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QMultiFlex-400™ Installation and Operating Handbook
6.2.22.16
Satellite Buffer Size
Range:
1 to 256 packets (Default 256)
Description:
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 or Traffic
Shaping 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.
6.2.22.17
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 performance at the
application level suffers due to excessive buffering of packets during periods of congestion,
leading to extremely high latency levels with old data being kept almost indefinitely in the
hope that extra bandwidth will become available and any overload will ease.
Buffer management in TCP/IP devices is typically passive and relies on the user setting
internal buffer sizes to be consistent with the needs of the application and its data rates.
However, TCP/IP can be bursty by nature and if the rate of arrival of packets at the satellite
modem exceeds its transmission capabilities then packets start to back up and will
eventually get dropped if the overload continues. The problem with this is that the end-to-
end packet delay can vary greatly, and data becomes increasingly stale as the backlog of
packets to be transmitted builds up.
Table 6-45 Satellite Buffer Size
6-58
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