Spirent SmartBits System, Reference Manual

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Glossary of Terms
Features and Terminology
SmartBits System Reference 261
VFDs (Variable Field Definitions)
Applications Supported:
SmartWindow, SmartLib
A VFD (Variable Field Definition) is a field within the frame whose value can be
manipulated—for example, incremented, decremented, or used in chunks. VFDs write
over the background fill pattern. Variable Fields are areas of packet data that can change
on a
per frame basis at wire speeds.
VFDs are available on Ethernet cards in the Traditional mode and on frame relay cards as
described below:
•
Ethernet SmartMetrics cards: The SmartMetrics mode does not support the use of
VFDs, except that in the SmartBits customizable stream VFD3 can be used to enter
the custom protocol header and payload. On SmartMetrics and TeraMetrics cards,
varied frame generation is accomplished via streams and “SmartFlows.”
•
Frame Relay cards: The frame relay cards support VFD1 and VFD2 in
SmartMetrics streams. They support VFD3 in limited fashion, where range is the total
number of bytes usable from the VFD3 buffer.
Three VFDs are supported for each Traditional card.
VFDs 1 & 2
Variable Fields 1 & 2 are up to 6 bytes long and can change on a per frame basis. Their
content can be:
•
Off . Variable field disabled .
•
Static . No change frame-to-frame .
•
Incrementing. Adding one to the value each frame.
•
Decrementing. Subtracting one from the value each frame.
•
Random. A random value each frame.
The content is defined by the State pull down menu.