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Appendix H. EDMAC CHANNEL
H.1
Overview
H.1.1
EDMAC
The Embedded Distant-end Monitor and Control (EDMAC) feature permits user access to the
M&C features of modems that are at the distant-end of a satellite link. This is accomplished by
adding extra information to your data, but in a manner that is otherwise transparent.
On the transmit side
The data is split into frames – each frame containing 1008 bits (except Rate 21/44 BPSK Turbo,
or when the data rates exceed 2048 kbps, where the frame length is 2928 bits, and Rate 5/16
BPSK Turbo where the frame length is 3072 bits). 48 bits in each frame are overhead, and the
rest of these bits are user data. This increases the rate of transmission by 5% (approximately
1.5% for the Turbo BPSK cases, and for all data rates greater than 2.048 Mbps). For example, if
the user data rate is 64 kbps, the actual transmission rate will now be at 67.2 kbps. Note that you
may also select EDMAC-2 framing, which always uses a 2928 bit frame, and yields a 1.6%
overhead for all modulation types and data rates.
At the start of each frame, a 12-bit synchronization word is added. This allows the demodulator to
find and lock to the start of frame. At regular intervals throughout the frame, additional data bytes
and flag bits are added (a further 36 bits in total). It is these additional bytes that convey the M&C
data.
When framing is used, the normal V.35 scrambler is no longer used. This V.35 approach is called
'self synchronizing' because no external information is required in the receiver in order for the de-
scrambling process to recover the original data.
The disadvantage of this method is that it multiplies errors. On average, if one bit error is present
at the input of the descrambler, three output errors are generated. However, there is an
alternative when the data is in a framed format: in this case, a different class of scrambler may be
used – one that uses the start of frame information to start the scrambling process at an exact
known state.
In the receiver, having synchronized to the frame, the de-scrambler can begin its processing at
exactly the right time. This method does not multiply errors, and therefore has a clear 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).
Appendix H
CDM-425 Advanced Satellite Modem
H–1
Revision 0
MN-CDM-425
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