Comtech EF Data CDM-625A - Manual (Page 645)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix H Revision 3
H–3
Now, a unit that has been designated an EDMAC master not only responds to its own unique
bus address, but it will also be configured to listen for the address that corresponds to its
EDMAC Slave. When a complete message packet has been received by the EDMAC Master, it will
begin to transmit this packet over the satellite channel, using the overhead bytes that become
available.
The ‘normal’ protocol for the message packet is not used over the satellite path, as
it is subject to errors. For this reason, a much more robust protocol is used which
incorporates extensive error checking.
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.)