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Quantum and Evolution Series Installation and Operating Handbook
8.12.9.2 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).
It defines the maximum amount of bandwidth, beyond the guaranteed bandwidth, that a
stream should be allocated, should spare bandwidth become available. Each BIR should
not be greater than the transmission data rate of the modem.
8.12.9.3 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 its input buffer waiting for transmission over satellite, then
the packets will be sent based on their priority, with the highest priority being sent first.
Packets are buffered up to a limit, after which packets may be dropped.
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).
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.
8.12.9.4 Stream Classification
How does the modem know which packets belong to which streams? Streams can be
classified using one of the four methods described in the following sections. These can be
selected using the Quality of service scheme dropdown control on the web user interface.
The result is that each incoming packet is assigned to one of a number of QoS Classes.
Data will be classified as belonging to the first class in the list for which a match is found
starting from the top. If no match is found then the packet is assigned a default class that
corresponds to a priority level of 0. The default data stream gets a BIR value of the
maximum transmit data rate of the modem.
8.12.9.4.1 IP Address
It is possible to classify a data stream based on either the source and/or the destination
address in the IP packet as well as the port number of the TCP or UDP header in the
packet.
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