Why Your Wi-Fi Device Ignores the Nearest Access Point

Users often complain that Wi-Fi is slow in certain spots. When the IT team investigates, they frequently find the user's device is connected to a distant access point (AP) instead of the one right next to it. In my experience, this happens most often on the 5 GHz band.

For this article, assume the AP cell sizes are designed correctly and the wireless drivers on the user devices are up to date.

Below are the two most common causes I have seen in the field.

1) The client device does not support every channel in the regulatory domain

Enterprise APs typically support every 5 GHz channel allowed in the country they are deployed in. In Singapore, for example, that is 25 channels:

• 36, 40, 44, 48

• 52, 56, 60, 64

• 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 144

• 149, 153, 157, 161, 165

  
  

Many client devices, however, do not support all of these channels. Channels 100–144 are DFS (Dynamic Frequency Selection) channels, which are shared with radar systems. To use them, a device must support radar detection and pass extra certification testing in each country. To keep costs down and certification simpler, some vendors, particularly makers of low-cost or specialized devices, choose not to support these channels.

Suppose the nearby AP is broadcasting the SSID on channel 100 and the client does not support channel 100. The client never sees the nearby AP at all. Instead, it connects to a farther AP that happens to be on channel 36, simply because that is a channel it can use. The result is a weak signal, a low data rate, and an unhappy user.

How to verify

Capture the client's Association Request in a wireless packet capture. Then check the "Supported Channels" information element (IE). Each entry lists a first channel and a number of channels,

Device Supporting all the channels

Device not supporting 100-144 channels as below

In the second capture, the sets are 36 (4 channels), 52 (4), 149 (4), and 165 (1). That covers 36–64 and 149–165, with nothing in the 100–144 range.

Recommendation

Verify channel support on all critical wireless devices, especially medical devices, before deployment. If such devices cannot use channels 100–144, either exclude those channels from the RF profile in the areas where the devices are used, or work with the vendor on a supported alternative.

2. Mixed channel widths: APs at 80 MHz and 40 MHz

   Observation

   In a network where some APs use 80 MHz channels and others use 40 MHz, clients connected to an 80 MHz AP tend not to roam to a 40 MHz AP. When searching for a roaming target, the client prefers another 80 MHz AP and only moves to a 40 MHz AP once no usable 80 MHz signal is left. The result is sticky-client behavior: the client stays on a distant 80 MHz AP even when a nearby 40 MHz AP offers a much stronger signal.

   This happens because many clients don't choose an AP based on signal strength alone. Their roaming logic estimates the throughput each candidate AP can offer, and an 80 MHz channel promises roughly twice the capacity of a 40 MHz channel. A weaker 80 MHz AP can therefore still "win" over a stronger 40 MHz AP, until the 80 MHz signal becomes too weak to use at all.

   
   

   An everyday analogy

   Think of an employee on a high salary. If offered a role that pays less, they will usually stay in their current job and only accept the lower-paying role when nothing at the same level is available. A client on an 80 MHz AP behaves the same way. Having experienced the higher capacity, it keeps looking for another 80 MHz AP and resists moving to a 40 MHz AP, even when that AP has the stronger signal.

   
   

   Recommendation

   Configure all APs in the same area with the same channel width. In typical enterprise deployments, 40 MHz is often the better choice for 5GHz, because it leaves more non-overlapping channels for the channel plan. Whichever width you choose, keep it consistent, so the client's roaming decision is driven by signal quality rather than by channel width.