COBHAM Sailor 250 FleetBroadband, Training Manual, Page: 33 / 270

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SAILOR FleetBroadBand
Training Manual
1
Subsequent FEC blocks may have different code rates for more
capable FleetBroadband Terminals. The layout algorithm tries
to minimise wasted space in the frame by grouping connections
for FleetBroadband Terminals with similar link conditions into the
same FEC blocks.
1.7.6 Return direction layout
Like the forward direction, the RNC prepares the return layout
at regular intervals, typically every 40 msec. or 80 msec. The
goal of the layout algorithm is to organise time slots for the
FleetBroadband Terminal bursts to squeeze the maximum
capacity out of each satellite channel over the period of the layout
interval. Slots for three burst types for the connections that the
scheduling algorithm has chosen to serve during the interval.
Each reserved slot is assigned to a specific connection belonging
to a single FleetBroadband Terminal. FleetBroadband Terminals
are permitted to send small amounts of signalling (such as status
messages) to the RNC along with return bursts to reduce the load
on the contention slots. Circuit switched connections (i.e. ISDN
and voice in the narrow beams) are allocated return direction
slots in a rigid layout every 20 msec. or 40 msec. respectively in
order to minimise delay jitter. Contention slots can be used by any
FleetBroadband Terminal for sending status messages or other
signalling. The quantity and grouping of contention slots in each
layout period is calculated by the RNC on the basis of the number of
FleetBroadband Terminals listening to the corresponding forward
bearer, their expected signalling requirements and the proportion
that require un-timed access. Slots reserved for packet traffic are
laid out around the circuit switched calls and contention capacity.
When the return direction layout is complete, it is broadcast to
all the FleetBroadband Terminals listening to the corresponding
forward bearer. A forward bearer may broadcast schedules for
more than one return channel to allow asymmetric traffic to be
carried more efficiently.