog-to-digital conversion processes for signals being multiplexed) and each channel usually occupies the same sequence position in successive frames.

Frame Relay
Frame relay is a network interface protocol defined in CCITT Recommendation 1.122 "Framework for additional packet mode bearer services," as a packet mode service. In effect it combines the statistical multiplexing and port sharing of X.25 packet switching with the high speed and low delay of time division multiplexing and circuit switching. Unlike X.25, frame relay implements no layer 3 protocols and only the so-called core layer 2 functions. It is a high-speed switching technology that achieves ten times the packet throughput of existing X.25 networks by eliminating two-thirds of the X.25 protocol complexity The basic units of information transferred are variable length frames, using only two bytes for header information. Delay for frame relay is lower than for X.25, but it is variable and larger than that experienced in circuit switched networks. This means that currently frame relay is not suitable for voice and video applications where excessive and variable delays are unacceptable.

Frequency
Frequency is acoustic waves and electrical signals might be made up of only a single tone, like a single note on a piano. In this case the signal waveform is made up of repeating identical "cycles" and is said to be of a single frequency, equal to the number of cycles that occur in one second of time. In communications, frequency was traditionally expressed in cycles per second, but is now expressed in hertz (Hz), still equal to one cycle per second. Thus, one thousand cycles per second is equal to one thousand hertz, or a kilohertz (kHz).

Frequency Division Multiplexing (FDM)
FDM divides the frequency bandwidth (spectrum) of a broadband transmission circuit into many subbands, each capable of supporting a single, full time communications channel on a non-interfering basis with other multiplexed channels. FDM multiplexing is generally suitable for use with analog carrier transmission systems.

FSS 
fully separated subsidiary

FTAM 
file transfer access & management

Full Duplex
Full duplex is a transmission path capable of transmitting signals in both directions simultaneously.

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G

GOSIP 
Government Open Systems Interconnection Profile

Grade Of Service (GOS)
GOS is an estimate of customer satisfaction with a particular aspect of service such as noise, echo or blocking. For example the noise grade of service is said to be 95% if, for a specified distribution of noise, 95% of the people judge the service to be good or better. In traffic networks, GOS defines the percentage of calls that receive no service (blocking) or poor service (long delays). GOS measures apply to all aspects of telecommunications networks. In many cases the literature equates GOS only with the probability of a blocked call. When used without further explanation, GOS generally refers to blocking probability.

Ground Start
Ground start is a supervisory signal given at certain coin telephones and PBXs by connecting one side of the line to ground.

Guided Media
Guided media is media that constrain electromagnetic or acoustic waves within boundaries established by their physical construction. Examples include paired metallic wire cable, coaxial cable, and fiber optic cable.

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H

Half Duplex
Half duplex is a transmission path capable of transmitting signals in both directions, but only in one direction at a time.

HDLC 
high-level data link control

Header
The header is control information appended to a segment of user data for control, synchronization, routing and sequencing of a transmitted data packet or frame.

Hertz (Hz)
Hertz is the measurement that distinguishes electromagnetic waveform energy; number of cycles, or complete waves, that pass a reference point per second; measurement of frequency, by which one hertz equals one cycle per second.

Horizontal Wiring
In a premises distribution system, horizontal wiring is the connection between the telecommunications outlet in work areas and the telecommunications closet.

Hub
In local area networks (LANs), a hub is a wiring concentrator equipment used in hierarchical star physical wiring topologies. Those directly connected to terminals or other user devices are of ten referred to as local hubs or concentrators. Central hubs are those at the highest hierarchical level. Hubs often provide the means for interconnecting 10BaseT, coaxial or fiber optic cable LAN segments. Intelligent hubs may implement multiport bridging and network management functions.

HVAC 
heating, ventilation and air conditioning

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I

I-MAC 
isonchronous media access controller

IDN 
integrated digital network

IEC 
International Electrotechnical Commission

Impairments
Impairments is the (e.g., transmission channel and signal impairments) degradation caused by practical limitations of channels, (e.g., signal level loss or attenuation, echo, various types of signal distortion, etc.) or interference induced from outside the channel (such as power-line hum or interference from heavy electrical machinery). The measurement of transmission impairments is an important aspect of predicting whether or not telecommunications systems will sustain the business applications they are intended to support. Signal-to-noise ratio, percent distortion, frequency response, and echo are measurements that define impairments most noticeable by users in analog voice systems.

