Quantum and Evolution Series Installation and Operating Handbook
8-12
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.
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.
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.
