COBHAM Sailor 250 FleetBroadband, Training Manual, Page: 29 / 270

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SAILOR FleetBroadBand
Training Manual
1
On each forward bearer the RNS plans the data and signalling to
be sent in advance, once every layout period. The layout period is
typically 40 ms or 80 ms.
System signalling and circuit switched connections require the
most rigid timing. Circuit switched calls are placed in the same
FEC blocks in consecutive frames so their position is fi xed before
deciding where to place other traffi c. The guaranteed bit rate
(streaming class) IP traffi c will be laid out next (the bit rate is
constant but averaged over a longer period so packets may not
appear in the same FEC block in consecutive frames). Lastly the
Standard Data packets, selected by the scheduling algorithm, is
slotted around the other traffi c and signalling.
1.7.2 Forward direction coding and multiplexing
A FleetBroadband Terminal tunes to a forward bearer and decodes
every FEC block on the bearer unless the code rate of the block is
too high. A FleetBroadband Terminal is permitted to save power
by receiving intermittently only when idle in a regional beam. The
RNS will never send data to a FleetBroadband Terminal in a
block with a code rate the FleetBroadband Terminal could not be
expected to receive given its link conditions.
A checksum is appended to every data unit. FleetBroadband
Terminals check all incoming data units against the checksum. If a
data unit fails the checksum the FleetBroadband Terminal discards
it and all that follow in the same FEC block. Only ISDN connections
are permitted to process data that failed the checksum because
discarding chunks of ISDN data would cause an unacceptably
high bit error rate for this type of service.
Figure 1-28: Payload
All data units are either addressed to a unique connection (i.e. a
circuit switched call or packet switched connection for a specifi c or
are broadcast to all FleetBroadband Terminals (e.g. bulletin board
signalling) or are ‘common signalling’ (e.g. paging messages).
The most common forward frame format in a narrow beam is
F80T4.5X8B. In this bearer type the 80 ms frame starts with a
unique word followed by eight individually turbo-coded blocks
(FEC blocks). Interspersed throughout the frame at equal intervals
(not shown) are pilot.