Glossary of Terms


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911: A three-digit telephone number to facilitate the reporting of an emergency requiring response by a public safety agency.

911 ADMINISTRATOR: The administrative jurisdiction of a particular 911 system. This could be a county/parish or city government, a special 911 or Emergency Communications District, a Council of Governments, an individual PSAP or other similar body.

911 CALL: Any telephone call that is made by dialing the digits "911".

911 CALL ANSWERER: Initial answerer of a 911 call. This individual may or may not be a dispatcher.

911 CENTER: See "PSAP".

911 SERVICE AREA: The geographic area that has been granted authority by a state or local governmental body to provide 911 service.

911 SYSTEM: A 911 system encompasses more than a three-digit telephone number which automatically connects the emergency call to a PSAP. A 911 system is the total emergency response capability involving the 911 emergency number, the dispatch function, radio equipment, staffing the PSAP facility, and the management of a coordinated effort for law enforcement, fire, and emergency medical aid.

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ABANDONED CALL: A call placed to 911 in which the caller disconnects before the call can be answered by the Public Safety Answering Point (PSAP) attendant. ACD:

See "Automatic Call Distributor.

ALI (AUTOMATIC LOCATION IDENTIFICATION): A system capability that enables an automatic display of information defining a geographical location (e.g., a street address) of the telephone used to place the 911 call; this feature is available in Enhanced 911 systems.

ALTERNATIVE ROUTING: Selective routing which allows 911 calls to be routed to a designated alternative location if all incoming 911 lines to the primary PSAP are busy or if the primary PSAP closes down for a period.

ANI (AUTOMATIC NUMBER IDENTIFICATION): A capability that enables the automatic delivery of the seven-digit number of the telephone used to place the 911 call; this feature is available in Enhanced 911 systems.

AUTOMATIC CALL DISTRIBUTOR (ACD): Equipment used to distribute large volumes of incoming calls in an approximate order of arrival to call answerers not already working on calls or to "hold" calls until call answerers become available.


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BASIC 911 SYSTEM: A telephone system which automatically connects a person dialing the digits "911" to an established PSAP through normal telephone service facilities.

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CAD: Computer Aided Dispatch.

CALL ANSWERER: The initial answerer of a 911 call sometimes referred to as a call taker.

CALL ANSWERING PERSONNEL: Any person engaged in or employed by a PSAP as a telecommunications operator whose responsibility is the receipt or processing of calls for emergency services provided by public safety, fire or emergency medical agencies and who receives or disseminates information relative to emergency assistance by telephone or radio.

CALL DETAIL RECORDING: Provides a written record by telephone number of all Enhanced 911 calls received by a PSAP.

CALLED PARTY HOLD: A telephone system feature that enables the Public Safety Answering Point to control the 911 call and to maintain a connection through the telephone system's switching facilities even if the 911 caller has hung up the telephone, or to permit the tracing of a call.

CALL RELAY METHOD: The 911 call is answered at the PSAP (where the pertinent information is gathered) and the call answerer relays the caller's information to the appropriate public or private safety agency for further action.

CALL ROUTING: The capability to selectively route the 911 call to the appropriate PSAP.

CALL TRANSFER METHOD: PSAP call answerer determines the appropriate responding agency and transfers the 911 caller to that agency.

CCS: Hundred second calls or the total amount of traffic in seconds divided by 100.

CENTRAL OFFICE (CO): Also called a wire center (a switching unit in a telephone system): the smallest subdivision in the telephone system which has relatively permanent geographic service boundaries.

COIN FREE ACCESS (CFA): Coin free dialing or no coin dial tone which enables a caller to dial 911 or "0" for the Operator on pay telephones without depositing money.

CO: See "Central Office."

CONTROL OFFICE: Also called a tandem switcher (a Bell system term for the telephone company central office which provides tandem switching capability for selectively routed 911 calls), the control office controls the switching of ANI and selective routing information to the PSAPs, provides standard ESS Speed Calling features, call transfer capability, and certain maintenance functions for each PSAP.

