This post is a continuation of another thread about roaming of Samsung mobile devices. I recommend first reading the content of that previous post.
Samsung – Mobile Devices – Roaming
At this stage, we already know quite a bit about devices from the Korean manufacturer. It’s time to focus on the star from the USA.
Factors Affecting Roaming
The most important factor that causes a device to want to connect to another access point is the received signal strength. When this value decreases to -70 dBm, scanning and searching for another candidate (access point) to which the device can connect begins.
By default, scanning takes place on all channels for both 2.4GHz and 5GHz frequencies, which is a very time-consuming process. Sometimes it takes several seconds.
We have the 802.11k protocol to help us, which allows the client to receive information from the access point about potential other candidates nearby that broadcast a similar wireless network. The first six entries in the report form the basis for scanning, significantly reducing roaming time.
The iOS system also takes into account the information provided by networks about channel usage and the number of connected clients. Unfortunately, I don’t have the details, but you can expect similar behavior to Samsung devices. The iOS system uses this information along with signal strength measurements (RSSI) to assess potential networks. Networks with higher scores offer better Wi-Fi performance.
Selecting a Roaming Candidate
If a device is actively transmitting or receiving data, the system selects an access point whose signal strength indicator is at least 8 dB higher than the signal strength of the current access point. For devices that are not transmitting or receiving data, a difference of 12 dB is applied.
Furthermore, for 5GHz networks, if multiple networks receive the same score, the iOS system will choose a network based on the following criteria:
- IEEE 802.11ax is preferred over IEEE 802.11ac.
- IEEE 802.11ac is preferred over IEEE 802.11n or IEEE 802.11a.
- IEEE 802.11n is preferred over IEEE 802.11a.
- An 80 MHz channel width is preferred over 40 MHz or 20 MHz channel widths.
- A 40 MHz channel width is preferred over a 20 MHz channel width.
For instance, if the RSSI (Received Signal Strength Indication) of the current connection drops to -75 dBm, and the user is actively transmitting data, the device will search for access points with an RSSI of -67 dBm or better. This is part of the iOS system’s strategy for optimizing network performance while roaming.
Summary
A weak signal strength is the basis for a device to consider connecting to another access point. Apple mobile devices consider -70 dBm as the threshold value. It’s also important to note that there must be a significant difference in signal strength for such a switch to occur.