Comtech EF Data CDM-550T - Manual (Page 101)

Chapter 11. EDMAC CHANNEL
11.1 Theory of Operation
As explained previously, EDMAC is the acronym for Embedded Distant-end Monitor and
Control. This is a feature which permits the user to access the M&C features of modems which
are at the distant-end of a satellite link.
This is accomplished by adding extra information to the user’s data, but in a manner which is
completely transparent to the user.
On the transmit side:
The data is split into frames - each frame containing 1008 bits (except Rate 21/44 BPSK Turbo,
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 the user’s data. This
increases the rate of transmission by 5% (approximately 1.6% for the Turbo BPSK cases).
For example, if the user’s data rate is 64 kbps, the actual transmission rate will now be at 67.2
kbps.
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
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, 3 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 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 seen by the user will degrade by a factor of 10log(1.05), or 0.21 dB
(0.07dB in the case of the two BPSK Turbo rates). The use of an externally synchronized
11–1