Wireless association: active vs passive scanning, & roaming
What is Association?
In wireless networks, association means wireless devices register with an AP/router to get access to the network. Association happens only in infrastructure mode, not in Ad Hoc mode.
To establish this association, a wireless device uses scanning methods to find an AP/router.
ACTIVE SCANNING
A wireless device broadcasts a probe signal to each channel in its frequency range and waits for an AP to respond. So the client initiates the search method.
- Step 1 — The device continually sends the signal searching for an AP.
- Step 2 — Both the APs respond by sending their SSIDs and some other information to the client.
- Step 3 — The client might send a request for an association to AP2 which may have better signals.
- Step 4 — AP2 sends a confirmation message with approval for association. The device can get full access to the network via AP2.
PASSIVE SCANNING
In passive scanning, the users’ device sits idle and does not initiate the search method. It only listens to the channels in a frequency range for available APs.
An AP broadcasts a beacon frame continually within its service area. This special frame contains APs SSID and other information.
- Step 1 — Both the APs broadcast a beacon frame continually within their service area. This special frame contains APs SSID and other information. A wireless device listens on all channels for beacons.
- Step 2 — The device can choose the AP to which it can have association. Suppose it chooses AP1. It sends its request for association with AP1.
- Step 3 — AP1 responds with approval for association. The device now has full access to the network.
Re-association and roaming
What happens if the device moves from one AP to another within one extended service set (ESS)? The process of a device to keep a connection with a different AP is called re-association. This allows a device to roam from one AP to another, within the same ESS without any hassle.
