Comtech EF Data CDM-625A - Manual (Page 644)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix H Revision 3
H–2
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.
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.