Comtech EF Data CDM-570 - Manual (Page 201)

11–1
Chapter 11. EDMAC CHANNEL
11.1 Theory of Operation
Embedded Distant-end Monitor And Control (EDMAC) is a feature that permits 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 user data in a manner that is completely 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 1.6% for the Turbo BPSK cases, and for all data rates greater than 2.048
Mbps). For example, if your 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 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 which 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, in the receiver, no external information is required in order for the
descrambling 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 which 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 descrambler 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 observable E
b
/N
0
will degrade by a factor of 10
log
(1.05),
or 0.21 dB (0.07 dB in the case of the two BPSK Turbo rates). The use of an externally
synchronized scrambler and descrambler almost exactly compensates for this degradation. The