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Why Do Wi-Fi Connection Speeds Keep Changing?

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Question: Why Do Wi-Fi Connection Speeds Keep Changing?
Wi-Fi networks support certain maximum connection speeds (data rates) depending on their configuration. However, the maximum speed of a Wi-Fi connection can automatically change over time.
Answer: This behavior is called dynamic rate scaling, a design feature of Wi-Fi networks. When a device initially connects to a network via Wi-Fi, its rated speed is calculated according to the current signal quality of the connection. The rated connection speed then automatically changes over time if necessary to maintain a reliable link between the devices. Wi-Fi dynamic rate scaling extends the range at which wireless devices can connect to each other in return for lower network performance at the longer distances.

802.11b/g/n Dynamic Rate Scaling

An 802.11g Wi-Fi device in close proximity to a network router will often connect at 54 Mbps. This maximum data rate is displayed the device's wireless configuration screens. Other 802.11g devices located further away from the router, or with obstructions in between, may connect at lower rates. As these devices move further away from the router, their rated connection speeds eventually get reduced by the scaling algorithm, while devices that move closer can have speed ratings increased (up to the maximum of 54 Mbps.

Wi-Fi devices have their rates scaled in pre-defined increments. For 802.11g, the defined ratings are (from highest to lowest)

  • 54 Mbps
  • 48 Mbps
  • 36 Mbps
  • 24 Mbps
  • 18 Mbps
  • 12 Mbps
  • 9 Mbps
  • 6 Mbps

Similarly, old 802.11b devices supported the following ratings

  • 11 Mbps
  • 5.5 Mbps
  • 2 Mbps
  • 1 Mbps

Controlling Dynamic Rate Scaling

Factors that determine which data rate is dynamically chosen for a Wi-Fi device at any given time are:
  • distance between the device and other Wi-Fi communication endpoints
  • radio interference in the path of the Wi-Fi device
  • physical obstructions in the path of the Wi-Fi device, that also interfere with signal quality
  • the power of the device's Wi-Fi radio transmitter/receiver
Wi-Fi home network equipment always utilizes rate scaling; a home network administrator cannot disable this feature.

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