Understanding Traffic Prioritization

The access point automatically prioritizes all data traffic that it forwards. The access point uses the WMM indicator, the 802.1p priority tag, and DiffServ code point (DSCP) to prioritize traffic. The access point also supports the SpectraLink Voice Priority (SVP) traffic classification.

WMM voice frames are not subject to 802.11n frame aggregation. This allows for low latency of each voice frame in a WMM voice traffic stream. Depending on the traffic on the network, this could give the appearance that aggregated WMM traffic—such as video—is being given higher priority than voice because the aggregated throughput of video traffic could be higher than the voice traffic. However, each unaggregated voice data frame is actually being assigned to a higher priority WMM queue.

The following sections define the rules for prioritizing frames.

• Frames Received on Wireless Medium
• Frames Received on Wired Medium
• DiffServ Marking Effects on Frame Priority

Frames Received on Wireless Medium

For packets received on the wireless medium the access point checks whether the frame contains WMM markings in the header. If the markings are present, the access point maps the WMM user priority (802.11e) to 802.1p priority as shown in Table 5.

Note that 802.1p priority 0 is given higher priority by the network than 802.1p priority 1 and 2.

If the incoming frame does not contain WMM markings, the access point checks whether the frame contains SpectraLink Voice Priority (SVP) protocol packets. Only IPv4 frames are checked for the SVP protocol packets. Any frame using the SVP protocol is assigned 802.1p priority 6.

If the frame does not contain SVP protocol packets, the access point examines the DSCP field. Only IPv4 frames are classified using the DSCP field. Table 6 maps DSCP values to the assigned 802.1p priorities.

Frames that do not fall into any of the categories in Table 6 are assigned 802.1p priority 0.

The access point has only one queue on the Ethernet port for packets to be transmitted to the wired side. This is based on the assumption that the Ethernet medium can handle more traffic than the access point can receive from the wireless side. Thus, an 802.11n access point must be connected to a Gigabit Ethernet port.

If the access point transmits the frame back into the wireless medium, then it uses the priority to select the appropriate egress queue in the same way as for traffic received from the wired network. The access point supports eight queues per radio for the packets to be transmitted to the wireless side.

Frames Received on Wired Medium

The access point can receive tagged and untagged frames on the wired medium. The access point prioritizes ingress traffic on the Ethernet port based on the 802.1p tag and the DSCP value. The Ethernet port prioritization is always enabled, even when WMM is disabled.

The frames destined to wireless clients are not tagged. The priority is only used for internal processing. If the frame uses the SVP protocol, then the 802.1p priority is set to 6. For other IPv4 frames the priority is assigned as shown in Table 7.

Untagged IPv4 frames use Table 7 to assign 802.1p priority. Untagged non-IPv4 frames are prefixed with a tag containing 802.1p priority equal to 0. Tagged frames that are not using the SVP protocol use the priority in the tag to direct the packet to the correct wireless egress queue.

The next step is to map the 802.1p priority to the appropriate egress queue. The mapping works differently depending on whether WMM is enabled on the access point. If WMM is not enabled, then non-SVP traffic is mapped to the same hardware egress queue. SVP traffic is mapped to a high priority hardware queue, which is configured using the following egress attributes:

• Arbitration interframe space (AIFS)=1
• Minimum contention window (cwMin)=0
• Maximum contention window (cwMax)=0
• Number of milliseconds of the longest burst (maxBurst)=1.5

When WMM is disabled, the transmitted 802.11 frames do not contain WMM markers.

When WMM is enabled, the SVP traffic is queued to the “voice” hardware queue. The other traffic is mapped to hardware queues based on the 802.1p priorities as shown in Table 8.

When WMM is supported by both the client and the access point, the frames contain WMM markers. If only the access point supports WMM then the frame is sent using the appropriate queue, but does not contain the WMM markers.

DiffServ Marking Effects on Frame Priority

The access point supports DiffServ as part of its client QoS feature, which allows for a frame to be marked as part of a configurable policy attribute action. A marking action can alter the 802.1p priority or the IP DSCP/precedence field of a frame to any of the values defined in the preceding tables.

Whenever DiffServ marks the 802.1p field of a frame, it also sets the internal frame priority to the same value. Whenever DiffServ marks the IP DSCP or IP Precedence field of a frame, it sets the internal frame priority to the same value as indicated in the 802.1p column in Table 7, although it does not actually change the contents of the 802.1p field in the frame itself. Note that an IP Precedence marking is interpreted according to its compatibility selector (CSx) code point value for purposes of referencing the DSCP mapping table. The updated internal frame priority is then used in the usual manner to determine the WMM queue mapping for frames traveling in the wired-to-wireless direction, or for frames received from the wireless medium that are forwarded to another wireless station within the same Basic Service Set (BSS).

Related Topics

• Junos OS CLI Reference. See the quality-of-service statement summary in the wlan Hierarchy and Statements.
• Junos OS J-Web Help. In the J-Web user interface, select Configure>Wireless LAN>Settings and click Help.