Paradise P300 specifications CAS Multiframe Summary

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`

1

2

3

4

Any Drop/Insert mode

without signalling

TS 32

TS 32

TS 32

TS 32

G.732 Drop/Insert

with CAS

CAS = 8

CAS = 8

CAS = 8

CAS = 16

T1-D4

RBS mode

TS 32

TS 32

CAS = 24

TS 32

T1-ESF

RBS Mode

TS 32

TS 32

CAS = 24

CAS = 8

P300H P300 Series Modem Installation and Operating Handbook Page 211