Bad Apple: A Single Slow Device Can Kill Your Wi-Fi

It’s true: The slower a device’s connection, the more Wi-Fi capacity it consumes. Worst case, it becomes a bad apple and makes the entire network suffer. Here’s how it happens and what you can do. Bad apples are prime examples of how problems that stem from poor wireless coverage can masquerade as speed issues.

First of all, this term comes from the English expression "One bad apple can spoil the whole bunch", it has nothing to do with Apple, the company.

What Makes a Bad Apple in Wi-Fi?

Multiple factors can cause devices to get poor coverage and become bad apples:

  • Older hardware and software that don't support the newer and faster standards for wireless communications.
  • Most obviously: being too far away from the nearest wireless access point.
  • Anything that generally blocks, interferes with, or weakens wireless signals; see also 10 things that block and interfere with Wi-Fi signals and Which Building Materials Can Block Wi-Fi Signals?
  • If you combine the above, the risk is of course even higher. In a home where a router in the living room is the only access point, for example, an older smartphone that is charging in a bedroom, could easily become a bad apple. If it is connected to the net and downloading updates, it does not matter that it is not in active use, it can still affect the speeds available to others on the same wireless network.

How Can a Single Device Ruin it for Everyone?

Airtime is measurement in percentages that describes the amount of time a wireless access point has available for sending and receiving to all clients, such as smartphones, laptops, and other devices connected to the wireless network.

The totality of airtime available is 100%. In practice, a certain portion of this is always lost in noise (interference) from other networks in the same area using the same Wi-Fi channel:

  • On the 2.4 GHz frequency band, which is the only one available to older clients, a densely populated neighborhood can assume that 40-50% of airtime has already been consumed by interference from the neighbor's network.
  • On the newer 5 GHz band, which has a shorter range, there is also far less interference, perhaps as much as 25% in very dense neighborhoods. With more airtime at our disposal, there is a lower risk that newer devices will become bad apples, so long as they do not encounter any other obstacles.

The worse the coverage for a client, the more time the access point will need to use to exchange data with it. This consumes airtime, so that the slower communication is for one client, the less airtime is left to share for the remaining clients on the same network.

A client with good coverage exchanges data quickly and consumes little airtime, but can still get into trouble when all airtime is used up by one or more bad apples.

How to Avoid Bad Apples

Bad apple: when one device on the network has poor coverage, it affects everyone on the network.
  • Turn off Wi-Fi on devices that aren't in use so they can't affect the network.
  • Generally speaking, anything that improves the coverage in the home, will reduce the risk of bad apples.
  • Note that some Wi-Fi extenders/repeaters/boosters work in such a way that they themselves may become bad apples if their coverage is insufficient. See also Five reasons not to buy a Wi-Fi repeater.
  • Our recommended solution is to set up a mesh network of AirTies wireless access points (three or more) that provide coverage throughout the home and automatically provide each client with the best performance available. Airties Smart WiFi: Full wireless coverage in the home.

Nerd's Corner: Airtime Calculation Formula

1 Mb divided by link speed in Mbps.


Having three laptops with link speeds of 10, 50, and 100 Mbps means:

1 Mb / 10 Mbps + 1 Mb / 50 Mbps + 1 Mb / 100 Mbps

10% + 2% + 1% = 13 % airtime is consumed

Add a bad apple--an old smartphone with 2 Mbps transfer rate = 1 Mb/2 Mbps = 50% airtime just for that smartphone.

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