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Chapter 19: Quality Of Service (QoS)
802.1p And DSCP Tags
information. The effect of this is to increase network speed and data throughput. The size of
packet bursts allowed (maximum burst length) is a configurable parameter.
19.1.3.5 Transmission Opportunity (TXOP) Interval For Client Stations
The Transmission Opportunity (TXOP) is an interval of time when a Wi-Fi Multimedia
(WMM) client station has the right to initiate transmissions onto the wireless medium
(WM).
19.1.4 802.1p And DSCP Tags
IEEE 802.1p is an extension of the IEEE 802 standard and is responsible for QoS provision.
The primary purpose of 802.1p is to prioritize network traffic at the data link/MAC layer.
802.1p offers the ability to filter multicast traffic to ensure it doesn't increase over layer 2
switched networks. It uses tag frames for the prioritization scheme. To be compliant with
this standard, layer 2 switches must be capable of grouping incoming LAN packets into sep-
arate traffic classes.
The 802.1p header includes a three-bit field for prioritization, which allows packets to be
grouped into various traffic classes. Eight priority levels are defined. The highest priority is
seven, which might go to network-critical traffic (voice). Higher priority packets are always
transmitted first. Lower priority packets are not transmitted if higher priority packets are still
in transmission, rather they are held in a queue until the higher packets have been success-
fully transmitted.The lowest priority level is zero, this is used as a best-effort default, it is
invoked automatically when no other value has been set.
Note: It is important to note that 802.1p will not work unless QoS and WMM are enabled.
WMM must be enabled on both the AP and on the client connecting to the AP.
The flow diagram in Figure 19.2 outlines the way in which tags are retrieved and traffic pri-
oritized on a network.
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Psion Teklogix 9160 G2 Wireless Gateway User Manual
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