Wireless Medium Access and QoS

What it is

Wireless medium access is the set of rules that decides which device may send on a shared channel at any moment.

All the devices on a channel share the same air. If two send at once, their signals collide and both are lost.

Quality of service (QoS) adds a way to give urgent traffic, such as voice, a better chance to send first.

How it works

Wi-Fi uses listen before talk with random delays, and it adds priority classes for QoS.

Half duplex

A Wi-Fi channel is half duplex. A device cannot send and listen at the same time on that channel, so it cannot detect a collision while it sends.

For this reason Wi-Fi uses collision avoidance, not collision detection as old shared Ethernet did.

CSMA/CA

CSMA/CA stands for carrier sense multiple access with collision avoidance. The steps are:

1. The device listens to see whether the channel is free.
2. If it is busy, the device waits until it is free.
3. The device then waits for a random backoff time. This lowers the chance that two devices start together.
4. The device sends the frame.
5. The receiver sends an acknowledgement. If the sender does not get one, it assumes a collision, and it tries again with a longer backoff.

The basic method is called the distributed coordination function (DCF). Every device follows the same rules, and there is no central controller.

Airtime, not equal speed

Devices take turns in time. A slow client takes more time to send the same amount of data, so it uses more airtime. A few slow clients can reduce the speed for everyone on the channel.

This is why removing low data rates and keeping the clients close to the access point helps.

The hidden node problem

Two clients may both hear the access point but not hear each other. Each one thinks the channel is free and both transmit, and the frames collide at the access point.

RTS/CTS reduces this. A client sends a short request to send, and the access point answers with a clear to send that all clients hear. The other clients then stay quiet for the time that is reserved.

/interface wireless set wlan1 hw-protection-mode=rts-cts hw-protection-threshold=500

• hw-protection-mode selects the protection method on the legacy wireless package, such as rts-cts.
• hw-protection-threshold is the frame size above which the protection is used.

RTS/CTS adds overhead for every protected frame. Use it only where hidden nodes cause trouble, such as a long outdoor link with many clients.

Timing on long links

The sender waits a limited time for the acknowledgement. On a long link the signal takes longer to travel, so the wait must be longer.

On the legacy wireless package the distance and the acknowledgement timeout settings handle this. A value that is too low breaks the link, and a value that is too high lowers throughput.

QoS and WMM

Without QoS, all frames have the same chance to send. WMM (Wi-Fi Multimedia) defines four access categories with different priority.

A higher category waits less before it sends. It gets a shorter random wait, so it wins the channel more often. This method comes from the 802.11e amendment and is called EDCA.

WMM only gives a better chance. It does not reserve bandwidth.

Marking the traffic

WMM needs to know the category of each frame. The category comes from the priority that is marked on the traffic, such as the DSCP value in the IP header or the priority in a VLAN tag.

On the legacy wireless package, the wmm-support setting can be disabled, enabled or required. In RouterOS you can set the packet priority with a firewall mangle rule using the set-priority action, and then WMM maps it to a category.

/interface wireless set wlan1 wmm-support=enabled
/ip firewall mangle add chain=forward protocol=udp dst-port=5060 action=set-priority new-priority=6

Why QoS is not a fix for a poor link

QoS helps only when the channel is busy. If the signal is weak or the noise is high, no priority setting can make up for it.