Inbound Signaling
Inbound signaling uses not only the same channel path as the voice traffic, but the same frequency range (band) used for the voice traffic.

Independent Telephone Company (ITC)
ITC is a local exchange carrier that is not one of the 22 divested Bell-operating companies. ITCs are not generally subject to the restrictions of the MFJ, although some of the larger ones are bound by separate consent decrees. Southern New England Telephone and Cincinnati Bell are generally considered ITCs from a regulatory point of view.

Integrated Services Digital Network (ISDN)
ISDN consists of a set of standards being developed by the CCITT and various U.S. standards setting organizations. The CCITT formal recommendations, adopted in October, 1984, first defined ISDN as ". . . a network, in general evolving from a telephony integrated digital network, that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice, to which users will have access by a limited set of standard multipurpose user-network interfaces." The concept of user access to an existing integrated digital network (IDN) underlies the ISDN.

Inter-LATA Services
Inter-LATA services are revenues, functions, etc., that relate to telecommunications originating in one LATA and terminating outside that LATA. An interexchange carrier (IXC) is a company which provides telecommunications services between LATAs. (the domain of IXCs).

Intermediate Cross Connects
In a premises distribution system, intermediate cross connects are cross-connects located telecommunications closets.

International Organization for Standardization (ISO)
ISO is a worldwide federation of national standards bodies (ISO National Bodies). The work of preparing international standards is normally carried out through ISO technical committees. Draft proposals for international standards adopted by the technical committees are circulated to the National Bodies for approval before their acceptance as Draft International Standards by the committee.

Internet
The internet is a large collection of connected networks, primarily in the United States, running the Internet suite of protocols. Sometimes referred to as the DARPA Internet, NSF/DARPA Internet, or the Federal Research Network.

Internet suite of protocols
The internet suite of protocols is a collection of computer-communication protocols originally developed under DARPA sponsorship, including the transmission control protocol/internet protocol (TCP/IP).

Interoffice Channel
In LEC tariffs, Interoffice Channel is the channel connecting two serving COs (more accurately serving wire centers). In IXC tariffs, the channel connecting two serving IXC POPs, interoffice transmission facilities used to connect LEC switching systems.

Intra-LATA
Intra-LATA are services, revenues, functions, etc., that relate to telecommunications originating and terminating within a single LATA (the domain of LECs).

IOC 
interoffice channel

IPX 
internetwork packet exchange

IP 
internet protocol

ISDN 
integrated services digital network

Isochronous Signals
Isochronous Signals are periodic signals in which the time interval that separates any two corresponding significant occurrences or level transitions is always equal to some unit interval or a multiple of that unit interval. For example, in digitized voice signals, ideally voice samples occur isochronously at precisely the sampling interval or frame rate. Packet data signals are not isochronous.

ISO 
International Organization for Standardization

ITC 
independent telephone company

ITU 
International Telecommunications Union

IVR 
integrated voice response

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J

JPEG 
Joint Photographic Experts Group

JTM 
job transfer manipulation

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K

Key Telephone System (KTS)
KTS is an arrangement of multi-line telephones and associated equipment that permits the station user to depress buttons (keys) to access different central office or PBX lines, as well as to perform other functions. Typical functions include answering or placing a call on a selected line, putting a call on hold, using the intercom feature between phones at the same location, or activating a signal buzzer.

KSU 
key service unit

KTS 
key telephone system

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L

Last Mile
The connection between the customer and the telephone company, also referred to as local loop. The last mile is made of copper or fiber based wire.

LAN (IEEE 802.3 10BaseT)
LAN (Local Area Network) is an Institute of Electrical and Electronic Engineers specification for a class of LANs using four-pair unshielded twisted pair (UTP) cable. See Local Area Network.

LAN 
local area network

LATA 
local access and transport area

LCR 
least cost routing

LDN
listed directory number; generally an organization's main telephone number that appears in the telephone book.

LDN 
listed directory number

LEC 
local exchange carrier

Line
See loop transmission facilities.

