M&C Connection - Comtech EF Data CDM-570A Installation And Operation Manual

The use of an externally synchronized scrambler and descrambler almost exactly compensates
for this degradation. The net effect is that you will see effectively identical BER performance,
whether or not framing is used.
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. Additionally, the start of frame permits the de-scrambler to correctly recover the data: your
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.
J.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 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 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
that is completely transparent.
This approach does not require any additional cabling; connection is made using the normal M&C
remote port, which is completely independent from the modem data port. Consequently, 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. It should be pointed out that, at 2.4 kbps, the effective throughput
of the overhead channel is only 14 asynchronous characters/second. For a message of 24 bytes,
the time between sending a poll request and receiving a response will be around 1.7 seconds.
(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 minimum response time will be around 5 seconds.)
EDMAC Channel
CDM-570A/570AL Satellite Modem with Optional Packet Processor
J–2
Revision 5
MN-CDM570A