Comtech EF Data CDM-570A - Manual (Page 556)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix J Revision 2
J–2
advantage over V.35 scrambling. This is fortunate, as there is a penalty to be paid for adding the
framing: by adding the extra 5% to the transmitted data rate, the effective Eb/No that is seen
will degrade by a factor of 10log(1.05), or 0.21 dB (0.07dB in the case of the two BPSK Turbo
rates or EDMAC-2).
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