This post is necessary. Many things will become clear! I didn’t know this myself.
It’s about roaming! So what? Specifically: It’s the switching of devices between access points. It might seem trivial, but it’s one of the most frequently reported issues in wireless networks, under the notion of “Wi-Fi doesn’t work.”
To ensure Wi-Fi works correctly, we need to be aware of the conditions under which our devices switch. Then, we should use this knowledge to properly design wireless networks. However, let’s start with the basics.
Cactors affecting roaming
For Samsung mobile devices, we can distinguish three factors that cause the device to want to connect to a different access point:
- Weak signal strength – the threshold value is a signal level of -75dBm. A drop to this value triggers roaming.
- Loss of beacon* packets – if a device does not receive a Beacon packet from the connected access point within 2 seconds, it triggers the device to roam to another access point. When the screen is off, this value is 6 seconds.
- Channel occupancy – through beacon packets, clients know the occupancy rate of the channel on which the access point operates, i.e., to which they are connected. When the channel occupancy exceeds 70 percent and the signal value is between -65dBm and -75 dBm, Samsung devices try to connect to another, less occupied, access point. This factor is supported by devices in the Samsung Galaxy S and Note series, starting from the Galaxy S8 version.
**The beacon frame is one of the management frames in WLAN networks based on the IEEE 802.11 standard. It contains all the information about the network. Beacon frames are transmitted periodically and are used to announce the presence of a wireless LAN. Beacon frames are transmitted by access points (AP).
When one of the factors mentioned above occurs, the device will switch to another access point whose signal is 10dBm stronger.
Active Scanning
You might be wondering, how does the device know which access point to switch to?
There are many methods. The basic one involves actively scanning available access points. We can distinguish between two types of scanning: full and partial. Full scanning involves scanning all available channels to find the best candidate (access point). Partial scanning involves scanning channels already known by the device, which broadcast the same SSID to which the device is connected. As you can guess, the latter method is much faster and more effective, especially when the connection parameters are weak. Remaining in such a state is not recommended and causes a lot of problems for users.
Here’s an interesting fact: active scanning of all available channels at a 5GHz frequency can take up to 2800ms, while scanning only 5 saved channels takes about 200ms. According to Samsung, active scanning of one channel takes about 40ms, and DFS channels even take around 130ms.
802.11k
Samsung mobile devices support various methods of obtaining information about potential roaming candidates before one of the factors is triggered. This includes protocols like 802.11k.
Roaming
During the switching process, the device cannot transmit any data until it connects to the new access point. To reduce this time, Samsung mobile devices support mechanisms like OKC (Opportunistic Key Caching) and the 802.11r protocol.
The 802.11v standard also allows for the exchange of information between the device and the access point to improve wireless network performance and optimize roaming in dynamically changing conditions.
Summary
The switching process is influenced by three factors: weak signal, loss of beacon packets, and high channel congestion. The device should quickly connect to another access point because data transmission is impossible during the switching process. The duration of this time depends on how information about potential access points is obtained and the actual switching process.