CO (Central Office) ◆ The site where the local Telco switches reside for the telephone system's call routing and all other functions. This is the Telco side of the local loop. If the distance between your location and the central office switch exceeds 18,000 feet and/or the signal loss exceeds 35.0 dB, a repeater must be installed to allow ISDN service.

Configuration Cycle ◆ A type of I/O cycle provided on the PCI bus to facilitate system configuration.

CPE (Customer-Premises Equipment) ◆ All equipment that resides past the telephone service demarcation point. In the U.S. this typically includes the NT1 and all ISDN devices. Customer equipment may also include inside wiring as well as telephone modems, and ISDN equip- ment.

CRC (Cyclic Redundancy Check) ◆ A bit errors detection technique that employs an algorithm to calculate a value for the information bits in a packet. The receiver, using the same algorithm, recalculates that value and compares it to the value received. If the two values do not agree, the transmitted packet is considered to be in error.

CSD (Circuit Switched Data) ◆ Adial-updatacommunicationschan- nel which, once established, looks like a transparent data pipe. Also, the type of ISDN service required to utilize this capability of an ISDN circuit. Contrast with CSV.

CSU/DSU (Channel Service Unit/ Data Service Unit) ◆ A device that incorporates the functions of a CSU and a DSU, and interfaces between a switched-56 (or Dedicated Digital Service) line and a user's data equipment.

CSV (Circuit Switched Voice) ◆ A dial-up communications circuit for voice grade communication. Also, the type of ISDN service required to use this capability of an ISDN circuit. Contrast with CSD.

CTS (Clear To Send) ◆ RS-232 (V.24) interface control signal (sent from the data communications equipment to the data terminal equipment on pin 5) indicating that the attached DTE may begin transmit- ting. Issued in response to the DTE’s request-to-send. (See RS-232.)

Data Link Control (DLC) ◆ A set of rules used by nodes on a data link (such as an SDLC link or a token ring) to accomplish an orderly exchange of information.

Data Link Control Identifier (DLCI) ◆ The numeric identifier of a frame-relay subport or PVC segment in a frame-relay network. Each subport in a single frame-relay port has a unique DLCI. The following table, excerpted from the American National Standards Institute

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