Q-Flex Satellite Modem Installation and Operating Handbook
7-24
7.8.9.3 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.
7.8.9.4 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 as well as the 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.
•
The Differentiated Services Code Point (DSCP) value in the IP packet header.
•
The 3-bit Priority Code Point field of an IEEE 802.1q VLAN tag (also referred to as
an IEEE 802.1p Priority Tag).
•
The 3-bit EXP field in an MPLS header.
•
The VLAN ID in a IEEE 802.1q VLAN tag.
•
The PID value in an MPEG2 transport stream packet.
The key classifiers are 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.
