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CDM-425 Advanced Satellite Modem
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At the distant-end, the EDMAC slave, configured for the correct address, receives these bytes,
and when a complete packet has been received, it will take the action requested, and then send
the appropriate response to the EDMAC Master, using the return overhead path on the satellite
link. The EDMAC Master assembles the complete packet, and transmits the response back to the
M&C User PC.
Apart from the round-trip satellite delay, the M&C User PC does not see any difference between
local and distant-end units – it sends out a packet, addressed to a particular unit, and gets back a
response. It can be seen that the EDMAC Master simply acts as forwarding service, in a manner
that is completely transparent.
This approach does not require any additional cabling; connection is made using the normal M&C
remote port. Furthermore, you do not have to worry about configuring the baud rate of the M&C
connection to match the lowest data rate modem in the system. The M&C system can have
mixed data-rate modems, from 18 kbps to 25 Mbps, and still run at speeds in excess of 19200
baud. It should be pointed out that, at 18 kbps, the effective throughput of the overhead channel
is only 56 asynchronous characters/second. For a message of 24 bytes, the time between
sending a poll request and receiving a response will be around 1 second. (Note that when
EDMAC-2 or either of the BPSK Turbo rates is in use, the overhead rate is reduced by a factor of
three, and therefore the response time will be around 3 seconds.)
Appendix H
H–3
MN-CDM-425
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