Agilent Technologies 622Vu Advisor, User Manual

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Appendix A
Glossary
622Vu Advisor
The user interface to the CellCommand OLE Automation server. This interface
program provides all the capabilities outlined in this Agilent 622Vu Advisor User’s Guide.
622Vu
Undercradle The Agilent Advisor undercradle analysis device. Comprises the logic used
to send, receive, and analyze ATM traffic as directed by your scripts, ping messages, and
QoSMeter sessions.
AAL
See ATM Adaptation Layer.
Adaptation Layer
See ATM Adaptation Layer.
Arrival Time
Applies to the capture window. Format is hh:mm:ss.nnnnnnnnn, shown to a
resolution of 33 ns. Initially displays relative to the start of the capture, but can be changed
via the View menu, to display a delta time between cells; or via the Edit menu, to establish a
particular cell as a baseline from which all other (relative) times are measured.
ATM
Asynchronous Transfer Mode — Group of communications protocols that support a wide
range of bit rates using a form of cell networking that transmits data in 53-byte cells. ATM
allows integration of voice, data, and video transmissions, and is the most widely studied form
of cell networking. It was initially developed by the CCITT and its successor, the ITU, as the
underlying form of transmission for B-ISDN. Subsequently, a group called the ATM Forum
has taken over some of the ATM design in an effort to provide standards that were lacking in
the CCITT version.
In this context, the term asynchronous refers to the fact that cells from any one source don’t
have to be spaced (within the data stream) at a set position within a recurring frame (as is
common in circuit switching).
ATM Adaptation Layer
Layer of ATM protocols responsible for packaging data into cells,
then reassembling it at the receiving end — that is, for adapting datagrams, voice samples, and
video frames into a format that can be handled by ATM. The individual protocols used in this
layer are referred to as AALs. As specified by the ITU, there are currently four AALs used to
package and reassemble normal traffic:
•
AAL1 supports connection-oriented services that require a constant bit rate, and have
specific timing and delay requirements.
•
AAL2 supports connection-oriented services that don’t require a constant bit rate; i.e.,
variable bit-rate services such as some video schemes.
•
AAL3/4 supports variable bit-rate transmissions that don’t require a bounded delay from
source to destination (connectionless and connection-oriented).
•
AAL5 is a simpler (and connection-oriented) version of AAL 3/4, suitable for most data
transmissions. Its simplicity comes at the expense of automatic error recovery and built-
in retransmission.
The ATM Forum is working on AAL 6, which will support a specific type of video-stream
transmission known as MPEG2.
Each AAL is divided into two parts: an SAR (Segmentation and Reassembly) sublayer and a
Convergence Sublayer (which itself comprises two separate sublayers). Beyond this brief
A: Glossary
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