Comtech EF Data SDM-300A - Manual (Page 233)

SDM-300A Satellite Modem Revision 6
Buffering SDM300A.IOM
8–5
8.4 Plesiochronous
The stability of station reference clocks is normally 1 x 10 -12 (derived from a
cesium standard). While the stability is exceptionally high, the two clocks are not
in synchronization with each other and will eventually pass by each other.
The clock used for the transmit signal is passed over the satellite, but will not be
used at the receive earth station where a national network derives its time locally.
A buffer will fill up with data using the clock from the satellite and will empty
using the local clock. The object of the buffer is to ensure that the buffer
overflows or underflows at regular, determinable intervals (typically every 40
days).
The buffer depth required (from center to end) will be:
Minimum slip period (seconds) * [stability of far end (transmit) clock + stability of
local clock]
For example:
Far end (transmit) clock stability 1 x 10 -9
Local (buffer) clock 1 x 10 -11
Minimum clock slip 40 days
Buffer Depth = (40 x 24 x 60 x 60) x (1 x 10 -9 + 1 x 10 -11 ) = 3.49 ms
Because the buffer will either fill or empty (depending on the frequency
relationship of the two clocks), the total buffer depth will be 2 x 3.49 ms = 6.98
ms.