Comtech EF Data SDM-650B - Manual (Page 89)

SDM-650B Satellite Modem Configuration
Rev. 5 3–47
3.3.1.3 Plesiochronous
The stability of station reference clocks is normally 10 -11 that is derived from a cesium
standard. When the stability is exceptionally high, the two clocks are not in sync with
each other. Eventually, the two reference clocks will pass by each other.
The clock used for the transmit signal passes over the satellite, but will not be used at the
receive earth station where a national network derives its timing locally. A buffer will fill
up with data using the clock from the satellite, and will empty using the local clock. The
buffer ensures that the buffer overflows or underflows at regular determinable intervals.
The intervals typically occur every 40 days.
The buffer depth required, from center to end, will be calculated as follows:
Buffer Depth = Minimum Slip Period (sec) x Stability of Far End (Transmit) Clock +
Stability of Local Clock
The following is an example on how to calculate a buffer depth:
Far end (transmit) clock stability 1 E-9
Local (buffer) clock stability 1E-11
Minimum clock slip 40 days
Buffer Depth = (40 x 24 x 60 x 60) x (1E-9 + 1E-11) = 3.49 ms
The buffer will either be filled or empty, depending upon the frequency relationship of
the two clocks. The total buffer depth will be 2 x 3.49 ms = 6.98 ms.
3.3.1.4 Frame/Multiframe Length
The depth of the receive buffer required has been discussed above and is applicable to all
unframed data.
Framed data (i.e., 2048 kbit/s G732 or 1544 kbit/s G733) is desirable to provide slips in
pre-defined locations. There are two advantages of organized slip locations:
• In relation to the frame, multiplexing equipment does not lose sync.
• Outages on any channel are kept to a minimum.
A 2048 kbit/s frame structure commonly used is G732. The frame structure has a length
of 256 bits, with 16 frames per multiframe (4096 bits total or 2 ms).