Cisco Cisco IOS Software Release 12.0(19)S

ACE—Access Control Entry (element of an ACL).

ACL— Access Control List. Access lists filter network traffic by controlling whether routed packets are 
forwarded or blocked at the router’s interfaces. The router examines each packet to determine whether 
to forward or drop the packet, based on the criteria specified within the access lists. See also EACL.

ADM—Add Drop Multiplexer. A multiplexer capable of extracting lower-rate signals from, and 
inserting lower-rate signals into, a higher-rate multiplexed signal without completely demultiplexing the 
signal.

ANSI—American National Standards Institute.

APS—automatic protection switching. SONET switching mechanism that routes traffic from working 
lines to protect them in case of a line card failure or fiber cut.

AS—Autonomous System. A group of routers under a common administration.

AU-4—Administrative Unit 4.

AU-3—Administrative Unit 3.

AUG—Administrative Unit Group.

BER—Bit Error Rate.

BERT—Bit Error Rate Test. Bit error rate is the probability that a bit error could occur on any given bit 
on a line.

CAR— Committed Access Rate. A Cisco IOS software feature that allows a stream of traffic to be rate 
limited and given a user-specified priority.

CDP—Cisco Discovery Protocol.

CEF—Cisco Express Forwarding. Layer 3 switching technology. CEF can also refer to central CEF 
mode, one of the two modes of CEF operation that enables a route processor to perform express 
forwarding.

COS—Class of Service: the process of treating one set of traffic differently from another set of traffic.

CRC—Cyclic Redundancy Check. A technique for using overhead bits to detect transmission errors.

DACS—Digital Access and Crossconnect System. AT&T’s term for a digital cross connect system.

dCEF—Distributed Cisco Express Forwarding. One of two modes of CEF operation that enables line 
cards to perform the express forwarding between port adapters. 

DE—Discard Eligibility: a bit in the Frame Relay header.

Dedicated Line—Communications line that is indefinitely reserved for transmissions, rather than 
switched as transmission is required.

DLCI— Data Link Connection Identifier. The ID on a packet which identifies it as belonging to a 
particular Frame Relay virtual circuit. In Frame Relay, multiple logical channels are multiplexed over a 
single physical channel. The DLCI says which of these logical channels a particular data frame belongs 
to.

DS1—Digital Signal Level 1. A U.S. standard for high-speed data transmission over a T1 line at a data 
rate of 1.544 megabits per second (Mbps).

DS3—Digital Signal Level 3. A U.S. standard for high-speed data transmission over a T3 line at a data 
rate of 44.736 Mbps.

DSU—Data Service Unit. Part of the customer premises equipment used to interface to a digital circuit. 
A DSU is effectively a high-speed modem (with data rates of 34 or 45 Mbps).