P300H
P300 Series Modem Installation and Operating Handbook
Page 95
The operator enters the end to end buffer size in milliseconds (INTELSAT documents typically specify
the buffer in terms of its end to end size). Should the capacity ever be exceeded and the buffer slip, then
it will slip by exactly half its capacity, back to the mid position.
For rates of 1544 kbps and 2048 kbps where the data is framed, the buffer is usually made to slip integer
multiples of the frame length (3ms and 2ms respectively). A user option sets if the buffer capacity should
automatically be incremented to the nearest integer multiple of frame length at these rates. This option is
available under
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.
A second option is available to automatically force the buffer to its mid position whenever the receive
traffic switches from failed to OK. This takes advantage of any outage, and ensures the buffer is centred
every time normal operation begins. This option is available under
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. This option can be disabled, as for minor outages the errors or sync
losses caused by centering the buffer may be larger than those caused by the outage.
In normal operation the buffer fill state can be viewed on the Received Detail Traffic Status screen
accessible from the operate menu by pressing Down Arrow. This screen also shows a count of
underflows and overflows (empty slips and full sips). Approximately equal counts indicate the buffer is
too small to accommodate the incoming Doppler shift (2ms is usually sufficient). One count significantly
higher than the other indicates that the buffer is slipping due to a large difference in the clock from the
satellite and the buffer output clock. With
typical
earth station clock accuracies, buffers never need be
larger than approximately 16ms.
The buffer can be manually centred by selecting
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.
