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P300H P300 Series Modem Installation and Operating Handbook Page 207
APPENDIX F: FRAMING AND DROP/INSERT OVERVIEW
IBS/SMS FRAMING
IBS/SMS Service Features
The INTELSAT definition of IBS framing (IESS 309) is intentionally very similar to Eutelsats SMS framing
(EESS 501 "SMS QPSK/FDMA System Specification"), as they are both designed to allow common
equipment to provide for both services. From here on we will refer to IBS framing, but the text applies
equally to SMS framing
Standard Features (specified in IBS/SMS definitions)
The addition of standard IBS framing increases the data rate by exactly 1/15th (approx 6.7%). IBS framing
is added to provide the following extra features compared to `closed network` links:
A Synchronous Scrambler, which replaces the self synchronising (V.35) scrambler used in most
closed network links. The closed network V.35 scrambler outputs 3 errors for every error received,
and so increases the error rate by a factor of 3. The IBS/SMS synchronous scrambler does not
multiply errors, and so give a factor of three BER performance increase over the V.35 scrambler.
A Backward Alarm facility, which raises a local deferred alarm if the distant end modem cannot
receive data transmitted from this station (ie a possible Tx problem this end).
A Low Rate Oversampled async ESC Channel (1/2000th the data rate), this is available on the
Aux port
of the modem. This results in async rates of <50 Baud at 64kbps and 110 Baud at
>256kbps.
A TS32 Multiframe Sequence to allow the transmission of satellite encryption control vectors, and
three 8 bit carrier ID’s (referred to as Station, Channel, and Spare ID).
Overhead channels to convey Channel Associated Signalling (CAS) corresponding to an extra
2kbps per 64kbps transmitted by use of a separate CAS Multiframe sequence (in TS16/48).
Timeslot ID Maintenance: which is the facility to maintain the identity of individual data streams
when multiplexed together into one aggregate rate carrier (eg two independent 64kbps circuits in
one 128kbps carrier).
Additional Features (Paradise products)
In addition to the basic INTELSAT definition above, the P300 Modems provide the following additional
features in IBS/SMS mode:
A high rate ESC channel carried in spare parts of the overhead. This allows up to 68% (22/32nd)
of the overhead to be used for ESC providing a 2400 Baud ESC with a 64kbps carrier, and
proportionally higher ESC rates with higher data rate carriers.
In addition to the INTELSAT low rate oversampled async ESC Channel (on the shared ESC/Aux
port), The P300 when fitted with the IDR option (standard on P300-IDR and above) also provides
a clock so the port may be used as a synchronous Aux channel. In addition the overhead usage
can be varied from the one bit per frame (TS32 bit 1) defined by INTELSAT, up to 21 bits per
frame, providing a synchronous Aux channel at up to 4.3% of the main data rate. This may be 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 & odd numbered frames with TS0 having different
functions in even & odd frames. Thankfully the IBS specification parted from the G.732 definition at this
point and instead of having even & 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,