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Q-Lite Satellite Modem Installation and Operating Handbook
6-64
6.2.4.1 Terrestrial Buffer Size
The terrestrial buffer is used to buffer IP packets coming into the IP terrestrial ports for
transmission over satellite. Satellite delay and the quality of the service in general can be
controlled by the size of this buffer in conjunction with setting the size of the satellite
buffer. The buffer should be set large enough to accommodate bursts of packets being
received by the modem. Setting the buffer larger than necessary could result in large
packet delays building up should more packets be sent to the modem than can be
transmitted.
The optimal size for the buffer depends on the link data rate, the packet sizes, the
number of packets and the specific application (some applications being able to tolerate
packet loss and/or delays more than others). When the buffer is full then received packets
will be dropped until space in the buffer is freed up.
It is generally desirable for the terrestrial buffer to be set so that packets are not dropped
unnecessarily before they have been assessed by the traffic shaper as to priority, etc. At
the same time, a large buffer could result in stale data being kept, which it might be better
to drop by making the buffer smaller so that only the most recent data is kept in an
overload situation.
6.2.4.2 Satellite Buffer Size
The satellite buffer is used to buffer IP packets ready for transmission over satellite. The
buffer is situated after all internal packet processing has been completed, including traffic
shaping and encapsulation. Satellite delay and the quality of the service in general can be
controlled by the size of this buffer. The buffer should be set large enough to even out
peaks and troughs in throughput that would result from setting an extremely small buffer.
Setting the buffer larger than necessary could result in large packet delays building up
should more packets be available than can be transmitted. The traffic shaper can be used
to ensure that the combined output from all classes of traffic does not exceed the
available satellite bandwidth, even when ACM (which dynamically adjusts the data rate) is
active.
The optimal size for the buffer depends on the link data rate, the packet sizes, the
number of packets and the specific application (some applications being able to tolerate
packet loss and/or delays more than others). When the buffer is full then new packets for
transmission will be dropped until space in the buffer is freed up.
However, as a general
rule, it is recommended that the satellite buffer size is set to 8 for most
applications, particularly when TCP acceleration is being used, unless an
alternative buffer size is found to give better performance. Setting the buffer too
large can cause throughput to oscillate when TCP acceleration is being used.
6.2.4.3 Active Queue Management
Although the terrestrial and satellite buffers can be tuned to match the specific needs of a
particular user application, passive buffer managed has inherent limitations and
drawbacks. Active Queue Management (AQM) is an intelligent and pro-active form of
TCP/IP queue management that overcomes the potential for inconsistent end-to-end
packet delays and the problems of 'buffer bloat'. Buffer bloat is where packet buffers in
the system are over-sized in order to try to prevent packet loss. The result is often that