Q-Flex Satellite Modem Installation and Operating Handbook
7-27
7.8.9.4.2 Diffserv DSCP Class
It is possible to classify data streams based on the Differentiated Services Code Point
(DSCP) value in the IP packet header. The DSCP class bits are the top three bits of the DS
field in an IP header (the other three bits of the field indicate drop precedence, which can
be mimicked using the Priority setting for the stream in the modem). Each packet passed to
the modem must have this field set to the appropriate value in order for the modem to
recognize the different data streams. The modem maps the eight possible DSCP classes
directly to eight equivalent internal modem classes (each of which can be allocated its own
CIR, BIR, etc.). DSCP Class 0 maps to internal modem QoS Class 0, etc., down to DSCP
Class 7 which maps to QoS Class 7. Class 7 has the highest priority.
An example traffic shaping scheme based on DSCP classification is shown in
Figure 7-12
.
Figure 7-12 Traffic Shaping Configuration Screen using DSCP
7.8.9.4.3 IEEE 802.1p Priority Tag
Classification may be done on the 3-bit Priority Code Point field of an IEEE 802.1q VLAN
tag (also referred to as an IEEE 802.1p Priority Tag). This is part of a 32-bit field inserted
into an Ethernet frame between the MAC address and length field.
The priority tag has eight possible values, each of which maps directly to an equivalent
internal class within the modem (for which a BIR, CIR, etc. can be set). Each packet
passed to the modem must have this field set to the appropriate value in order for the
modem to recognize the different data streams.
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Note that the modem includes support for IEEE 802.1p (priority tagging) as an alternative
QoS scheme to traffic shaping in its own right. In this case the packets must already be
tagged at the point of entry to the modem. It is a layer 2 feature that uses 8 classes of data
