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P300H P300 Series Modem Installation and Operating Handbook Page 92
Buffer size (end-end): [16]ms
(0-99ms) ??ms (YES)
Change, Rx, Buffer/Clocking, Buffer Size Menu
6.7.6 Change, Rx, Buffer / Clocking, BUFFER SIZE Menu
This menu is only available of the Rx Clk is not set to Satellite (ie Buffer Disable) !
Refer also to the following applications sections for helpful notes:
8.1 "Doppler & Plesiochronous Buffering" on page 140
8.2 "Determining Clocking Schemes & Buffer Size" on page 141
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 CHANGE, USER-OPT, OPERATION, TRANSFER, BUFFER MF SLIP.
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 CHANGE, USER-OPT, OPERATION, ACTIONS,
BUFFER AUTO CENTRE. 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 ACTIONS, BUFFER CENTRE.