Wireless Advanced Settings · Topic 3 of 6
WMM Traffic Priority
Learn how WMM gives wireless traffic four priority classes, how RouterOS maps packet priority to them, and how to enable wmm-support and set priority.
- 4 min
- WMM has four access categories: voice, video, best effort and background.
- RouterOS packet priority runs from 0 to 7 and maps to the categories.
- Priorities 6 and 7 map to voice, 4 and 5 to video, 0 and 3 to best effort, and 1 and 2 to background.
What it is
WMM (Wi-Fi Multimedia) gives wireless traffic four priority classes, so voice and video can get through ahead of bulk data.
Wi-Fi is a shared medium, so every device waits for its turn. Without priorities, a large download and a voice call wait in the same queue.
WMM lets the radio treat the two differently. Higher classes wait less before they send, so they see lower delay.
How it works
A packet carries a priority, the radio maps it to an access category, and each category competes with different timing.
The four access categories
WMM defines four access categories. Each has its own queue and its own channel access timing.
| Access category | Typical traffic |
|---|---|
| Voice | Calls and other very delay sensitive traffic |
| Video | Streaming and conferencing |
| Best effort | Normal data |
| Background | Bulk transfers and backups |
The categories do not reserve bandwidth. They only change how often each queue wins when devices compete. A busy voice class can still delay others, and an overloaded air still gives poor service to everyone.
Priority to category mapping
A packet carries a priority from 0 to 7 inside RouterOS. The wireless interface maps that priority to an access category with the standard mapping:
| Priority | Access category |
|---|---|
| 1 and 2 | Background |
| 0 and 3 | Best effort |
| 4 and 5 | Video |
| 6 and 7 | Voice |
Priority 0 is the default for ordinary traffic and lands in best effort. To put traffic into another class, you must set its priority.
Enabling WMM
The setting is on the wireless interface:
/interface wireless set wlan1 wmm-support=enabled| Value | Meaning |
|---|---|
disabled | WMM is not used |
enabled | WMM is used with clients that support it |
required | Only clients that support WMM can connect |
Use required with care. A client without WMM support cannot join an interface that requires it.
WMM helps only when the clients also support it, so check your client devices before you choose required.
Setting priority on packets
The priority is set with the firewall in the mangle table. For example, to give a voice flow priority 6 as it leaves a routed wireless interface. If the interface is a bridge port, the rule must match the bridge instead:
/ip firewall mangle add chain=postrouting out-interface=wlan1 protocol=udp dst-port=5060 action=set-priority new-priority=6Other rules can set the priority from the DSCP value in the packet, or from the priority field of a VLAN tag. Put the rule on a chain that the packet passes before it leaves the wireless interface. Check the counters of the rule to confirm that traffic matches.
What WMM does not do
WMM does not limit rates and does not give a minimum bandwidth. For rate control you use queues, which are a separate topic.
WMM only changes who wins when the radio contends for the air.
It also acts only on the wireless hop. A priority set for the wireless interface has no effect on a wired link beyond it.
A simple test
- Enable
wmm-supporton the access point. - Add a mangle rule that sets a high priority for one traffic type.
- Check the rule counters to see matching packets.
- Load the link with bulk traffic and compare the delay of the priority flow before and after.
Keep the number of high priority flows small. If everything is voice class, nothing gets an advantage.
What to remember for the exam
Six facts cover WMM.
- WMM has four access categories: voice, video, best effort and background.
- RouterOS packet priority runs from 0 to 7 and maps to the categories.
- Priorities 6 and 7 map to voice, 4 and 5 to video, 0 and 3 to best effort, and 1 and 2 to background.
wmm-supportcan bedisabled,enabledorrequired.- Packet priority is set with a mangle rule using
action=set-priority. - WMM changes the waiting time on the air and does not reserve bandwidth.
Common mistakes
Four errors are typical.
Expecting reserved bandwidth. WMM does not guarantee a rate.
Forgetting to set priority. With no mangle rule, traffic stays in best effort.
Using required for mixed clients. Clients without WMM are locked out.
Marking everything high. When every flow is voice class, no flow stands out.
Check yourself
Work through these three questions.
Which access category do priorities 6 and 7 map to?
They map to voice, the class with the shortest waiting time. Priorities 4 and 5 map to video.
What does
wmm-support=requireddo?It allows only clients that support WMM to connect. Clients without WMM support are refused.
How do you give a traffic flow a higher WMM class?
Use a mangle rule with
action=set-priorityand a new priority such as 6. The wireless interface then maps that priority to the voice class.
Topic 3 of 6 in Wireless Advanced Settings
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