Comtech EF Data CDM-550T - Manual (Page 102)

CDM-550T Satellite Modem Revision 3
EDMAC Channel MN/CDM550T.IOM
scrambler and descrambler almost exactly compensates for this degradation. The net effect is that
the user will see effectively identical BER performance whether framing is used or not.
On the receive side:
When the demodulator locks to the incoming carrier, it must go through the additional step of
searching for, and locking to the synchronization word. This uniquely identifies the start of frame,
and permits the extraction of the overhead bytes and flag bits at the correct position within the
frame. In addition, the start of frame permits the de-scrambler to correctly recover the data. The
user’s data is extracted, and sent through additional processing, in the normal manner. The
extracted overhead bytes are examined to determine if they contain valid M&C bytes.
11.2 M&C Connection
Data to be transmitted to the distant-end is sent to a local unit via the remote control port. A
message for the distant-end is indistinguishable from a ‘local’ message - it has the same structure
and content, only the address will identify it as being for a distant-end unit.
Before the M&C data can be successfully transmitted and received, pairs of units must be split
into EDMAC Masters and EDMAC Slaves. Masters are local to the M&C Computer, and Slaves
are distant-end.
Now, a unit which 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 which 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 which
become available.
Note: 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 Computer.
Apart from the round-trip satellite delay, the M&C Computer 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 which is completely transparent.
This approach does not require any additional cabling - connection is made using the normal
M&C remote port. Furthermore, the user does 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 2.4 kbps to 2048 kbps, and still run at speeds in excess
of 19,200 baud. It should be pointed out that at 2.4 kbps, the effective throughput of the
overhead channel is only 11 async characters/second. For a message of 24 bytes, the time
11–2