Local Access And Transport Area (LATA)
LATA is a geographic area (called an "exchange" or "exchange area" in the MFJ) within each BOC's franchised area that has been established by a BOC in accordance with the provisions of the MFJ for the purpose of defining the territory within which a BOC may offer its telecommunications services. In 1989, there were 198 LATAs, also referred to as market service areas (MSAs), in the United States.

Local Area Network (LAN)
LAN is a premises high-speed (typically in the range of 10 Mbps) data communications system wherein all segments of the transmission medium (typically coaxial cable, twisted pair or optical fiber) are contained within an office or campus environment.

Local Channel
In LEC tariffs, a local channel is the local loop that connects customer premises to serving LEC wire centers. In IXC tariffs, the network components (transmission, switching, other) used to connect customer premises to serving IXC POPs.Local Loop The lines between a customer and the telephone company's central office, also referred to as the last mile. Local loops use copper or fiber based wire.

Loop Length
Typically, loop length is a complete electrical circuit; the pair of wires that winds its way from the central office to the telephone set or system at the customer's premises.

Loop Signaling
Loop signaling is a method of signaling over dc circuit paths that utilizes the metallic loop formed by the line or trunk conductors and terminating circuits.

Loop Start
Loop start is a supervisory signal given at a telephone or PBX in response to closing the loop's DC 
current path.

Loop Transmission Facilities
Loop transmission facilities connect switching systems to customer premises equipment throughout the serving area. A loop is a transmission path between a customer's premises and a LEC central office. The most common form of loop, a pair of wires, is also called a line. A loop" can be derived from digital loop carrier (DLC) systems also referred to as subscriber loop carrier (SLC) systems.

LSI 
large-scale integrated circuit

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M

MAAP 
maintenance & administration panels

MACSTAR 
multiple access customer station rearrangement

MAC 
media access control or moves, adds, and changes

Main Cross-Connects
In a premises distribution system, main cross-connects are the cross connects located in an equipment room.

MAN 
metropolitan area network

MCU 
mobile control unit

Media
See transmission medium; yielded media; unguided media.

Message Telecommunications Service (MTS)
MTS is a non-private-line intrastate and interstate long-distance that uses in whole or in part the public switched telephone network (PSTN).

MFJ 
Modification of Final Judgment

MFOTS 
Military Fiber-Optic Transmission System

MF 
multiple frequency

MHS 
message handling system

MIB 
management information base

MIC
Material Identification Code

Microwave
In telecommunications, microwaves are frequencies above 1 GHz.

MIPS 
million instructions per second

MRC
The monthly amount that you pay for your service. For colocation, this is a combination of your monthly bandwidth usage calculated from the previous month, your rack space charges and any additional service or management fees as requested.

Modems (MOdulator/DEModulators)
Modems are devices that transform digital signals generated by data terminal equipments (DTEs) to analog signal formats, suitable for transmission through the extensive, world-wide connectivity of public and private, switched (dial up) and non-switched telephone voice networks.

Modification Of Final Judgment (MFJ)
MFJ is a ruling issued by U.S. District Court Judge Harold Greene which concluded the U. S. Justice Department's antitrust suit against AT&T by modification of an earlier (1956) consent decree's final judgment.

Modulation
Modulation is the process of varying certain parameters of a carrier signal i.e., a signal suitable for modulation by an information signal by means of another signal (the modulating or information bearing signal).

MPEG 
Moving Pictures Experts Group

MSS 
metropolitan switching system

MTSO 
mobile telephone switching office

MTS 
message telecommunications service

Multiline Telephone
A multiline telephone is a telephone that incorporates visual displays and switches (keys) that permit the station user to access more than one central office or other line and to perform other desired functions. Typical functions include answering or originating a call on a selected line, putting a call on hold, operating an intercom feature, a buzzer, etc. Displays can indicate busy, ringing and message waiting status.

Multimode Optical Fiber(S)
Multimode fibers, with much wider cores than single mode fibers, allow light to enter at various angles, and reflect (bounce off of) core-clad boundaries as electromagnetic (light) wave propagates from transmitter to receiver. From a technical performance trade-off point of view, single mode fiber exhibits bandwidths of up to 100,000 MHz (MHz = 1,000,000 hertz or cycles per second = one megahertz) while multimode band width is in the range of 1,000 to 2,000 MHz (1,000 MHz = one billion hertz = one gigahertz = 1 GHz). See optical fiber(s); single modefiber(s).

