COBHAM Sailor 250 FleetBroadband, Training Manual, Page: 31 / 270

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SAILOR FleetBroadBand
Training Manual
1
In the return direction the BGAN air interface operates in TDMA
(Time Division Multiple Access). A range of bearer types are
supported in the return direction. FleetBroadband Terminals
transmit bearers in bursts of either 5 msec. or 20 msec. in duration.
There are four possible symbol rates, two modulation schemes
and a variable coding scheme that is used to improve the data rate
in favourable link conditions.
1.7.4 Return direction coding, multiplexing and
synchronisation
Figure 1-32: Return bearer
Each FleetBroadband Terminal return burst is sent at a coding rate
appropriate to the FleetBroadband Terminals currently reported
link conditions (the link adaptation mechanism is described later
in this document) FleetBroadband Terminals transmit traffi c bursts
when instructed to by a schedule of timeslots broadcast by the RNS
(note that there is no return direction traffi c in the global beam so,
no return schedule is broadcast in the global beam). The schedule
tells the FleetBroadband Terminals which return direction timeslots
are reserved for each connection and what bearer type should
be used for each. The return schedule advertises a proportion of
timeslots available for all FleetBroadband Terminals to use i.e.
contention capacity for random access. BGAN network uses a
slotted ALOHA mechanism for random access.
1.7.5 Forward direction layout
On each forward bearer the RNC plans the data and signalling to
be sent in advance, once every layout period. The layout period is
typically 40 msec. or 80 msec.
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
variable bit rate packets selected by the scheduling algorithm is
slotted around the other traffi c and signalling.