RF Environment, Roaming and Interference

What it is

The RF environment is everything that changes a radio signal between the access point and the client.

Walls, furniture, people and other radios all affect the signal. Some weaken it and some add noise.

This page also covers roaming, which is how a client moves between access points, because interference and roaming problems often look alike.

How it works

Signals bend, bounce and fade, and clients decide when to move.

How a signal behaves

A radio wave meets objects and reacts in several ways.

Multipath happens when a signal reaches the receiver along several routes. The copies arrive at slightly different times and can cancel each other, which causes fading.

Materials matter. Metal reflects, water absorbs, and thick concrete weakens the signal a great deal. Glass and thin wood weaken it less.

Interference and noise

Noise is unwanted energy on the channel. The noise floor is the level of that background energy.

Co channel interference lowers throughput but the devices still take turns. Adjacent channel interference is usually worse, because the signals are corrupted rather than shared. This is why 2.4 GHz networks use channels 1, 6 and 11.

Client roaming

Roaming is a client moving its association from one access point to another. The client decides when to roam, not the network.

For roaming to work, the access points should use the same SSID and the same security settings. The cells should overlap a little, so the client hears the next access point before it loses the first.

A sticky client holds on to a distant access point although a nearer one is available. The result is a weak signal and low rates.

Helping clients roam

Several tools improve roaming.

• A signal limit on the access list stops a client from staying on an access point with a weak signal. The access list page covers this.
• 802.11r speeds up the reauthentication step when a client moves, and the wifi driver security profile carries the related ft setting.
• 802.11k lets the access point tell a client about nearby access points.
• 802.11v lets the network suggest that a client moves to a better access point. The wifi driver has steering settings for these two.

The client must support the standard for the feature to help, and some clients ignore suggestions.

Station roaming (legacy wireless package, RouterOS v6)

On the legacy wireless package a station interface has the station-roaming setting. It is a RouterOS feature of the client side, so it works without any change on the access points.

/interface wireless set wlan1 mode=station station-roaming=enabled

With it enabled, the station keeps scanning in the background while connected. If it finds an access point with the same SSID and a better signal, it moves to that access point.

• It helps a mobile station, such as a vehicle unit, that passes through several cells.
• It applies only to station modes and to the 802.11 protocol.
• The access points must share the SSID and the security settings.
• The background scan briefly takes the radio off its channel, so traffic can stutter.

Checking the environment

You can see noise and neighbours from the router.

/interface wireless scan wlan1
/interface wireless monitor wlan1
/interface wireless registration-table print

• scan lists nearby networks and their channels and signal.
• monitor shows the current frequency and noise floor.
• registration-table print shows connected clients and their signal.