Multiplexing
Multiplexing is a technique that enables a number of communications channels to be combined into a single broadband signal and transmitted over a single circuit. At the receiving terminal, demultiplexing of the broadband signal separates and recovers the original channels. Multiplexing makes more efficient use of transmission capacity to achieve a low per channel cost. Two basic multiplexing methods used in telecommunications systems, are frequency division multiplexing (FDM) and time division multiplexing (TDM).

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N

NBEC 
non-Bell exchange carrier

NCP 
network control point

NCTE 
network channel terminating equipment

Network Control Point (NCP)
In virtual private networks, the Network Contol Point is a centralized database that stores a subscriber's unique VPN definition. Highly sophisticated, this database screens every call and applies call processing control in accordance with customer-defined requirements.

Network Operating System (NOS)
NOS is software that controls the execution of network programs and modules. Structurally, networking software comprises multiple modules, most residing in network servers, but some must be installed in each terminal/station that can access network resources. Peer-to-peer NOSs permit any terminal/station to act as a resource server or a client, and can be based on Microsoft's Disk operating system (MS DOS) designed for IBM and compatible PCs. Since MS DOS is not designed to run multiple programs and respond to many simultaneous users, most NOSs designed for large networks with dedicated servers/superservers, have a multitasking and multi-user architecture. Advanced NOS products support network management, diagnostics, and administration, as well as, primary server, client, de vice and external network driver functions.

Network Services
Network Services are specified sets of information transfer capabilities furnished to users between telecommunications network points of termination. Network services categories include access and transport, public and private, and switched and non-switched.

NIC 
network interface card

NIST 
National Institute of Standards and Technology

NI 
network interface

NOC 
network operations center

NOS 
network operating system

NPA 
numbering plan area

NRC
One time charges that are incurred at the initiation of service. These charges generally cover labor costs and the cost of any equipment required to implementyour service.

NSEP 
National Security & Emergency Preparedness

NTSC 
National Television System Committee

NT 
network termination

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O

OA&M 
operation administration & maintenance

OCC 
other common carriers

Ohms Restriction
Ohms Restriction is the same as Ohms resistance. Ohms are measures of resistance. A resistance of one Ohm allows one Ampere of current to pass at the electric potential of one Volt.

Open Systems Interconnection (OSI) standards
OSI standards are standards for the exchange of information among systems that are "open" to one another by virtue of incorporating ISO standards. The OSI reference model segments communications functions into seven layers. Each layer relies on the next lower layer to provide more primitive functions and, in turn, provides services to support the next higher layer.

Operating Telephone Company
An operating telephone company is any Bell-operating company or independent telephone company (termed exchange carrier in the MFJ) operating in North America.

Optical Fiber(s)
Optical fibers are light guides for electromagnetic waves in the infrared and visible light spectrum composed of concentric cylinders made of dielectric materials with different indices of refraction (i.e., velocity of propagation normalized to the velocity of light in free space). At the center is a core comprising the glass or plastic strand or fiber in which a light wave travels. A low index of refraction clad surrounds the core and is itself enclosed in a light-absorbing jacket that prevents interference among multi-fiber cables. Multi-fiber cable can be purchased with between 2 and 136 fibers.

OPX 
off-premises extension

OSC 
operating system control

OSI 
open systems interconnection

OSS 
operations support systems or operational support system

Out Of Band Signaling
Out of band signaling uses the same channel path as the voice traffic but signaling is in a frequency band outside that used for the voice traffic. In digital systems, out-of-band signaling may take the appearance of an allocated bit position or a dedicated channel or time slot.

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P

P-MAC 
packet media access controller

PAD 
packet assembler-disassembler

PCB 
printed circuit board

PDS 
premises distribution system

Personal Communications
Personal communications provides at least one human operator with direct terminal access and real time or near real time interactive communications with a remote human opera tor or an information system resource. Personal communications can refer to a broad range of services, systems and equipment, e.g., facsimile machines, landline telephones, cellular tele phone systems and emerging personal communication system (PCS) adjuncts, and a variety of radio systems including pagers, hand-held remote data entry terminals, and autonomous citizen-band-like radio systems.

