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P300H P300 Series Modem Installation and Operating Handbook Page 152
8.9 V.35 SCRAMBLERS
In all cases, setting the P300 Modem scramblers to Normal will implement the correct scrambling to match
the configuration you have the modem in (ie Closed network, IBS, or IDR, with or without Reed-Solomon).
Only read this section if you are having trouble interworking two manufacturers equipment which you think
is scrambler related (or just for interest !).
Note that there are no known incompatibilities between any manufacturers synchronous IBS or
synchronous Reed-Solomon scrambling. The issues relate only to V.35 scrambling, used typically on
closed network or IDR links without Reed-Solomon.
History
Note: This is the authors understanding from 15 years working in the industry.
There was only intended to be one V.35 scrambler, but the CCITT who made the original specification were
the first of many organisations to get it wrong. To ensure there was no ambiguity they printed a written
description together with an example schematic. However the schematic they supplied behaved differently
from the written description (well done boys), and there immediately grew two opposing camps, those who
had worked to the schematic and those who had worked to the written description !
The difference between the two implementations was only in the action to take when the `adverse state`
is detected, ie what to do if a long string of zeroes has been output. In such a case a counter injected an
opposing bit to generate a transition (and break the cycle of all zeroes) with a counter in the corresponding
descrambler cancelling out this injected opposing bit. The difference between the two implementations is
in the count length of this adverse state detector counter, one is 30 counts and the other 31.
The net result of inter-operating equipment from the two differing V.35 camps (schematic / written
description) was a background pattern dependent error rate in the order of 1 x 10 . Note that as the
-8
background error rate is pattern dependent, there is no guarantee you will see it with regular test equipment
such as Firebirds etc which generate repeated fixed patterns. In most satellite communications such a
background error rate might be masked by the channel noise, but in some carrier line up procedures a
circuit may be required to operate error free for 12/24 hours. For higher data rate circuits this proved
impossible to achieve with the differing scramblers, and so tests were performed `on the bench`.
INTELSAT were called in to adjudicate the argument, and as both camps were technically correct someone
was going to be upset. INTELSAT decided in favour of the larger camp, with a Japanese company being
the only member in the opposing camp. This was unfortunate as it happened that the losers were the only
company to have the V.35 scrambler buried deep within as ASIC at the time (and therefore a nightmare
to change), whereas every other manufacturer had it implemented in hardware, and could have made cuts
/ straps to resolve the issue !
The definition finally approved by INTELSAT became known as "INTELSAT V.35 scrambling", with the
other definition derived from the original CCITT spec as "CCITT V.35 scrambling". INTELSAT of course
approve only INTELSAT V.35 scrambling !
Other Derivatives
As mentioned earlier, the CCITT were only the first of many companies to mess up the V.35 scrambling.
At least two other companies also implemented a V.35 scrambler which was accidentally different to the
CCITT definition (ie didn't match the schematic or the written description). These companies now offer
corrected INTELSAT V.35 implementations, but in order to maintain backwards compatibility they support
their original V.35 attempts as well. To maximise compatibility with other manufactures equipment, in
addition to both INTELSAT and CCITT, Paradise modems implement two of these other V.35 schemes,
these modes are referred to as "FDC" and "Linkabit".