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P300H P300 Series Modem Installation and Operating Handbook Page 209
Operating over satellite without TS ID maintenance is comparable to using an external Drop/Insert mux
interfaced in our 2 x 64kbps example to the modem at 128kbps. Once demultiplexed to 128kbps the start
and end boundaries of each timeslot are lost in the continuous 128kbps data stream. Without the TS ID
maintenance there would be 16 different ways (2 x 8 bits per TS) that the two data streams could be
inserted back into the terrestrial PCM bearer timeslots at the receive end. One correct way, one with
channels reversed (8 bits shifted), and 14 ways (shifts of 1-7, 8-15 bits) in which parts of each channel
appeared in the channel. Without TS ID maintenance then, Drop/Insert can only useful if all the data
dropped / inserted comprises one single data stream of N x 64kbps from a single source.
Note: Typically D/I is tested with a PRBS from a Firebird in all relevant timeslots, this is a guaranteed way
to NOT TEST TS ID Maintenance, as it is not required for such a test (because all the data comprises one
single data stream from a single source, the PRBS generator). Instead to verify TS ID maintenance insert
the PRBS into any subset of the dropped timeslots (eg just one). If the Timeslot ID is not maintained, you
are likely to get something else back in the equivalent receive timeslot instead of the PRBS (probably the
idle code from one of the other dropped timeslots). If the Timeslot ID is maintained, you should
consistently get the PRBS back in this same subset or single timeslot. Bear in mind that without TS ID
maintenance, when dropping N timeslots, statistically it will still work one time in 8xN, when the random
timeslot alignment happens to be correct !
Implementation of Timeslot ID Maintenance
The requirement is to provide:
1) Timeslot boundary maintenance (start/end of each timeslots 8 bits) within the data stream.
2) Timeslot identity (ID) maintenance, ie which 8 bits relate to which timeslot (first, second, third etc) within
the data stream.
The data from each timeslot appears as a set of 8 bits from each terrestrial frame. Timeslot boundary
maintenance is provided by placing the data into the IBS frame, timeslot aligned after TS0. When TS0 is
located, the next 8 bits represent 8 bits from one terrestrial timeslot, the following 8 from the next etc.
Timeslot Identity Maintenance again relies on relative position to TS0. Data from the first timeslot of the
group of timeslots is always placed in the first timeslot after TS0, the second of the group in the second TS,
and so forth. Once the 8 bits of data from every timeslot have been placed in the frame, the process
repeats until the frame is full. For the following frame to be similarly formatted the timeslot data must fit into
the 60 available data timeslots of the satellite frame exactly (so that the next data is from the first TS again).
This brings in a complication.....
Each IBS frame has 60 timeslots available for data, and so in order to provide timeslot ID maintenance, the
number of timeslots `N` must fit into 60. This limits the values of N to:
1 Frame (ie 60 available data timeslots): N = 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30
This is less than ideal, and so a few further values are accommodated by mapping them into two frames:
2 Frames (ie 120 available data timeslots): N = 8, 24
Finally, to support one further rate, four frame are used:
4 Frames (ie 240 available data timeslots): N = 16
The remaining values of N cannot have Timeslot ID Maintained by groups of 1, 2, or 4 frames, these are
collectively know as the `Odd values of N`:
`Odd values of N`: N = 7, 9, 11, 13, 14, 17, 18, 19, 21, 22, 23, 25, 26, 27, 28, 29, 31
Most previous equipment could not provide timeslot ID maintenance for these `odd values of N`, however
the P300 when equipped with the Extended D/I feature uses a derivative of the IBS CAS Multiframe (see
later section "CAS Multiframe") to provide timeslot ID maintenance for even these remaining values of N.