Quantum and Evolution Series Installation and Operating Handbook
8-10
8.2 Framing and Drop/Insert Overview
8.2.1
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. Where a reference is
made to IBS framing 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 that 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 thereby increases the error rate by a factor of 3. The
IBS/SMS synchronous scrambler does not multiply errors and therefore gives a factor
of three BER performance increase over the V.35 scrambler.
∋
A backward alarm facility that raises a local deferred alarm if the distant end modem
cannot receive data transmitted from this station (i.e. there may be a Tx problem at this
end of the link).
∋
A
low rate
oversampled asynchronous ESC channel (1/2000th the data rate) that is
available on the Aux port of the modem. This results in asynchronous rates of less than
110 Baud at greater than 256kbps.
∋
A TS32 multiframe sequence to allow the transmission of satellite encryption control
vectors and three 8-bit carrier IDs (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 maintains the identity of individual data streams when
multiplexed together into one aggregate rate carrier (e.g. two independent 64kbps
circuits in one 128kbps carrier).
Additional Features (Paradise products)
In addition to the basic Intelsat definition above, the modem provides 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 modem when fitted with the IDR option also provides a clock so the
port may be used as a synchronous Aux channel. 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