PHY 
physical-layer protocol

PMD 
physical-layer media-dependent

Point-of-presence (POP)
A POP is a physical location within a LATA that an IXC establishes for the purpose of gaining access to BOC/LEC networks within the LATA using LEC provided access services. An IXC may have more than one POP within a LATA and the POP may support public and private, switched and non switched services.

POP 
point-of-presence

POT 
point of termination

PPSN 
public packet switched network

Premises Distribution System (PDS)
PDS is the transmission network inside a building or among a group of buildings, for example an office park or a campus. PDS is used in this book as a generic term although AT&T used it to describe a specific product offering. The PDS connects desktop and other station equipment with common host equipment, (e.g., switches, computers and building automation systems), and to external telecommunications networks.

Private Branch Exchange (PBX)
A PBX is a premises switching system, serving a commercial or government organization, and usually located on that organization's premises. PBXs provide telecommunications services on the premises or campus, (e.g., internal calling and other services), and access to public and private telecommunications network services.

Private Network
A private network is a network made up of circuits and, sometimes, switching equipment, for the exclusive use of one organization.

PRI 
primary rate interface

Protocols
Protocols are strict procedures for the initiation, maintenance and termination of data communications. Protocols define the syntax (arrangements, formats and patterns of bits and bytes) and the semantics (system control, information context or meaning of patterns of bits or bytes) of exchanged data, as well as numerous other characteristics (data rates, timing, etc.).

PSN 
packet switched network

PSPDN 
packet switched public data network

PSTN 
public switched telecommunications network

PTT 
postal, telephone, and telegraph

Public Switched Telephone Network (PSTN)
A Public Switched Telephone Network (PSTN) denotes those portions of the LEC and IXC networks that provide public switched telephone network services.

PUC 
public utility commission

Pulse Code Modulation (PCM)
Pulse Code Modulation (PCM) is a modulation scheme involving conversion of a signal from analog to digital form by means of coding. See also modulation.

PVC 
permanent virtual circuit

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Q

Quantizing Noise
In any analog to digital conversion process, e.g., PCM, quantizing noise is the difference between the converted binary value and the actual analog signal's amplitude.

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R

RBHC 
regional Bell holding company

RBS 
robbed bit signaling

Regional Bell Holding Company (RBHC)
RBHC is one of 7 regional companies created by the AT&T divestiture to assume ownership of the Bell operating companies. They are Ameritech, Bell Atlantic, Bell South, NYNEX, Pacific Telesis, Southwestern Bell and US West.

Repeater
In digital transmission, a repeater is equipment that receives a pulse train, amplifies it, retimes it, and then reconstructs the signal for retransmission. In IEEE 802 local area network (LAN) standards, a repeater is essentially two transceivers joined back to back and attached to two adjacent LAN segments. See transceiver.

RFC 
request for comment

RFE
Request for Engineering

RFI 
radio frequency interference

RFP 
request for proposal

RF 
radio frequency

Routers
In IEEE 802 local area network (LAN) standards, routers are devices that connect autonomous networks of like architecture at the network layer (layer 3). Unlike a bridge which operates transparently to communicating end-terminals at the logical link layer (layer 2), a router reacts only to packets addressed to it by either a terminal or another router. Routers perform packet (as 
opposed to frame) routing and forwarding functions; they can select one of many potential paths based on transit delay, network congestion or other criteria. How routers perform their functions is largely determined by the protocols implemented in the networks they interconnect.

RSU 
remote switching unit

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S

SAFENET 
survivable adaptable fiber-optic embedded network

Satellite Communications
Satellite communications entails microwave radio, line-of-sight propagation from a transmitting earth terminal (i.e., usually ground-based but potentially ship or airborne) through the atmosphere and outer space media to a satellite, and back to earthbound receiving terminals. In essence, satellites are equivalent to orbiting microwave repeaters.

Scenario
In SWAT, a scenario is an iteration of an RFW project. A scenario holds the design detail work units that are associated with the project. By creating multiple scenarios, you can compare pricing and scheduling options. A scenario has three different statuses. A Working status indicates that the scenario is under review. A Posted status indicates that a scenario has been identified as the official design. A Final Posted status indicates that a scenario is the "as-built" version of the project.

