Glossary
531
application layer
Layer 7 of the OSI reference network model, supplying
services to application procedures (such as electronic mail or file transfer)
that are outside the OSI model. This layer chooses and determines the avail-
ability of communicating partners along with the resources necessary to
make the connection, coordinates partnering applications, and forms a con-
sensus on procedures for controlling data integrity and error recovery.
ARA
AppleTalk Remote Access: A protocol for Macintosh users estab-
lishing their access to resources and data from a remote AppleTalk location.
area
A logical, rather than physical, set of segments (based on either
CLNS, DECnet, or OSPF) along with their attached devices. Areas are com-
monly connected to others using routers to create a single autonomous
system. See also: autonomous system.
ARM
Asynchronous Response Mode: An HDLC communication mode
using one primary station and at least one additional station, in which trans-
mission can be initiated from either the primary or one of the secondary
units.
ARP
Address Resolution Protocol: Defined in RFC 826, the protocol that
traces IP addresses to MAC addresses. See also: RARP.
ASBR
Autonomous System Boundary Router: An area border router
placed between an OSPF autonomous system and a non-OSPF network that
operates both OSPF and an additional routing protocol, such as RIP. ASBRs
must be located in a non-stub OSPF area. See also: ABR, non-stub area,
and OSPF.
ASCII
American Standard Code for Information Interchange: An 8-bit
code for representing characters, consisting of seven data bits plus one
parity bit.
ASICs
Application-Specific Integrated Circuits: Used in layer 2 switches to
make filtering decisions. The ASIC looks in the filter table of MAC addresses
and determines which port the destination hardware address of a received
hardware address is destined for. The frame will be allowed to traverse only
that one segment. If the hardware address is unknown, the frame is for-
warded out all ports.
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