Telecom & VoIP Glossary

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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 Communication
A data communications method in which bits are sent
without using a clock signal for synchronization.
Instead, each character is transmitted surrounded by a
start and stop bit that designates the beginning and
ending points of the information. This as opposed to
synchronous communication where blocks of data are
transmitted using a synchronizing clock.

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.

Audio Menu
A verbal choice provided by a recording over the phone. Audio choice menus are common in automated attendant, IVR and fax-on-demand systems. They are prompts for caller input. Audio menus can instruct you to speak commands or press keys on a touch-tone keypad as commands.

Audio Response Unit (ARU)
A computer telephony system incorporating voice storeandforward technology. There are passive and interactive ARUs. Passive ARUs simply play out messages while interactive ARUs play messages based on input from callers.

Audio Teleconferencing or Audio Conferencing
The original technology used for audio teleconferencing was based on PBX (Private Branch Exchange) conferencing circuits. Setting up conference calls through the PBX is cumbersome, the voice quality degrades as the number of people on a call increases, and there are capacity limitations, so specialized conference bridges were developed to improve capacity and voice quality. Conference bridges, however, require trained operator intervention to schedule and invoke most features. As a result, individual corporations found the cost of ownership prohibitive, and the market for such products has been concentrated on service bureau providers. Today’s PC-based systems provide the freedom of conference bridges. By installing a conference server on your voice networks, you can set up, attend, and manage your own conferences over any touch-tone telephone. Additionally, users can schedule meetings using desktop software from their e-mail systems, or from a web browser.

AWG 
American Wire Gauge.

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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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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 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 Subscriber Line (DSL)
A high speed digital switched service that uses existing copper pairs to connect subscriber CPE (customer premises equipment) to the CO (central office). DSL handles more data downstream (data flowing towards the subscriber) than upstream (flowing towards the network).

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-1
The designation for the 2.048 Mbps ITU standard for Europe’s 30-channel digital telephone service. It is the European version of T-1 (DS-1). The bandwidth is divided into 2 signaling channels (channels 15 and 31 starting from 0) and 30 bearer channels (voice channels). A&B bit signaling (robbed bit signaling) is not used here. E-1 uses one of the control channels for signaling and the other for clock synchronization.

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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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.

Fax Server
A computer based fax machine. Fax servers are “shared use” devices, typically installed on a LAN. Clients on the LAN can use the fax server from their PCs in much the same way they share a network-based (shared) printer. Faxes can be generated by users at their workstations and “printed” to the fax server for  transmission. Likewise, fax servers can route incoming faxes to printers, file server directories, or to individual users. Fax servers save users from having to print documents, carry them to the fax machine, and subsequently wait for them to be transmitted after creating a cover page.

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
In data communications, Frame Relay is a packet switching method that uses available bandwidth only when it is needed. This fast packet switching method is efficient enough to transmit voice communications with the proper network management.

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

FX 
foreign exchange

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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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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

Inter-Asterisk eXchange (IAX or IAX2)
IAX is the Inter-Asterisk eXchange protocol used by Asterisk, a dual licensed open source and commercial PBX server from Digium and other softswitches and PBXs. It is used to enable VoIP connections between servers, and between servers and clients that also use the IAX protocol. IAX now most commonly refers to IAX2, the second version of the IAX protocol. The original IAX protocol has been deprecated almost universally in favor of IAX2.

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 consists of the world’s combined public IPbased packet-switched networks. The Internet is an outgrowth and combination of a variety of university and government sponsored computer networks. Federal and private sector subsidies supported the DARPA-NET, NSFnet (National Sciences Foundation,) and thousands of other subnetworks, which were used to do inter-agency research and communication. Today, the Internet is made up of millions upon millions of computers and subnetworks works—almost entirely supported by commercial funds except in countries where deregulation has not occurred. The Internet is the substrate and chief communications backbone for the world wide web (WWW).

Internet Telephony
Any means of transmitting the human voice (real-time or near real-time) over the Internet. There are several components: 1) On the client side, a multimedia-equipped PC with special client software will digitize your voice. This can be done with a voice modem or other voice encoding method; 2) A direct or dial-up connection to the Internet allows your voice to be transmitted in packet form to its destination; 3) Connection with the far side is achieved by IP address search, common servers or beacons to identify the called party (and to “ring” that person’s phone); 4) A similar arrangement on the far end completes the call and allows both parties to speak. There are also PSTN/Internet gateways that allow regular telephone callers to make phone-to- Internet-to-phone connections. There are PC-to-phone connections and phone-to-PC connections.

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

Internet Service Provider (ISP)
A business that provides subscriber-based access to the Internet. Subscribers can be individuals or businesses. According to Jack Rickard, publisher of Boardwatch Magazine, ISPs operate at the fourth or lowest level of the Internet. At the third level, regional providers aggregate traffic from lower-order ISPs to the second, backbone level. The highest level in North America is the NAP (Network Access Point), which acts as peer-topeer interconnection points for the largest backbones. There are three ”official” NAPs located in San Francisco, California; Chicago, Illinois; and Pennsauken, New Jersey. ISPs use Internet routers, servers and Rrack-mounted modems to provide a variety of services, including web site hosting, FTP service, e-mail accounts, unified messaging, audio and video broadcasting, and—in some cases—Internet telephony and fax gateway services.

