| Cable | one or more conductors within a protective insulating cover |
| Cable head | common receiving equipment in a cable television network which receives terrestrial and satellite transmissions for distribution across the network |
| Caching | temporary storage of data where it can be retrieved rapidly, the cache is usually updated frequently |
| Call congestion | failure of a telecommunications network to accept a bid to establish a call |
| Call drop-out | unintentional disconnection of a call by a mobile terminal, usually as a result of the loss of signal strength at the terminal |
| Carrier | a radio or microwave signal transmitted from one earth station, through the satellite, to one or more earth stations |
| Carrier | holder of a telecommunications carrier licence; a continuous electromagnetic wave which can be modulated by a signal to carry information |
| Casegrain | a reflector antenna geometry employing a primary parabolic dish and a secondary convex hyperbolic reflector placed close to the main mirror's focus |
| C band | frequency band between 4 and 8 GHz, normally 6 GHz is used for uplinks to the satellite and 4 GHz is used for the downlink |
| Cell relay | network transmission format using small packets of uniform size called cells; these fixed length cells can be processed and transmitted by hardware at very high speeds; acts as the basis for SMDS Interface Protocol and ATM |
| Central Office | local telephone company office which connects to all local loops (subscriber lines) in a given area and where circuit switching of customer lines occurs |
| Centre fed dish | reflector antenna with the focal point of the dish on the axis of the dish in front of the reflector; the feed is located at the focus |
| Channel | the one-way simplex path from one earth station, through the satellite, to another earth station; an electrical path between two devices |
| Channel aggregation | the combination of multiple physical channels into one logical channel of greater bandwidth; also termed bonding |
| Checksum | a numerical value that produces a predetermined constant when added to the sum of a group of bytes, used to verify that no degradation has occurred |
| Chrominance | the part of a colour television signal carrying the colour information |
| Churn | transfer customers from one service provider to another |
| Circuit | a two-way duplex path from one earth station, through the satellite, to another earth station - equivalent to two channels or two half-circuits |
| Circuit switched network | network that establishes a physical circuit temporarily, until it receives a disconnect signal |
| Circuit switching | a networking technique in which a physical connection is established between the sender and receiver for the duration of the transmission |
| Clamp | a video processing circuit which removes the energy dispersal signal from a video waveform |
| Clarke orbit | an archaic expression for the geostationary orbit, so named because it was first suggested by Arthus C Clarke in 1945 |
| Client/server | a distributed system model of computing which brings computing power to the desktop, with users (clients) accessing resources from servers |
| Clock | a source of digital timing signals; also the timing signal itself, usually in the form of a continuous series of timing pulses |
| Closed user group | group of users which operate a private network |
| Cluster | in a VSAT system an outbound and its associated inbounds |
| Codec | device for coding and decoding a digital signal |
| Collision | in Aloha, the transmission of two packets of data by different earth stations which overlap in time and so cannot be received without error |
| Co-location | the technique of using the same orbital position for two or more satellites |
| Combined access TDMA | is a synchronised multi-access protocol with both contention and fixed assigned access |
| Committed Information Rate | the transport speed a frame relay network will maintain between service locations |
| Common carrier | an organisation which operates communications circuits used by other people |
| Communications protocol | a set of procedures which control how a data communications link or network operates |
| Companding | technique of reducing the data rate required for transmission by processing the signal to reduce the amount of redundant information |
| Composite video | television signal containing chrominance, luminance and synch information |
| Compression | technique of coding a digital signal to reduce the amount of data required to represent that signal |
| Concentration | a channel sharing scheme in which a number of input channels share a smaller number of output channels |
| Concentrator | a device that combines the signals from several data sources into a single transmission line |
| Conditional access | the technique of encrypting broadcasts and restricting the provision of decoders and keys to those authorised to receive the transmissions |
| Conditioning | changes or adjustments made to equipment to improve transmission characteristics |
| Conformal array | an antenna that is contoured to the surface of the structure carrying it, for example an antenna built into the skin of an aircraft |
| Congestion | the condition in which excessive network traffic results in reduced performance |
| Connectionless communications | a form of packet switching that relies on global addresses in each packet rather than on predefined virtual circuits; connectionless transmission is characterised by unsolicited and unacknowledged transmissions from one point to another; because it does not require circuit set up or teardown, and does not require confirmation that messages were received correctly, it has less overhead than connection-oriented transmission |
| Connectionless Network Service | packet switched network based on connectionless communications |
| Connection-oriented communications | a form of packet switching that requires a circuit from source to destination to be established before data can be transferred |
| Constellation | a group of similar satellites which operate together to provide a regional or global service |
| Contention | network access method in which devices compete for the right to access the physical medium |
| Contention TDMA | a variant of Aloha (slotted Aloha) in which packet transmissions occur in predefined, and centrally synchronised, time slots; at the transmitting earth station, packets of data are transmitted in the next time slot; in the event of a collision, the data is retransmitted |
| Control character | a character that is used as a signal to control a transmission, rather than being part of the transmission being sent |
| Control word | the key used in a descrambler |
| Conus | contiguous US - all the states of the USA except for Hawaii and Alaska |
| Coordination area | the area associated with an earth station outside of which another earth station sharing the same frequency band neither causes nor is subject to interference greater than a specified level |
| Cross modulation | signal distortion in which the modulation from one or more RF carriers modulates another carrier, generally when passing through a non linear unit such as a power amplifier |
| Cross strapping | uplinking in one frequency band and beam type and downlinking in another frequency band and beam type |
| Cross talk | interference caused by the unwanted leakage of signal between two independent channels |
| Customer Premises Equipment | terminating equipment, such as terminals, phones and modems, installed at customer sites and connected to the company network |
| Cyclic redundancy check | an error detection scheme which encodes the bits of a message, by treating them as a binary polynomial, into a group of (usually) 16 bits which are placed at the end of the message |