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DATA MANAGEMENT SYSTEM: A system of manual procedures and computer programs used to create, store and update the data required to provide selectively routed 911 service.

DDD (Direct Distance Dialing): Telephone service which permits subscribers to dial their own long distance calls.

DEFAULT ROUTING: A selective routing feature which allows 911 calls to be routed to a designated alternate location (the default PSAP) if the incoming 911 call cannot be selectively routed due to ANI failure, garbled digits, or other causes which may prevent selective routing.

DIRECT DISPATCH METHOD: 911 call answering and radio dispatching functions for a particular agency are both performed at the PSAP.

DISPATCH CENTER/RADIO DISPATCH CENTER: The location from which a public or private safety agency's mobile units are dispatched.

DISPATCHER: An individual who uses radio or other means to dispatch public or private safety agency's resources. This person may or may not function as a 911 call answerer.

DISPLAY AND TRANSFER UNIT: A special console and associated common equipment that allows display of ANI numbers at the 911 call answer's position and is used by the call answerer to activate the fixed transfer and/or selective transfer features. This equipment is associated with Enhanced 911.


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ECHO: Return of transmitted data.

EMERGENCY ALERT SYSTEMS (EAS): Radio or television based broadcast of emergency event information.

EMERGENCY CALL: A telephone request for service which requires immediate action to prevent loss of life, reduce bodily injury, prevent or reduce loss of property and other emergency situations determined by local policy.

EMS: Emergency Medical Services.

EMD (Emergency Medical Dispatch): Critical medical advice offered by specially trained 911 call answering personnel. Advice follows approved protocols, given in logical sequence, for such conditions as heart attacks, choking, and child birth. Protocols are administered by phone until the arrival of emergency medical personnel on the scene.

END OFFICE: The central office(s) from which the 911 calls originate.

ENHANCED 911 (E911): Enhanced 911 systems is the general term referring to emergency telephone systems with specific electronically controlled features, such as ALI, ANI, Selective Routing, and which use the MSAG geofile. Most Enhanced 911 systems implemented to date have been provided by the local Bell operating companies, although there are other vendors who make equipment (e.g., computers, ACD, and PSAP equipment) which is often part of the overall Enhanced 911 system.

ESS (Electronic Switching System): A central office with programmable telephone switching logic.

ESN (Emergency Service Number): Defines the selective agencies which are served by a particular telephone number. Also see ESZ.

ESZ (Emergency Service Zone): Defines a geographical territory consisting of a specific combination of law enforcement, fire and EMS coverage areas. Also see ESN.

EXCHANGE: A defined geographic area served by one or more central offices in which the telephone company furnishes services.

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FIXED TRANSFER: A selective routing feature which allows the call answerer to transfer 911 calls to secondary PSAPs by use of a single button. Each button corresponds to a designated secondary PSAP.

FORCED DISCONNECT: A telephone feature which allows the PSAP to break or disconnect a telephone connection and avoid caller jamming of 911 lines.

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GIS (Geographic Information System): A graphical mapping system where the graphical information is related or matched to data about that information. In this system a house shown on a map might be related to a data base that lists all pertinent information about the house and the owner/occupant.

GPS (Global Positioning System): A constellation of 24 satellites orbiting the earth at 10,900 nautical miles. These satellites emit signals that can be monitored by special receivers and those signals used to locate the geographical position of the receiver.

GRADE OF SERVICE: A grade of service is the probability (P) expressed as decimal fraction of a telephone call being blocked
by busy lines. For example, P.01 is the grade of service reflecting the probability that one call out of one hundred calls will be uncompleted (or busy).

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HEARING CARRY OVER (HCO): A method utilizes both voice and text communications on the same call, allowing a person who is speech impaired to listen to the other party's conversation and respond by typing via a TTY or other means of text communications.

HIGHWAY CALL BOX: A telephone enclosed in a box and placed along a highway that allows a motorist to summon emergency and non emergency assistance.

