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SDM-300L3 Satellite Modem
Clocking Options
Slave Site:
1. Set Configuration/Interface/Loop Timing to ON.
2. Set Configuration/Interface/TX Clock Source to SCT (LOOP).
Or TX Terrestrial, Only if the user equipment can provide the proper slaved clock to the
modem.
3. Set Configuration/Interface/Buffer Clock to RX Satellite (Buffer Bypass).
4. The modem can be placed in loop timing under EIA-422 or V.35 operation. Therefore, a
master/slave relationship easily occurs. This causes the RX clock to be placed on the ST
line to the terrestrial equipment. The customer equipment can then clock the TX data to
the modem using the ST line that has been derived from the RX recovered clock.
6.7
Frame/Multiframe Length
The depth of the receive buffer required is applicable to all unframed data.
When the data is framed (such as 2048 kbps G732 or 1544 kbps G733), it is desirable to
provide slips in predefined locations. The advantage of organized slip locations (in
relation to the frame) is that multiplexing equipment does not lose sync and outages on
any channel are kept to a minimum.
A 2048 kbps frame structure commonly used is G732. This has a frame length of 256 bits
with 16 frames per multiframe (4096 bits total, or 2 ms).
6.7.1
Multiples of the Frame Length
If this setting is set to NONE, the user can choose any buffer depth.
6.7.2
Total Buffer Length
T1 and E1 framing structure under G.704 is available. When this is selected, the buffer
length is restricted to the size of the buffer. Using the examples from the three previous
sections, the total buffer depth (end to end) will be:
Doppler + Plesiochronous (rounded up to the nearest multiframe)
1.15 ms + 6.98 ms = 8.13 ms
If the frame length is 2 ms, then the nearest multiframe will be 10 ms, or 20,480 bits.
6–17
Revision 1
MN/SDM300L3.IOM
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