In a Wi-Fi context, mobility means the ability of a client device to roam, or switch, from one access point (AP) to another while still maintaining an active network connection. When roaming is effective, client devices can move from one area to another and roam from AP to AP without disrupting applications that require a persistent network connection.
A Wi-Fi client device will roam from one AP to another when its current connection becomes suboptimal. As the client moves away from the AP to which it is connected, the strength of the signal decreases and RF interference in the area generally increases. Due to the decreased signal strength and increased interference, some transmitted data packets are not received, forcing the sender to retry the transmission. To maintain the Wi-Fi connection, the client and AP negotiate a lower data rate. If the client continues to move away from the AP, then it eventually reaches the edge of coverage for that AP, where the Wi-Fi connection can be maintained only at the lowest data rate supported by that AP. Beyond the AP edge of coverage, the client is out of range, and the connection with the AP is lost. Therefore, to prevent loss of coverage, the client will switch to an AP with stronger signal, thus preventing a drop in connectivity.
While the physical movement of a client is the most common trigger for roaming, it is also possible for a stationary device to roam. Suppose that a stationary device is within range of two APs and connected to one of them. If a patient, doctor, nurse or hospital visitor positions his or her body between the device and the AP it is connected to, their body may block enough of the Wi-Fi signal that the client is forced to roam to the second AP to avoid losing its connection.
The process of roaming is more complex that just switching from one AP to another. There are four steps involved in the process:
- Evaluate: Determine if the connection with the current AP is less than optimal.
- Scan: If the connection is less than optimal, then scan the vicinity for other APs.
- Select: Determine if any AP within range is likely to provide a better connection than the current AP.
- Roam or Repeat: Roam from the current AP to the best candidate in the vicinity, or determine when to repeat the process beginning at step 1 again.
The most common way to evaluate the current connection is to compare the strength of the RF signal from the current AP to a configured threshold value. If the signal strength is weaker than the threshold, then the client must look for an AP that offers a better connection.