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P300H
P300 Modem Installation and Operating Handbook
Page 199
Channel Associated Signalling
The IBS specification includes an option for equipment to support CAS, and defines a mechanism whereby
the CAS (the 2kbps signalling channel per timeslot) can be carried within the 6.7% IBS satellite overhead.
This option is fully implemented within the P300 equipped with the Extended D/I feature (standard on P300
TCM), and is further explained in the following section on "CAS Multiframe".
Robbed Bit Signalling (RBS)
Historically PCM bearers using Robbed Bit Signalling could not be used with Drop/Insert, as over standard
IBS links the location of the signalling information (ie which timeslots have a bit overwritten with signalling)
was lost. Even with Timeslot ID Maintenance (which preserved timeslot sequence and timeslot start / end
boundaries) RBS was still lost. To preserve RBS not only is TS ID maintenance required, but also the
timeslots required placing back into the correct frames within the terrestrial multiframe (so in T1-D4 for
example the timeslots containing the RBS are placed in frames 6 & 12 of the terrestrial multiframe).
INTELSATs IBS definition does not specify a mechanism to achieve this, so the P300 equipped with the
Extended D/I feature (standard on P300 TCM) uses an extension of the Timeslot ID Maintenance scheme
to provide RBS support with Drop/Insert over `thin route` satellite links. It does this by treating not just the
timeslots from one frame as a group of `N` timeslots, but the group from the whole terrestrial multiframe
as a block within which timeslot identity has to be maintained. In effect if two timeslots were dropped from
T1-D4 and T1-ESF bearers with RBS support enabled, although the aggregate data rate would be only
128kbps plus 6.7% overhead, the frame and multiframe structure would be as to support N=24 and N=48
respectively (N x 12 for D4, and N x 24 for ESF as D4/ESF have 12/24 frames per multiframe, relative to
which RBS is placed). As with the support for `Odd values of N`, the CAS Multiframe is again used to
provide support for these extended timeslot sets, and hence support the Robbed Bit Signalling.
CAS Multiframe
INTELSAT have defined a satellite CAS Multiframe to allow Channel Associated Signalling to be transferred
across the satellite link between E1 PCM bearers within the IBS satellite overhead. The P300 equipped
with the Extended D/I feature (standard on P300 TCM) uses the CAS Multiframe definition to support CAS,
and also to provide Timeslot ID Maintenance for `Odd values of N` (N = 7, 9, 11, 13, etc), which in turn
allows the Modem to also support Robbed Bit Signalling (RBS). The CAS multiframe is entirely
independent of the TS32 Multiframe which has already been described.
The CAS Multiframe consists of data & frame alignment words in TS16 and TS48 of the IBS frame. A CAS
multiframe starts off as a unique word of [00001y11] in TS16, and comprises a further 15 signalling data
timeslots in TS48 & TS16 stretching over 8 frames. When carrying CAS, the 15 signalling data timeslots
carry pairs of the `abcd` signalling nibbles from the terrestrial TS16. When not carrying CAS but where the
CAS Multiframe format is being used for solely for timeslot ID maintenance for `Odd values of N`, these 15
signalling data timeslots are available for the high rate ESC.
Similar to the `fitting` of the main data into the 60 timeslots available for data per IBS frame, the `abcd`
signalling information has to be `fitted` into the 30 available nibbles in the 15 signalling data timeslots in
TS16 / TS48 following the CAS MF unique word in TS16. There are certain values of `N` for which this will
work, ie where 30/N gives an integer. These include N = 1, 2, 3, 5, 6, 10, 15, 30. In these cases then the
CAS multiframe is 8 frames in duration with the first frame containing CAS-MF unique word in TS16 and
pairs of `abcd` signalling nibbles in TS48, and the following seven frames contain pairs of `abcd` signalling
nibbles in TS16 and TS48.
For values of N which are not supported by an 8 frame CAS multiframe (with room for 30 nibbles of
signalling), the multiframe is simply extended in length by multiples of 8 frames until it reaches a value
where it can accommodate an integer multiple of the signalling information in one multiframe. Although the
multiframe is extended in multiples of 8 frames giving a further 30 locations to carry signalling each 8
frames, the CAS-MF unique word is not repeated but is instead replaced by a [00000000] pack byte in
TS16 of every eighth frame after the original CAS-MF unique word. This keeps the same ratio of 30
signalling nibbles every 8 frames, no matter how long the CAS multiframe actually is.