Planning and Installation Guide
Call-Waiting Tone The tone that is presented to a user with call waiting when a second call
Caller For documentation purposes, an outside caller—a person calling the telephone
system from outside. See also End User.
CallerID A technique for transmitting the calling party’s telephone number and (optionally)
name to equipment enabled to handle this feature; also called CLI in Europe.
Centrex A name for advanced telephone services provided by the local telephone company.
It usually requires a connection to a special telephone system but provides services such as
voice mail and call forwarding.
CLASS Custom Local Area Signalling Services, a family of telephone services offered from
local telephone companies, usually for a monthly fee; includes features such as CallerID,
Call Waiting, call return, repeat dialing, call rejection, call trace, priority ringing, and
selective call forwarding.
Class of Service Abbreviated as CoS or COS; a set of features and privileges associated with a
particular user or extension, used for grouping similar users together.
CO Central Office; the building where the telephone company’s telephone switching
equipment that services the local area is located.
CO Line See Trunk.
Conference Three or more parties joined together in a single call, such that each party can
hear and be heard by the others.
DHCP Dynamic Host Configuration Protocol, a protocol for downloading network
information (such as IP addresses) to client workstations.
DID Direct Inward Dial, a signaling mechanism used by telephone companies to indicate to
a customer’s PBX what telephone number was dialed by the calling party. It can be used
with analog lines but is used mostly with digital (that is, T-1) connections.
DTMF Dual-Tone Multi-Frequency, a technique of providing two tones for each button on a
telephone to signal dialing digits; also known as Touch Tone.
End User For documentation purposes, a person using the telephone system from the
inside, such as from an extension or a call control application, as opposed to a caller who
dials in from outside the system; often shortened to “user.” See also Caller.
Erlang Formula A mathematical way of predicting a randomly arriving workload (such as
telephone calls) based on known information (such as average call duration). Although
traditionally used in telephone traffic engineering to determine the required number of
trunks, Erlang formulas have applications in call center staffing as well.
External Call A telephone call directed to or from outside the telephone system, and over the
Public Switched Telephone Network (PSTN).
FSK Frequency Shift Key, a modulation technique used with low-speed modems; also used
with CallerID and message-waiting lamp indicators.