How far will your AP reach?

I am frequently asked when presenting on Wi-Fi, how far will your access point reach.  My response varies, but is typically something along the lines of, how far will your Wi-Fi client reach?

The worse response I hear is, “oh it will cover x square feet”.  Really?  Is that indoors or outdoors?  What if you have concrete walls versus sheetrock?

A better standard response that I hear often is that it varies from environment to environment.  Depending on the wall types and other environmental factors, the signal level will greatly vary.  For example, a building with a wide-open floor plan like a warehouse, will receive a much larger coverage area, than say a school that has been built with concrete walls.

Another common response that I hear is that density also plays a role in deciding how large an access points coverage area should be.  If you are not expecting a large amount of people to be using the Wi-Fi network all at once, and you are looking more to just have complete coverage, you would design using larger coverage cells.  However, if you are planning to have a large amount of people congregated in one area connected to Wi-Fi, smaller coverage cells are more appropriate so that you can split the load across multiple access points.

However, back to my original response, I always point back to the client devices.  In my experience, the client device is normally the weakest link and should always be a consideration.  Keeping in mind that in mobility, devices are designed to preserve battery life.  This means that they are not going to be transmitting at full power.  Typically, the smaller the device, the smaller the battery, the lower the transmit power.

Imagine this scenario:

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If you turn the access point up to the highest available power setting, you will be transmitting at 30 dBm.  This will provide a good size coverage cell.  However, if your client device is only capable of transmitting at 15dBm, even though you have a large coverage cell, the client devices will not be able to transmit back to the access point.  This means that the packets will not be acknowledged be either side , and both devices will keep retransmitting packets which will result in very poor performance.  Would you design your network in such a way that client devices are not able to transmit back to the AP?

Truth is, all of these responses need to be taken into consideration when determining how much area an access point will cover.  Knowing the environment will help you to determine how far the Wi-Fi signal will propagate.  Knowing the quantity of devices, will help you to understand how large you want your coverage cells to be.  Knowing the capability of your clients will help to determine your weakest link.

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