ITC 
independent telephone company

ITU 
International Telecommunications Union

Interactive Voice Response (IVR)
In computer telephony, IVR is a horizontal application wherein computer-based information is accessed over the phone by using a telephone instead of a computer. An IVR platform uses computer telephony components to translate callers’ touch-tones or voice commands into computer queries after the callers listen to an audio menu. For example: “Please enter your account number using the touch-tones on your telephone.” These queries are then “fetched” by the IVR platform from the host computer. In some cases, the information resides in the same platform (self-hosted). The information is converted into voice commands that are spoken over the phone to the caller.

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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

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.

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.

Messaging
In computer telephony, any means of storing and forwarding messages. This includes fax mail, voice mail, and broadcast messaging. This horizontal application is the most popular of all voice solutions. Messaging systems provide for the storing and forwarding of “nonreal time” communication. For example, a recorded voice message can be stored for later playback either locally or remotely, or a fax can be received and stored before it is re-transmitted to the ultimate recipient. Messages can vary in content and media type—the distinction being that they are recorded or stored for pick up in the future.

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

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

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

Packet
A logically grouped unit of data. Packets contain a payload (the information to be transmitted), originator, destination, and synchronization information. The idea with packets is to transmit them over a network so each individual packet can be sent along the most optimal route to its destination. Packets are constructed on one end of the communication and de-constructed on the receiving end based on the header addressing information at the front of each packet. Routers in the network will store and forward packets based on network delays, errors, and re-transmittal requests from the receiving end.

Packet Switching
A means of economically sending and receiving data over multiple network channels. Packet switching takes data and breaks it down into packets—small bundles of information containing the payload and routing information. The packets are then transmitted to the receiving end, where they are converted back to the original data format. One feature of packet switching is that packets
can be received out of order and then be quickly arranged into the correct order. There are slow packet switching networks—like the old SNA networks—and fast packet networks based on Frame Relay and ATM. Although traditionally used for data, packet networks— especially well-managed ones—are suitable for real-time transmission of voice and video.

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.

Post Office Protocol (POP)
An Internet standard for storage and retrieval of email messages.

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) or
Private Automatic Branch Exchange (PABX)
In telephony, a PBX system behaves as a customer’s premises over trunk lines (thus the term branch). At first, PBXs mimicked a small telephone company switchboard. Users would use an operator to make telephone calls to the PSTN (public switched telephone network). Now, users dial directly, without using an operator; computer telephony platforms such as automated attendants are able to route incoming calls automatically, too.

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

Public Switched Telephone Network (PSTN)
The world’s combined public circuit-switched telephone networks make up the PSTN. At one time, the PSTN consisted of analog telephone systems, now the PSTN is almost entirely digital, and includes mobile telephones along with fixed telephones.

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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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

Real-Time
Communications wherein perceptible delays between the sender and receiver are minimal and easily tolerated are considered to take place in real-time. Regular telephone calls are real time. Point-to-point fax transmissions are near to real-time. Voice messaging is not real-time.

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.

Registered Jack-11 (RJ-11)
The designation for connecting a tip and ring circuit to a standard, modular, 6-position jack.

Registered Jack-45 (RJ-45)
Eight-position modular connector used for data transmission over standard twisted or flat pairs.

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 needs. 

Service Provider
A company that provides services to Internet, telephone, and mobile phone users.

Signaling System #7 (SSY7)
The basis for routing traffic with out-of-band signaling. Its forerunner, CCIS (Common Channel Interoffice Signaling), used 4.8 kbps data links to transmit call set up and tear down messages to switching office adjunct computers and packet switches. SS7 in itself is not a network service offering, but rather the underlying infrastructure upon which many existing and proposed offerings are based. For example, local Basic Rate ISDN (BRI) services can tap into SS7, so 64 kbps packetized data can be routed with the help of the network’s out-of-band signaling capability. In addition, nationwide Primary Rate ISDN (PRI) services can use the same backbone.

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).

Session Initiation Protocol (SIP)
The Session Initiation Protocol (SIP) is a signalling protocol used for establishing sessions in an IP network. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session. The ability to establish these sessions means that a host of innovative services become possible, such as voice-enriched e-commerce, web page click-to-dial, Instant Messaging with buddy lists, and IP Centrex services.

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.

Speech Recognition
Speech recognition describes a technology that enable callers to speak words that are used to control applications.

Store and Forward
The method for storing a message or transmission for later playback or transmission. As opposed to real-time communication, store and forward is the basis for all messaging systems, including email, fax-on-demand, unified messaging, etc. In data communications, store and forward applies to momentary buffering of packets or other data strings.