SDH 
synchronous digital hierarchy

SDN 
software defined network

Server
In a network, a server is equipment that makes available file, database, printing, facsimile, communications or other services to client terminals/stations with access to the network. A gateway is a server that permits client terminal/station access to external communications networks and/or information systems.

Service Management System (SMS)
In virtual private networks, SMS is a facility used to build and maintain a VPN database allowing customers to program specific functions for unique business applications. The SMS contains complete specifications of customer defined private network specifications including location data, numbering plan, features, screening actions, authorization codes, calling privileges, etc. This information is downloaded (transmitted) to network control points (NCPs) which implement its instructions on a customer-by-customer basis.

Service Order (SO)
A service order is a record that describes a customer request to establish, change, or terminate a service. The service order contains all information required to meet a customer's need . 

SF 
single frequency

Shielded Twisted Pair
Shielded twisted pair is twisted copper paired wire cable with an outer metallic sheath surrounding insulated conductors. See unshielded twisted pair.

Signal
A signal is usually a time-dependent value attached to an energy propagating phenomenon used to convey information, for example, an audio or sound signal in which the data is characterized in terms of loudness and pitch.

Signaling
Signaling is the process of generating and exchanging information between components of a telecommunications system to establish, monitor, or release connections (call handling functions) and to control related network and system operations (other functions).

Signaling System No. 7, SS #7
SS#7 is an international common channel signaling system recommendations established by the CCITT.

Simple Network Management Protocol (SNMP)
SNMP is the application protocol offering network management service in the Internet suite of protocols. A structure for formatting messages and transmitting information between reporting devices (agents) and data collection programs. Developed jointly by the Department of Defense, industry and the academic community as part of the TCP/IP protocol suite; ratified as an Internet standard in Request for (Comment (RFC) 1098.

Simplex
Simplex is a transmission path capable of transmitting signals in only one direction.

Single Mode Optical Fiber(s)
Single mode optical fibers have sufficiently small core diameters in relation to the wavelength (frequency) of operation that electromagnetic (light) wave is constrained to travel in only one transverse path from transmitter to receiver. This requires the utmost in angular alignment of light emitting devices at points where light enters the fiber and results in higher transmitter/termination costs than multimode fiber systems. See multimode optical fiber(s).

SMDR 
station message detail record

SMDS 
switched multimegabit data system

SMT 
station management technology

SNA 
Systems Network Architecture

SNI 
subscriber network interface

SNMP 
Simple network management protocol. See Simple Network Management Protocol (SNMP).

SO
Service Order

SONET 
synchronous optical network

Space Division
Space division is a switch that implements the switch matrix using a physical, electrical, spatial link. Where older space division switches used electro-mechanical mechanisms with metallic contacts, modern space-division switches are implemented electronically using integrated circuits. (Usually denoted by "S" in combined time and space division switches).

Special Services
Special services are any of a variety of LEC and IXC switched, non-switched, or special rate services that are either separate from public telephone service or contribute to certain aspects of public telephone service. Examples include PBX tie trunks, foreign exchange (FX) and private line services. These services are important to business telecommunication planners/users.

SSN 
switched service network

SS 
signaling system

Station Equipment
Station equipment is a component of telecommunications systems such as a telephone or data terminal, generally located on the user's premises. Its function is to transmit and receive user information (traffic), and to exchange control information with the network to access communications services.

STM 
synchronous transfer mode

STP 
shielded twisted pair or signaling transfer/point

Superframe Format (SF)
Superframe format is a framing format (D3/D4--mode 3), the most widely used T1 carrier framing format in which the bipolar bit stream is organized into superframes each consisting of 12 frames. To ensure timing, the signal must consist of at least one "1" bit in every 15 bits and at least :3 " 1" bits in every 24 bits. See extended superframe.

Supervisory Signals
Supervisory signals are signals used to indicate or control the states of circuits involved in a particular switched connection. A supervisory signal indicates to equipment, to an operator, or to a user that a particular state in the call has been reached and may simplify the need for action.

Switch Matrices
Switch matrices are the mechanism that provides signal paths between its input and output terminations. Modern matrices are electronic and involve either time or space division switching. A time division switch employs a TDM process, in a time-slot interchange (TSI) arrangement. In space division, a physical, electrical, spatial link is established through the switch matrix. Whereas older space division switches used electro-mechanical mechanisms with metallic contacts, modern space-division switches are implemented electronically using integrated circuits.

