B-4
Appendix B: Glossary
AH Authentication Header. One of the IPSec protocols that can encapsulate
packets sent over a VPN tunnel. AH uses authentication algorithms to ensure
the integrity of the packet contents. AH authenticates the entire IPSec packet,
including the delivery IP header. See also IPSec.
ALG Application Level Gateway. A protocol that acts as a proxy server between a
trusted client behind a firewall and an untrusted client. ALGs analyze and filter
packets at the OSI Application Layer and provide applications the special
services that they need to function through a firewall. Each application must
have its own ALG.
AMI Alternate Mark Inversion. A line-coding scheme used with T1 and E1 connec-
tions. Logical zeros are transmitted as zero voltage, and logical ones are
transmitted as pulses with alternating polarity.
Analog A continuously varying electrical sinusoidal signal. This signal type is used for
voice or data transmission.
ANI Automatic Number Identification. A service that provides the receiver of a
telephone call with the number of the calling phone. Also known as Caller ID.
ANSI American National Standards Institute. An organization that fosters the devel-
opment of technology standards in the United States. For more information
on ANSI, visit the ANSI Web site at http://www.ansi.org/.
AO/DI Always On/Dynamic ISDN. A form of ISDN connection that allows the BRI D
channel to be used for a low-speed data connection. Because the D channel
is always active, this connection is considered always on.
Application Layer Layer 7 of the OSI model. This layer supports application services for file
transfers, e-mail, and other network software services. Telnet and FTP are
applications that work at the Application Layer.
ARCFour A symmetric encryption algorithm supported by IP Security (IPSec), the
industry standard for virtual private networks (VPNs). ARCFour is a stream
cipher that supports keys ranging from 8 to 2048 bits in length.
ARM Asynchronous Response Mode. ARM designates a type of High-Level Data-
Link Control (HDLC) connection between a primary and secondary device,
during which the secondary device can initiate a transmission, but the primary
device controls the establishment and termination of the link. See also HDLC.
ARP Address Resolution Protocol. A Network Layer Ethernet protocol used to
convert a network IP address into a physical address. A host that wants to
obtain a physical address broadcasts an ARP request onto the TCP/IP network.
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