Cache - Generally, a place to temporarily store something. Regarding computers and telecommunications, it is a place where data is temporarily stored during transmission.

Call - This is a connection between two or more users with the intent of exchanging information.

Call Back - This is a security method with which a user calls into a system and requests to be called back by the system at a particular, predetermined number.

Caller - Anyone who makes a call.

Caller ID - The service that allows someone being called to see the caller's name and phone number.

Call Progress - This is the status of the telephone line. It can be busy, ringing, not ringing, etc.

Call Waiting - This is a service that tells you if someone is trying to call you when you are already using the line (i.e., talking with someone else).

Capacitor - This is an electronic device for storing electric charge.

Capture Effect - This effect occurs when two signals are received that are the same (or nearly the same) frequency. The stronger signal will appear in the output.

C-Band - The portion of the electromagnetic spectrum allotted for satellite transmission; it is the 4 to 8 GHz frequency transmission range. Specifically, the 3.7 to 4.2 GHz band is used as the downlink, and the 5.925 to 6.425 GHz band serves as the uplink.

Card - This term refers to a computer peripheral that can be connected internally (i.e., directly), without any cables, to a computer, such as a modem.

Caret - This is the character "^".

Carrier - This term refers to: (1) a company, such as a local telephone company, that provides circuits dedicated to communication, and (2) an electrical signal capable of being modulated to carry information.

Carrier Detect (CD) - This is a light on most modems that signifies, when lit, that the modem is connected.

Carrier Frequency - The main frequency on which data, a voice, or video signal is sent. Common microwave and satellite communications transmitters operate in the band from one to fourteen GHz.

Carrier Signal - The basic radio, television, or telephony center of a frequency transmission signal.

Cassegrain Antenna - An antenna that places a subreflector at the focal point to reflect energy to or from a feed located at the apex of the main reflector.

CB - This stands for Citizen's Band and is low-power (up to 4 Watts), public radio that does not require FCC approval.

CD - See Carrier Detect.

CDMA - This stands for Code Division Multiple Access and refers to a multiple-access scheme where stations use wide-spectrum modulations and orthogonal codes to reduce the chance of interfering with each other.

Central Office (CO) - This is a place where lines are joined to switching equipment (which control connections).

Channel - A frequency band, 6 MHz wide in the US, in which a specific broadcast signal is transmitted. Channel frequencies are determined in the US by the Federal Communications Commission (FCC).

Character - This is a letter, numeral, or symbol.

Characters per Second (CPS) - This is the number of characters that can be sent over the phone lines in 1 second.

Chat - A mode that allows two or more people to communicate with each other in real time over a network using modems.

Checksum - This is the sum of a group of data items that is used for error checking. Received information packets must have the same number of bits present during the initial transmission.

Chip - This is the physical structure of an integrated circuit.

Circuit - This is an electrical device that provides a path for electrical current to flow, or more generally, it is a journey or route all the way around a particular place or area.

Circular Polarization - Some satellites transmit both right-hand rotating and left-hand rotating signals simultaneously on the same frequency. This technique effectively doubles the channel carrying capacity of the satellite.

City Code - Some foreign countries require you to dial a city code before the phone number you are trying to reach.

Clamp - A circuit that removes the energy dispersal signal component from the video waveform.

Clarke Orbit - This is the circular orbit in space 22,237 miles from the surface of the earth at which geosynchronous satellites are placed to revolve around the earth. It is named after Arthur C. Clarke, who first postulated its existence. Satellites in these orbits move at speeds that cause their angular velocity to match the angular rate of the rotation of the earth, causing them to appear stationary when viewed from a particular point on Earth.

Clear to Send (CTS) - This is when a receiving modem tells the calling modem.

C/N (Carrier to Noise Ratio) - This is the carrier-to-noise-ratio, which is the ratio of the received carrier power to the noise power in a given bandwidth; it is expressed in decibels (dB), which is a logarithmic scale.