Q-Lite Satellite Modem Installation and Operating Handbook
7-26
(3-bit field) to prioritise packets. The modem uses the tags to decide which packet to
transmit over satellite first when it has more than one packet to send.
Two modes are supported, namely Strict Priority Queuing and Fair Weighting Queuing:
•
Strict priority queuing: packets are queued for transmission based solely on
their priority – highest will always be sent first.
•
Fair-weighting queuing: higher priority transmitted first but lower priority
packets are given a percentage of the bandwidth to stop total starvation.
The 8 QoS priority levels are mapped to three TCP/IP queues. Packets with highest QoS
priority (level 7) are sent to high priority TCP/IP queue. Delay-sensitive packets (QoS
levels 6 and 5) are sent to the medium priority queue. The remainder (QoS levels 4 to 0)
are sent to the low priority TCP/IP queue. For Strict priority queuing, all packets in high
priority queue are processed before any in medium priority queue which in turn are
processed before any in the low priority queue. For Fair-weighting queuing, for every 4
packets sent from high priority queue, 2 are sent from medium queue and 1 from low
priority queue.
]
The example shown in
Figure 7-13
is a traffic shaping scheme based on priority tagging.