Quality of Service with WMM-How Voice

Quality of Service with WMM-How Voice and Data Are Kept Separate

The first challenge is to address the unique nature of voice. Unlike data, which is usually carried over protocols such as TCP that are good at making sure they take the available bandwidth and nothing more, ensuring a continuous stream of data no matter what the network conditions, voice is picky. One packet every 20 milliseconds. No more, no less. The packets cannot be late, or the call becomes unusable as the callers are forced to wait for maddening periods before they hear the other side of their conversation come through. The packets cannot arrive unpredictably, or else the buffers on the phones overrun and the call becomes choppy and impossible to hear. And, of course, every lost packet is lost time and lost sounds or words.

On Ethernet, as we have seen, the notion of 802.1p or Diffserv can be used to give prioritization for voice traffic over data. When the routers or switches are congested, the voice packets get to move through priority queues, ahead of the data traffic, thus ensuring that their resources do not get starved, while still allowing the TCP-based data traffic to continue, albeit at a possibly lesser rate.

A similar principle applies to Wi-Fi. The Wi-Fi Multimedia (WMM) specification lays out a method for Wi-Fi networks to also prioritize traffic according to four common classes of service, each known as an access category (AC):

• AC_VO: highest-priority voice traffic
• AC_VI: medium-priority video traffic
• AC_BE: standard-priority data traffic, also known as "best effort"
• AC_BK: background traffic, that may be disposed of when the network is congested

The underscore between the AC and the two-letter abbreviation is a part of the correct designation, unfortunately. You may note that the term "best effort" applies to only one of the four categories. Please keep in mind that all four access categories of Wi-Fi are really best effort, but that the higher-priority categories get a better effort than the lower ones. We'll discuss the consequences of this shortly.

The access category for each packet is specified using either 802.1p tagging, when available and supported by the access point, or by the use of Diffserv Code Points (DSCP), which are carried in the IP header of each packet. DSCP is the more common protocol, because the per-packet tags do not require any complexity on the wired network, and are able to survive multiple router hops with ease. In other words, DSCP tags survive crossing through every network equipment that is not aware of DSCP tags, whereas 802.1p requires 802.1p-aware links throughout the network, all carried over 802.1Q VLAN links.

There are eight DSCP tags, which map to the four access categories. The application that generates the traffic is responsible for filling in the DSCP tag. The standard mapping is given in Table 1.

Table 1: DSCP tags and AC mappings 

DSCP	TOS Field Value	Priority	Traffic Type	AC
0×38 (56)	0×E0 (224)	7	Voice	AC_VO
0×30 (48)	0×C0 (192)	6	Voice	AC_VO
0×28 (40)	0×A0 (160)	5	Video	AC_VI
0×20 (32)	0×80 (128)	4	Video	AC_VI
0×18 (24)	0×60 (96)	3	Best Effort	AC_BE
0×10 (16)	0×40 (64)	2	Background	AC_BK
0×08 (8)	0×20 (32)	1	Background	AC_BK
0×00 (0)	0×00 (0)	0	Best Effort	AC_BE

There are a few things to note here. First is that the eight "priorities"—again, the correct term, unfortunately—map to only four truly different classes. There is no difference in quality of service between Priority 7 and Priority 6 traffic. This was done to simplify the design of Wi-Fi, in which it was felt that four classes are enough. The next thing to note is that the many packet capture analyzers will still show the one-byte DSCP field in the IP header as the older TOS interpretation. Therefore, the values in the TOS column will be meaningless in the old TOS interpretation, but you can look for those specific values and map them back to the necessary ACs. Even the DSCP field itself has a lot of possibilities; nonetheless, you should count on only the previous eight values as having any meaning for Wi-Fi, unless the documentation in your equipment explicitly states otherwise. Finally, note that the default value of 0 maps to best effort data, as does the Priority 3 (DSCP 0×18) value. This strange inversion, where background traffic, with an actual lower over-the-air priority, has a higher Priority code value than the default best effort traffic, can cause some confusion when used; thankfully, most applications do not use Priority 3 and its use is not recommended here as well.

A word of warning about DSCP and WMM. The DSCP codes listed in Table 1 are neither Expedited Forwarding or Assured Forwarding codes, but rather use the backward-compatibility requirement in DSCP for TOS precedence. TOS precedence uses the top three bits of the DSCP to represent the priorities in Table 6.1, and assign other meanings to the lower bits. If a device is using the one-byte DSCP field as a TOS field, WMM devices may or may not ignore the lower bits, and so can sometimes give no quality-of-service for tagged packets. Further complicating the situation are endpoints that generate Expedited Forwarding DSCP tags (with code value of 46). Expedited Forwarding is the tag that devices use when they want to provide higher quality of service in general, and thus will usually mark all quality-of-service packets as EF, and all best effort packets with DSCP of 0. The EF code of 46 maps, however, to the Priority value of 5—a video, not voice, category. Thus, WMM devices may map all packets tagged with Expedited Forwarding as video. A wireless protocol analyzer shows exactly what the mapping is for by looking at the value of the TID/Access Category field in the WMM header. 

