Wavetek ANT-20, Руководство по эксплуатации

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Broadband Analyzer/Generator ANT-20/ANT-20E
TB-4 Technical Background
1.3.2 “On-Off” source model
The “On-Off” source model generates test cells in bursts. Fill cells are inserted in the gaps
(see Fig. TB-3). The desired traffic profile can be influenced using the following parameters:
•
Peak Cell Rate
•
Mean Cell Rate
•
Burst Size
•
Cell Jitter
“Peak Cell Rate” specifies the nominal time spacing between test cells within a burst,
∆
t.
“Mean Cell Rate” specifies the mean cell rate.
“Burst Size” determines the length of a burst. The burst period to be used is calculated by the
instrument automatically. It is largely based on the ratio of “Peak Cell Rate” to “Mean Cell Rate”.
The “Cell Jitter” parameter is used to cause a variation in the nominal cell arrival times. The
profile used for this corresponds to an increasing ramp function. where the length of the ramp
corresponds to the “Burst Size”.
The ramp rises until the cell arrival is advanced by the entered value. The gap between bursts
is increased correspondingly to ensure that the correct mean cell rate is reached.
Fig. TB-3 Principles of “On-Off” source model
The specified jitter is superimposed on the source before multiplexing. Due to the unavoidable
effects of multiplexing, the traffic profile actually transmitted may be modified again.
This source model is particularly suitable for testing VBR and SBR traffic contracts. The profile
of the superimposed cell jitter makes it easy to test a traffic contract in respect of its Cell Delay
Variation Tolerance (CDVT
PCR
).
∆
t
∆
t’
without Jitter
with super-
Cell Jitter
. . .
Filling cells
t
t
imposed
Burst Size
Jitter