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P300H P300 Series Modem Installation and Operating Handbook Page 208
TS16, TS32 (same as G.732 TS0 odd), and TS48 (same as G.732 TS16 odd), which leaves 60 timeslots
available for normal data.
An IBS Frame is constructed as follows:
TS0 consists of a spare bit (X), followed by a 7 bit frame alignment word ie [X0011011].
TS16 & TS48 are defined to convey Channel Associated Signalling (CAS, when required), and are
normally forced to all ones [11111111] when CAS is not being transmitted. CAS is covered
separately later.
TS32 conveys the Low rate ESC (E), the Backward Alarm (B), and the TS32 Multiframe sequence (M).
If IBS satellite link encryption is in use (very, very rare) then it also conveys the encryption control
vectors otherwise these bits are spare (X as shown). One bit is also forced to a logic 1 for frame
alignment, resulting in a content of [E1BMXXXX]
A standard IBS frame then is 64 x 8 (ie 512) bits long.
TS32 Multiframe
A 64 bit multiframe pattern is placed one bit at a time into the `M` bit of TS32.When the 64 bits have been
sent (one bit at a time over 64 frames) the sequence restarts indicating the end of one multiframe and the
beginning of the next. The 64 bit TS32 multiframe sequence also contains 3 eight bit user fields nominated
names of Station ID, Channel ID & Spare ID. The TS32 multiframe then is 64 x 512 (ie 32768) bits long.
Synchronous IBS Scrambler
The synchronous IBS scrambler is restarted at the beginning of every multiframe. Note this is not every
frame, as it requires a reasonable length of the scrambling pattern to be used to ensure the final data
appears random. The scrambling pattern itself is 2 -1 (32767) bits long and runs throughout the frame.
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Scrambling is however disabled for TS0 & TS32 (otherwise the distant end could never find the frame sync,
then TS32 multiframe sync, in order to find out where the scrambler starts !).
Backward Alarm
The backward alarm is a single bit of TS32 which is connected to a summary `Rx Fail` signal from the
demodulator / deframer. If at either end of the link the Rx path fails for any reason, then this backward
alarm bit of TS32 is raised on the outgoing Tx signal, indicating to the distant end that there is a Rx failure
this end. This could be caused by any manner of Rx fault (LNA, downconverter, demod) at this end, but it
is a warning that the fault might equally exist at the Tx end (modulator, upconverter, HPA etc). The
backward alarm is normally a `deferred alarm`, meaning it should be checked but it is not necessarily a
failure of the traffic path at this end.
Low Rate ESC Channel
The low rate ESC channel is a very low rate. One bit of TS32 is allocated to carry the ESC information, and
so the rate is one bit per frame (ie 1/64x8), one 512th the framed data rate or 1/480th the unframed rate.
For example at 64kbps the ESC channel is a synchronous rate of 133bps. However, INTELSAT did not
define a synchronous interface to the channel, only an oversampled asynchronous interface (ie a data line
without a clock line). This means that the Baud rate of the async channel must be one third to one quarter
of the synchronous rate to ensure each Baud period of the async word is sampled 3 or 4 times (necessary
otherwise the reconstructed async character cannot be read at the far end). This is a very poor use of the
bandwidth, but the definition was made to allow the facility to be added to then current equipment with
minimal hardware implications. The INTELSAT defined limit is 1/2000th of the data rate, but in practice this
can be pushed to about 1/1500th of the data rate (not quite enough to get a reliable 50 Baud with a 64kbps
carrier ! ).
Timeslot ID Maintenance
Timeslot identity maintenance is the facility to maintain the identity of individual data streams when
multiplexed together into one aggregate rate carrier (eg in its simplest form two independent 64kbps circuits
in one 128kbps carrier). Typically such individual data streams comprise of 64kbps data channels from `N`
timeslots of terrestrial PCM bearers, interfaced to the IBS/SMS framing via the Drop/Insert multiplexer.