Switching
Switching refers to the process of connecting appropriate lines and/or trunks to form a desired communications path between two station sets, or more generally, any two arbitrary points in a telecommunications network. Included are all kinds of related functions such as signaling, monitoring the status of circuits, translating address to routing instructions, alternate routing, testing circuits for busy conditions, and detecting and recording troubles.

Switching Systems
Switching systems are interconnect transmission facilities at various network locations and route traffic through a network.

Systems (Manual Circuits)
In Plant Records and Assignment, these terms refer to circuits whose network elements have been assigned manually rather than by automatic assignment.

Systems Network Architecture (SNA)
SNA is IBM's proprietary description of the logical structure, formats, protocols, and operational sequences for transmitting information units through and controlling network configuration and operation.

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1. Determine your daily amount of anticipated data transfer.
   
   Consider the number of interactive visitors, the amount of data each will transfer and the frequency of the transfer.
2. Multiply that number by 30.5 (the average number of days in a month)
   
   Ex. Your web site consistently has 5000 unique visitors per day. The average visitor transfers about 1.5 MBs of data (Yes MegaBytes). There are 30.5 days in a month. Therefore we have 5000 * 1.5 MB * 30.5 = 228,750 MBs of transfer per month.
3. Calculate the number of GBs (GigaBytes) of transfer per month.
   
   Each 1000 MBs is equivalent to 1 GB. From the above example, we have 228,750 MB / 1000 MB/GB = 228.75 GB per month of transfer.
4. Determine the approximate connection speed required to support the level of transfer.
   
   In the above case, we take 228.75 GB / 340 GB/1 Mbps = .673 Mbps or 673 Kbps to support the indicated transfer at 100% utilization.
5. Allow for growth by calculating 40% overhead availability.
   
   This is critical since bandwidth utilization is NOT constant over the course of the day. There are peak times of utilization. We recommend that average utilization remain at or below 60% of the total capacity. In the above case, we have confirmed usage of .673 Mbps. We need to determine the factor necessary to calculate our actual need. Using 60%, we know that 100/60 = 1.667 so we multiply .673 * 1.667 = 1.12 Mbps. Based upon our services, you would need between 1 Mbps and 2 Mbps to provide effective service for your 5000 visitors per month.

Transceiver
A transceiver is a generic term describing a device that can both transmit and receive. In IEEE 802 local area network (LAN) standards, a transceiver consists of a transmitter, receiver, power converter, and, for CSMA/CD LANs, collision detector and jabber detector capabilities. The transmitter receives signals from an attached terminal's network interface card (NIC) and transmits them to the coaxial cable or other LAN medium. The receiver receives signals from the medium and transmits them via the transceiver cable and NIC to the attached terminal. The jabber detector is a timer circuit that protects the LAN from a continuously transmitting terminal.

Transfer Mode
Transfer mode is a generic term for switching and multiplexing aspects of broadband integrated services digital networks (BISDN), adopted by CCITT Study Group XVIII.

Transmission control protocol/internet protocol (TCP/IP)
See "Internet suite of protocols" and "TCP/IP".

Transmission Facilities
Transmission facilities provide the communication paths that carry user and network control information between nodes in a network. In general, transmission facilities consist of a medium (e.g., free space, the atmosphere, copper or fiber optic cable) and electronic equipment located at points along the medium. This equipment amplifies (analog systems) or regenerates (digital systems) signals, provides termination functions at points where transmission facilities connect to switching systems, and may provide the means to combine many separate sets of call information into a single "multiplexed" signal to enhance the transmission efficiency.

Transmission Impairments
Transmission impairments is the degradation caused by practical limitations of channels (e.g., signal level loss due to attenuation, echo, various types of signal distortion, etc.), or interference induced from outside the channel (such as power-line hum or interference from heavy electrical machinery).

Transmission Medium
Transmission medium is any material substance or "free space" (i.e., a vacuum) that can be, or is, used for the propagation of suitable signals, usually in the form of electromagnetic (including light waves), or acoustic waves, from one point to another; unguided in the case of free space or gaseous media or guided by a boundary of material substance.