This mapping can be configured on some devices. However, changing these values from the defaults can cause problems with the more advanced pieces of WMM, such as WMM Power Save and WMM Admission Control, so it is not recommended to make those changes. (The specific problem that would happen is that the mobile device is required to know what priority the other side of the call will be sending to it, and if the network changes it in between, then the protocols will get confused and not put the downstream traffic into the right buckets.)

Once the Wi-Fi device—the access point or the client—has the packet and knows its tag, it will assign the packet into one of four priority queues, based on the access categories. However, these queues are not like their wired Ethernet brethren. That is because it is not enough that voice be prioritized over data within the device; voice must also be prioritized over the air.

To achieve this, WMM changes the backoff procedure. Instead of each device waiting a random time less than some interval fixed in the standard, each device's access category gets to contend for the air individually. Furthermore, to get the over-the-air prioritization, higher quality-of-service access categories, such as voice, get more aggressive access parameters.

Each access category get four parameters that each determine how much priority the traffic in that category gets over the air, compared to the other categories. The first parameter is a unique per-packet minimum wait time called the Arbitration Interframe Spacing (AIFS). This parameter is the minimum amount of time that a packet in this category must wait before it can even start to back off. The longer the AIFS, the more a packet must wait, and the more it is likely that a higher-priority packet will have finished its backoff cycle and started transmitting. The key about the AIFS is that it is counted after every time the medium is busy. That means that a packet with a very high AIFS could wait a very long time, because the amount of time spent waiting for an AIFS does not count if the medium becomes busy in the meantime. The AIFS is measured in units of the number of slots, and thus is also called the AIFSn (AIFS number).

The second value is the minimum backoff CW, called the CWmin. This sets the minimum number of slots that the backoff counter for this particular AC must start with. As with pre-WMM Wi-Fi, the CW is not the exact number of slots that the client must wait, but the maximum number of slots that the packet must wait: the packet waits a random number of slots less than this value. The difference is that there is a different CW min for each access category. The CWmin is still measured in slots, but communicated to the client from the access point as the exponent of the power of two that it must equal. This exponent is called the ECWmin. Thus, if the ECWmin for video is 3, then the AC must pick a random number between 0 and 2³ − 1 = 7 slots. The CWmin is just as powerful as the AIFS in distinguishing traffic, by making access more aggressive by capping the number of slots the AC must wait to send its traffic.

The third parameter is similar to the minimum backoff CW, and is called the CWmax, or the maximum backoff CW. If you recall, the CW is required to double every time thesender fails to get an acknowledgement for a frame. However, that doubling is capped by the CWmax. This parameter is far mess powerful for controlling how much priority one AC gets over the other. As with the CWmin, there is a different CWmax for each AC.

The last parameter is how many microseconds the AC can burst out packets, before it has to yield the channel. This is known as the Transmit Opportunity Limit (TXOP Limit), and is measured in units of 32 microseconds (although user interfaces may show the microsecond equivalent). This notion of TXOPs is new with WMM, and is designed to allow for this bursting. For voice, bursting is usually not necessary or useful, because voice packets come on regular, well-spaced intervals, and rarely come back-to-back in properly functioning networks.

The access point has the ability to set these four AC parameters for every device in the network, by broadcasting the parameters to all of the clients. Every client, thus, has to share the same parameters. The access point may also have a different set for itself. Some access points set these values by themselves to optimize network access; others expose them to the user, who can manually override the defaults. The method that WMM uses to set these values to the clients is through the WMM Parameter Set information element, a structure that is present in every beacon, and can be seen clearly with a wireless packet capture system. Table 2 has the defaults for the WMM parameters.

Table 2: Common default values for the WMM parameters for 802.11 

AC	Client		Access Point		CWmin	TXOP limit
	AIFS	CWmax	AIFS	CWmax		802.11b	802.11agn
Background (BK)	7	210− 1 = 1023	7	210 − 1 = 1023	24− 1 = 15	0μs	0μs
Best Effort (BE)	3	210− 1 = 102	3	26− 1 = 63	24− 1 = 15	0μs	0μs
Video (VI)	2	24− 1 = 15	1	24− 1 = 15	23− 1 = 7	6016μs	3008μs
Voice (VO)	2	23− 1 = 7	1	23− 1 = 7	22− 1 = 3	3264μs	1504μs