Wavetek ANT-20, Operating Manual

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ANT-20/ANT-20E Broadband Analyzer/Generator
Technical Background TB-3
1.3.1 “Constant Bit Rate” source model
The “Constant Bit Rate” source model generates test cells at equidistant intervals in time, with
fill cells inserted in the spaces (see Fig. TB-2). The desired traffic profile can be affected using
the following parameters:
•
Peak Cell Rate
•
Cell Jitter
“Peak Cell Rate” specifies the nominal time spacing between test cells,
∆
t.
The “Cell Jitter” parameter is used to alter the nominal time spacing (--->
∆
t’) and thus cause a
variation in the nominal cell arrival times. The profile used for this corresponds to an increasing
ramp function.
The ramp rises until the cell arrival is advanced by the entered value. A gap then occurs in the
cell arrival times to ensure that the correct mean cell rate is reached. The period of the ramp is
based on the “Peak Cell Rate” setting and is calculated by the instrument automatically.
Fig. TB-2 Principles of “Constant Bit Rate” 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 CBR, DBR and UBR 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
Jitter