Comtech EF Data CDM-500 Especificaciones descargar pdf (Pagina 58)

Rev. 1.3 8–1

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

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