Comtech EF Data CDM-500 - Manual (Page 58)

Rev. 1.3 8–1
8
Chapter 8.
EDMAC CHANNEL
8.1 Theory Of Operation
As explained earlier, EDMAC is an 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. 960 of these bits are the
user’s data, the remaining 48 bits are overhead. This increases the rate of transmission by
5%. (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 synchronised to the frame, the de-scrambler can
begin its processing at exactly the right time. This method does not multiply errors, and