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IDLE CIRCUIT TONE APPLICATION: Telephone system feature which applies a distinctive tone to the 911 call answerer to indicate the calling party has hung up. This tone may indicate whether the calling party has hung up before or after the PSAP answers.

IMPLEMENTATION: Activity between formal approval and a given system design and commencement of operations.

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JURISDICTIONAL ADDRESS: An MSAG valid address for the physical location of a subscriber access line, which has been assigned by the jurisdiction's local addressing authority; i.e., planning department, zoning department, etc. and is used for 911 emergency dispatching.

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KEY TELEPHONE EQUIPMENT: An instrument that has the capability of multiple line terminations. Most phones in offices which have six, ten, or more "buttons" are examples of key telephones. Each line can be accessed by depressing one of the buttons or "keys."

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LIGHT EMITTING DIODE (LED): Lamps used for display of information. Commonly used on telephone sets to indicate line status.

LOCAL AREA NETWORK (LAN): A transmission network encompassing a limited area, such as a single building or several buildings in close proximity.

LOGGING RECORDER: A device that records date/time, voice communications and other transactions involved in the processing of calls to a PSAP.

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MAIN STATION: A telephone that is connected directly to a central office and has a unique telephone number. It is not an extension station.

MANAGEMENT INFORMATION SYSTEM (MIS): A program that collects, stores and collates data into reports enabling interpretation and evaluation of performance, trends, traffic capacities, etc.

MANUAL TRANSFER: A call routing feature which allows the call answerer to transfer an incoming call by pressing a single button and dialing either a telephone number or a two-digit Speed Calling code.

MASTER STREET ADDRESS GUIDE (MSAG): The computerized geographical file which consists of all streets and address ranges within the 911 system area. This data base is the key to the selective routing capability of Enhanced 911 systems, and requires constant updating after the initial file is established.

MISROUTE: An E911 call routed to an incorrect PSAP due to network discrepancy; i.e., wrong ESN in selective routing database, incorrect switch translations.

MOBILE: Use of a wireless-connected device with the capability of movement during the call.

MODEM: An interface device which allows digital data signals to be transmitted over analog telephones lines.

MULTI-PARTY LINE: A local subscriber line terminating on the switching equipment that serves two or more main subscriber locations.

MUNICIPAL ADDRESSING OFFICER: A representative designated by a community participating in the E-911 system responsible for all issues involving the development and maintenance of address information for the E-911 addressing and routing databases.

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NATIONAL EMERGENCY NUMBER ASSOCIATION (NENA): The National Emergency Number Association is a not-for-profit corporation established in 1982 to further the goal of "One Nation-One Number." NENA is a networking source and promotes research, planning and training. NENA strives to educate, set standards and provide certification programs, legislative representation and technical assistance for implementing and managing 911 systems.

NETWORK: 1. A series of points interconnected by communications channels. 2. The switched telephone network is the network of telephone lines normally used for dialed telephone calls. 3. A private line network of communications channels confined to the use of one customer.

NIGHT SERVICE: An arrangement allowing an answering agency to close its operations and have all 911 calls to it forwarded to a pre-selected alternate agency.

NO-COIN DIALING: See "Coin Free Access".

NOMADIC VoIP CALL: Call generated by a VoIP user other than their originally provisioned fixed location using the terminal equipment from that location (i.e.: VoIP handset, laptop, VoIP terminal, PC)

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ONE STAGE SYSTEM: The same individual(s) at the PSAP answers both 911 calls and functions as a dispatcher.

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PBX (Private Branch Exchange): A telephone switchboard with many stations not individually identifiable to the telephone utilities switching network. (Also called a PABX.)

PERSONAL DIGITAL ASSISTANT (PDA): Small, handheld device used to store address book information, telephone numbers, personal contacts and other personal information.