Transport Services
Transport Services are network switching, transmission and related services that support information transfer capabilities between originating and terminating access service facilities.

Trunk
In a network, a trunk is a communications path connecting two switching systems used to establish end-to-end connections between customers.

Twisted Pair
Twisted pair is the most common type of transmission medium, consisting of two insulated copper wires twisted together. The twists or lays are varied in length to reduce the potential for interference between pairs. In cables greater than 25 pair, the twisted pairs are grouped and bound together in a common cable sheath. See unshielded twisted pair.

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U

U - Unit of measurement for Colocation
1U = 1.75" of Vertical Rack Space 2U = 3.5" of Vertical Rack Space 3U = 5.25" of Vertical Rack Space 4U = 7.00" of Vertical Rack Space. This measurement is used to indicate the total amount of rack space occupied by your colocated equipment.

UIS 
Universal Information Services

Unguided Media
Unguided media is any medium in which boundary effects between "free space" and material substances are absent. The "free space" medium may or may not include a gas or vapor. Unguided media including the earth's atmosphere and outer space support terrestrial and satellite radio and optical transmission.

UNI 
user-network interface

Unshielded Twisted Pair (UTP)
UTP is a two wood pulp or plastic insulated copper conductors (wires), twisted together into pairs, capable of propagating electromagnetic waves. The twists, or lays, are varied in length to reduce the potential for signal interference between pairs, in multi-pair cables. Wire sizes range from 26 to l9 gauge (i.e., 0.016 to 0.036 inch in diameter) and are typically manufactured in cables of from 2 to 3600 pairs. Shielded twisted pair cable is similar to UTP, but the twisted pairs 
are surrounded by a cylindrical metallic conductor that is clad with an insulating sheath. See cable; conductor.

UTP 
Unshielded twisted pair. See Unshielded Twisted Pair.

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V

VAD 
value-added distributor

VAN 
value-added network

VAR 
value-added reseller

VBR 
variable bit rate

VCI 
virtual circuit identifier

VCS 
virtual circuit switch

Very Small Aperture Terminal (VSAT)
VSATs are earth terminals using small antennas (1.5 6 feet in diameter). This technology typically operates in the Ku band (11/14 GHz), and Ka band (20/30 GHz).

Video Conferencing
Video conferencing is the real-time, usually two-way transmission of voice and images between two or more locations. Today, both voice and video analog signals are digitized by video codices before transmission which can involve wide bandwidths. To conserve bandwidth, some systems employ "freeze frame," where a television screen is only "repainted" every few seconds. Codecs for higher quality full motion video attempt to minimize bandwidth requirements by taking advantage of intervals with relatively little motion (which require smaller bandwidths), and by trading off smooth motion tracking and picture resolution.

Virtual Private Networks (VPN)
VPNs are services using public network facilities augmented by network control point and service management system facilities wherein traffic is routed through the public network under computer control in a manner that makes VPN service indistinguishable from dedicated facilities based private networks. Customers can define, change and control network resources with the same or more flexibility as afforded by facilities based private networks.

VPI 
virtual path identifier

VPN 
virtual private network

VRU 
voice response unit

VSAT 
very small aperture terminal

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W

WAN 
wide area network

WARC 
World Administrative Radio Consortium

WATS 
Wide-area telecommunications services. See Wide-area telecommunications services.

Waveform
amplitude (magnitude) versus time representation of signals.

Wide-Area Telecommunications Services (WATS)
WATS is a service permitting customers to make (OUTWATS) or receive (INWATS) long distance voice or data calls and to have them billed on a bulk rather than an individual call basis. The service is provided by means of special private access lines connected to WATS equipped central offices. A single access line permits inward or outward service but not both.

Wink-Start
Wink-Start is a supervisory signal that consists of an off-hook followed by an on-hook signal, exchanged between two switching systems. The wink-start signal is generated by the called switch to indicate to the calling switch that it is ready to receive address signal digits.

WO
Work Order

Work Area
In a premises distribution system, a work area is an area containing stations and the connections between those stations and their telecommunications (information) outlets.

Work Order
A work order is the basic engineering document that adds, modifies, or removes network components (capital assets). It is the tracking mechanism that all telecommunication companies use to add or remove components from their capital base.

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