Guide to Telecom Lingo

Don’t know the difference between PBX and Voice over IP? Confused about Primary Rate Interface (PRI)and POTS? Here's a handy glossary of common business telecommunication terms.

Access Line
A software-based line in the switching center switch that provides access to the public switched network for each Centrex system. The quantity of the Centrex access lines can vary and is designed to meet the unique needs of each individual Centrex group.
Refers to an electrical signal that varies continuously over an infinite range of voltage or current values, as opposed to a digital signal, which varies discretely between two values, usually one and zero. Analog signals can be viewed as sine waves of various sizes. The public network was initially built around analog technology. Digital facilities are rapidly replacing older analog facilities in most parts of the country.
Analog Line
A standard switched telephone line. Analog usually indicates that any type of plain old telephone can use the line. To transmit data over an analog line, a modem is required to convert digital computer signals into analog signals that are then transmitted and received through this phone line. Local service providers are rapidly replacing analog lines with digital lines which require special terminal equipment, but many parts of the country still utilize analog facilities.
The difference between the highest and lowest frequencies of a transmission channel. Bandwidth is a measure of the information capacity of the transmission channel and is also referred to as the speed of the line. Bandwidth varies with the type and method of transmission and is often expressed in bits per second or kilobits per second.
Bits Per Second
A basic unit of measurement for data transmission speed, usually represented as bps. For example, Kbps stands for kilobits per second or a thousand bits per second.
The inability to complete a connection between two lines because of a network busy condition. Blocking can be caused by insufficient equipment, processor capacity or lines.
Central Office
The traditional term for a facility that contains the local telephone switch that services public, private, and business customers and where customer lines are interconnected. Also referred to as an exchange, switching center, end office, or local central office wire center.
A service offered by local service providers that allows every subscriber to be directly dialed from the outside and provides basic business telephone system functions without requiring common equipment at the customer's premise. Centrex switching equipment is usually located in the central office.
Refers to communications procedures, techniques, or equipment where information is encoded as either a binary "1" or "0". Information is represented in discrete binary form, discontinuous in time, as opposed to the analog representation of information in variable, but continuous, wave forms. The telecommunications industry is moving rapidly away from analog to digital technology because digital is faster, more reliable, and more cost-effective.
Direct Inward Dialing
Allows incoming calls from the public network to be completed to specific station lines without attendant assistance. DID is available on all Centrex stations. DID is also available on a PBX but requires extra trunking and special software/hardware.
A calling area served by a service provider's switching center. An exchange will encompass one or more NXX codes, which are the first three digits of your local phone number.
A physical transmission path between two or more points, provided by a service provider, local telephone company, or a long distance company.
Integrated Services Digital Network (ISDN)
A specific type of switch-based digital telecommunications service. There are two types of ISDN service in North America: Basic Rate Interface and Primary Rate Interface. ISDN is characterized by end-to-end digital technology, out of band signaling and standard interfaces.
Key System
A type of telecommunications system located on a customer's premises and interconnected to the public network via central office or switching center lines. A key system is characterized by proprietary station equipment that comes with buttons or keys to access features such as call hold, intercom and conferencing. The key service unit (KSU) is the main operating unity of a key telephone system. Key systems are commonly used by small organizations.
Line Class Code
Defines the calling capabilities of each station line, including the capability to call or be called by another station or the outside world. Each station must be assigned a line class code. The code also indicates the type of rate and toll diversion associated with the station.
Local Exchange Carrier
Another name for the local telephone company servicing a customer's business or home. These consist of the seven Regional bell Operating Companies and the independent local telephone companies.
Local Loop
A telephone line from the switching center switch to a telephone set, computer or other piece of terminating equipment.
Network Interface
A physical point of demarcation, usually on a customer's premise, that divides the customer's telecommunications equipment from the telephone company's switching equipment. Generally, the customer is responsible for caring for the equipment and wiring on the premises side of the network interface, and the telecom service provider maintains everything on the switching center side.
Off Premise Extension
A telephone extension located somewhere other than where the main switch is located. An OPX may be used to connect a remote site to the main business location.
Plain Old Telephone Service
(POTS) Conventional telephone service, usually consisting of a single telephone set and access to the public switched network.
Point of Presence (POP)
The physical access location within a LATA of a long distance or other interLATA common carrier. ThePOP is the point to which the local telephone company terminates subscriber's lines for long distance communications.
Primary Rate Interface
An ISDN service that carriers 24 streams (channels) of information over a single set of telephone wires. Twenty three of the channels re called "bearer" or "B" channels, each with a transmission rate of up to 64Kps. The B channel can carry digitized voice, data, images, and video. The twenty-fourth channel is reserved for "signaling" and is called the "D" channel. The D channel also has a transmission rate of up to 64 Kbps. ISDN PRI is intended for termination to an ISDN equipped PBX or other ISDN equipped switch.
Private Branch Exchange
Commonly referred to as a PBX, this is a telephone switch that is owned or leased by an end user customer. It is located entirely on a customer's premises. A PBX allows internal dialing from station to station and supports connection to the public network for incoming and outgoing calls. PBX's are generally designed for larger organizations.
Private Facility
A communications path between two switching systems, intended for exclusive use only by the individual user company who purchased it, as opposed to the switched facilities that are shared by many users in the public network. A private line is also referred to as a leased line or a private trunk. They may be provided in groups of trunks and are shared by all members of the user group who are given access authorization. Access to private facilities from a Centrex station is generally gained by using an access code.
Proprietary Telephone Sets
Types of telephone sets designed to work only with a specific telecommunications system. They cannot be used with other systems.
The telephone set in the telephone system or the point where business equipment interfaces with the channel on a private line. When referring to Centrex, station is defined as the internal Centrex group telephone number used to designate an individual telephone user.
Station Line (Centrex)
The communications path between the Centrex station and the switching center switch.
A digital transmission facility, usually operating at an aggregate data rate of 1.544 Mbps or above. A T-1 can be multiplexed into 24 voice channels. T-1s are usually supplied by a telecom service provider. They are used to connect directly to a switch, such as a PBX, or to a LAN, via a gateway. Also know as a DS-1.
A single transmission channel between two points, both of which are switching centers. For instance, the telephone line that extends from the service provider's switching center to a customer's PBX system is called a trunk. A trunk is usually shared by many users and accessed by dialing an access number.
Uniform Call Distribution
A feature that spreads calls coming in on a group of lines as smoothly as possible so that all stations handle relatively similar loads. Most call distrubution systems also provide for queuing of incoming calls. Typically the incoming call with the longest hold time is represented for service first.