POSTAL ADDRESS: Address recognized and used by the United States Postal Service (USPS) for delivery of mail. It may be an address with a house number and street name or may also consist of other USPS acceptable delivery options, such as Army Post Office (APO), or Fleet Post Office (FPO). Postal addressing may contain variants of abbreviations (Avenue or Ave, Street or St.) that the Postal Service recognizes as acceptable postal addresses. Postal addresses reflect the name of the community assigned by the USPS to the correct zip code.

PRIMARY PSAP: The initial answering location for 911 calls in a selectively routed 911 system.

PRIVATE LINE: A telephone line used only for communication between two points and which does not connect with the public telephone system.

PSAP (Public Safety Answering Point): The initial answering location of a 911 call. Sometimes called a 911 center.

PUBLIC OR PRIVATE SAFETY AGENCY: Any unit of state or local government, a special purpose district, or a private firm which provides or has authority to provide fire fighting, police, ambulance, or emergency medical services.

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QUEUING: Queuing is an automatic process by which calls are presented in a predefined sequence to a call taker.

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Permits the answering point to ring the hung up telephone on a held circuit; this feature is useful when a calling party has failed to provide all necessary information to the answering point before hanging up.

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SECONDARY PSAP: A location to which 911 calls are transferred from the primary PSAP.

SELECTIVE ROUTING (SR): A telephone system feature that enables all 911 calls originating from with a defined geographical region to be answered at a pre-designated PSAP: a component on an Enhanced 911 system.

SELECTIVE TRANSFER: Another term for a transfer which allows the call answerers to transfer an incoming call by pressing a single button. For example, one button would transfer calls for fire, and another button would be use for police and a third button would be used for EMS.

SERVICE ADDRESS: The physical location of a subscriber access line. Service Address is the recommended address for 911 use. (May be different from the listed address or billing address).

SERVING CENTRAL OFFICE: The telephone company's central office area in which a PSAP is located.

SPEED CALLING: Dialing two digits causes telephone equipment to automatically ring a pre-designated seven-digit number.

SWITCHED NETWORK: A complex of diversified channels and equipment that automatically routes communications between the calling and called person or data equipment.

SWITCHHOOK STATUS INDICATION: Allows the PSAP to monitor, by means of supervisory lamps, the status of a calling party being held. Indicates whether the call party still is connected, is on hold, or has disconnected.

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TANDEM TRUNKING: An arrangement where a telephone line connection has one or more immediate switching points which are required or permitted (usually on a controlled dial pulse) basis before reaching the final destination (called party).

TARIFF: A document filed by a telephone company with the state telephone utility regulatory commission which lists the communication services offered by the company and gives a schedule of rates and charges.

TDD (Telecommunications Device for the Deaf): Any time of instrument, such as typewriter keyboard connected to the caller's phone and involving special equipment at the PSAP which allows an emergency call to be made without speaking. Sometimes referred to as a "TTY".

TELECOMMUNICATOR: Interchangeable with 911 operator, dispatcher, call receiver, emergency communicator, public safety communications specialist.

TELEPHONE LINE: A telephone line from a telephone utility's central office that is connected to key or non-key telephone equipment.

TERMINAL EQUIPMENT: Telephone call answering and transfer equipment.

TRUNK: A circuit used for connecting a subscriber in a central office to all other services in/out of the switching equipment (e.g., Long Distance Trunk, Operator Trunk, etc.).

TTY (Teletypewriter): See "TDD".

TWO-STAGE SYSTEM: The 911 call answering and radio dispatching functions are performed by two separate individuals.

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UNINTERRUPTIBLE POWER SYSTEM (UPS): The capability of providing a continuing source of power without regard to the interruption or loss of commercial power.

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VOICE OVER INTERNET PROTOCOL, VOICE OVER IP (VoIP): Provides distinct packetized voice information in digital format using the Internet Protocol. The IP address assigned to the user's telephone number may be static or dynamic.

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WIDE AREA NETWORK (WAN): Network using common carrier-provided lines that covers an extended geographical area.

WIRELESS SERVICE PROVIDER (WSP): Cellular, satellite or other radio based telephony or data transport commercial entity.

WORLD WIDE WEB (WWW): The public internet.

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