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P300H P300 Series Modem Installation and Operating Handbook Page 211
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.
The CAS Multiframe structure is of variable length (in multiples of 8 frames), and this feature lends itself
to providing timeslot ID Maintenance for `Odd value of N`, ie values which the fixed length TS32 multiframe
cannot accommodate. This is an efficient use of the overhead as when not sending signalling information
15 out of every 16 TS16/TS48 locations are still available for high rate ESC, even when the CAS
multiframe is required to support `Odd values of N` or Robbed Bit Signalling.
CAS MULTIFRAME SUMMARY
TS Identity Maintenance Method & Length of CAS Multiframe (in frames)
`N` Any Drop/Insert mode G.732 Drop/Insert T1-D4 T1-ESF
without signalling with CAS RBS mode RBS Mode
1 TS 32 CAS = 8 TS 32 TS 32
2 TS 32 CAS = 8 TS 32 TS 32
3 TS 32 CAS = 8 CAS = 24 CAS = 24
4 TS 32 CAS = 16 TS 32 CAS = 8