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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.
H.1.2
Drop & Insert ++
A new variation of EDMAC is available with D&I++ framing. With this, each frame contains 2944
bits, with 64 overhead bits and 2880 user data bits. The portion of the overhead used for the
EDMAC link performs identically to that of the EDMAC frame, but because D&I++ uses a smaller
overhead, the two modes are not compatible with each other.
H.1.3
EDMAC-3
EDMAC-3 is another variation of EDMAC, which uses the same rate exchange as the original
EDMAC frame:
•
21/20 (5%) at 2048 kbps and below
•
61/60 (1.6%) above 2048 kbps
With EDMAC-3, however, the EDMAC channel operates at 1/3 the rate of original EDMAC
because most of the overhead is dedicated to carrying the remote modem's complete status
information (including AUPC), so that it is available to the near-end modem at nearly real-time
speed. EDMAC-3 is best suited for SNMP proxy applications.
H.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 User PC, 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.
Appendix H
CDM-425 Advanced Satellite Modem
H–2
Revision 0
MN-CDM-425
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