Glossary

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.

CS (Controlled Slip) A CS is the replication or deletion of the pay- load bits of the DS1 frame. A Controlled Slip may be performed when there is a difference between the timing of a synchronous receiving terminal and the received signal.

CSD (Circuit Switched Data) A dial-up data communications channel 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 which 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.

D channel One of the three standard channels on a BRI line. At 16 Kbps, a D channel carries all signaling information and can carry low- speed packet data such as X.25 packet switching.

demarc (demarcation point) The point on a customer’s premises where the line from the telephone company meets the customer’s wiring at the customer’s premises.

Directory Number Your seven digit telephone number (without the area code), as found in the telephone directory. Each BRI connection can have up to two directory numbers, one for each B channel.

DMA (Direct Memory Access) A fast method of moving data between two subsystems without processor intervention.

DMS/DMS100 The name of a central office switch manufactured by Northern Telecom. These switches use Custom (proprietary) or National ISDN-1 (NI-1) software.

DSU See CSU/DSU.

ETS (European Telecommunications Standard)

ETSI ETSI is the European Telecommunications Standards Insti- tute.

iSPAN PRI PCI ISDN Users Guide

135