ND SatCom SKYWAN IDU 7000, Manual

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2010-10-26 Network Design and Engineering Guide 35
General Carrier Design
Essential SkyWAN
®
Satellite Link Layer Features
To minimize jitter for real time applications the allocation of stream slots for a station will be
done as presented in figure 2-18. Once the capacity is allocated the position of the assigned
stream slots in the frame will be maintained.
Another feature of streaming capacity allocation is that the capacity will be assigned in a semi-
permanent way: As long as the real-time service (e.g. voice call, video session) is still up, the
master will assign the stream slots continuously to the station until the station signals that serv-
ice is terminated. This ensures the service quality of running real-time services irrespective of
the congestion state of the network. If the stream slots on a carrier are already allocated to sta-
tions, new stream slot requests will be rejected by the master. This causes a blocking of the
real-time service!
Not every data slot in a frame may be assigned as stream slot. Generally the amount of avail-
able stream slots per channel may be restricted by configuration. This makes sense if one
wants to avoid the situation that high priority real time traffic is occupying 100% of the available
slots on a carrier, thus disrupting non real time application for an indefinite time.
The system limitation for the number of stream slots is equal to the number of data slots minus
one (because one time slot is needed for potential key exchange for link encryption) on carriers
without request bursts. For carriers with request bursts additionally the slots needed for the
ranging subframe may not be allocated as streaming slots.
Dynamic Slots
Dynamic Slots are triggered from data stored in any other transmit queue type (Control or Non
Real Time):
-
Usage: a station needs resources to transmit data which are not time critical.
- If specified, stations may get access to free resources without even asking for it: the IDU
’Free Slot Assignment’ feature, see description above.
- Examples: File Transfer, Web Browsing.
In contrast to streaming slots, dynamic slots may be allocated in arbitrary positions of the
frame. If there are more requests for dynamic slots than available, the master allocates the
slots via a fair sharing procedure. In any case requests for streaming slots up to the configured
maximum will be served with higher priority than dynamic slots.
Figure 2-18 Slot Assignment Differences