Quantum and Evolution Series Installation and Operating Handbook
8-11
used for example with external or internal BER testers to provide a continuous
measurement of the live BER.
IBS/SMS
Definition
Frame Format
IBS framing is a derivative of the framing used on 2048kbps PCM bearers within Europe
specified in ITU-G.732. G.732 specifies a frame of 32 timeslots (each timeslot is 8 bits of
data) and the timeslots are labelled TS0 - TS31. TS0 and TS16 are reserved for special
functions, leaving the remaining 30 timeslots available for data, ie 30 data timeslots with 2
overhead timeslots, a ratio of 32/30 or an increase of 1/15th. G.732 uses an awkward
definition of having even and odd numbered frames with TS0 having different functions in
even and odd frames. Thankfully the IBS specification parted from the G.732 definition at
this point and instead of having even and odd frames of 32 timeslots, it defined one longer
frame equivalent to an even then an odd G.732 frame. An IBS frame therefore has 64
timeslots, with special functions for TS0, 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 and 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
15
-1 (32767) bits
long and runs throughout the frame. 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
