Q-Lite Satellite Modem Installation and Operating Handbook
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level of bandwidth that is guaranteed under all normal circumstances where the equipment
is operating correctly.
The sum of all CIRs for all classified data streams cannot be more that the transmission
data rate of the modem.
7.8.10.2 Maximum Bandwidth
If excess bandwidth becomes available at any point (i.e. one or more streams do not
require their allocated bandwidth), or some of the overall bandwidth has not been allocated
to any particular stream, then it is not wasted and it can be allocated in a controlled manner
between potentially competing streams.
This setting is called the
Burst Information Rate
(BIR)
specified in bits per second (bps)
(or as a percentage). It defines the maximum amount of bandwidth, beyond the guaranteed
bandwidth, that a stream should be allocated, should spare bandwidth become available.
Each BIR must be less than or equal to the transmission data rate of the modem.
7.8.10.3 Priority
What happens when excess bandwidth does become available (i.e. all guaranteed
bandwidths are being met and there is spare capacity) in the situation where several
streams have BIRs set (meaning that they are all potentially competing for the same
excess bandwidth)? This is determined by the stream
Priority
setting.
In this case, the allocation of the spare bandwidth between competing schemes will be
done based on the priority level allocated to each stream. This is done on an absolute
basis: if 256kbps of bandwidth is spare and two streams both want an additional 256kbps
then all 256kbps will be allocated to the stream with the higher priority.
The priority setting also controls latency and jitter. In the situation where the transmit
modem has several packets in different priority queues for transmission over satellite, then
the packets will be sent based on their priority, with the packets from the highest priority
queue being sent first.
The priority value ranges from zero to seven, with zero being the highest priority (note that
this is the opposite order of prioritization to IEEE 802.1p priority tagging where seven is the
highest priority).
A default data stream exists for any packets not explicitly part of a defined data stream.
These get assigned the lowest available priority, namely, seven.
7.8.10.4 Stream Classification
How does the modem know which packets belong to which streams? Streams can be
classified using one of the following methods:
•
The source and/or the destination address in the IP packet along with the source
and/or the destination port number of the TCP or UDP header in the packet. Any or
all of these can be used in any combination at the same time, including using a
range of port numbers.
