Wi-Fi channel planning in 2.4 GHz - Part 2

In my last post, I talked about Wi-Fi channel planning in 2.4 GHz. The main consideration with this frequency band is that there are only three non-overlapping channels and that careful planning needs to happen in order to avoid co-channel interference.

After reading the article, someone emailed me several great questions and suggested a follow up post. Here is what they asked:

You say in this post that co-channel interference can occur when 2 nearby APs are operating on the same channel. I wonder if you can expand on that. It raised some questions for me. First, why doesn't one AP act as "boaster" to another? Second, does that mean that an enterprise can only have 3 APs in a physical space? If so, what's the size limitation?

Regarding the first question, one access point cannot act as a "boaster" to another as the station (i.e. laptop) needs to build an association to a specific Access Point. This association process is similar to plugging your computer into a specific Ethernet switch port if you were using wired networking.

Therefore, once a wireless station is associated to an Access Point, it has a physical layer link. Because this specific path/link is established, another access point cannot process data without the station disassociating from the original AP and associating with the second - this is roughly analogous to unplugging your cable from one port and plugging it into a second.

Regarding the second question, because of co-channel interference, an enterprise can only have 3 APs in a given physical space (if they are using 2.4 GHz). The size limitation depends on many factors including the output power of the access point, if any special antennas are being used, and the construction materials of the area. Typically, an AP with an omni-directional antenna in general office space covers about 5,000 square feet.

This becomes an issue in high-density situations as each Access Point can only handle a certain number of stations. My rule here is that a typical AP can handle 20-25 stations, depending on bandwidth requirements. Therefore, organizations run into a problem where they would like to have more than 60-75 users in a given space (such as conference centers, meeting rooms, theatres, multipurpose rooms, cafeterias or gymnasiums).

Don't fret - there is another option, which is to use wireless devices in the 5 GHz frequency band. 5 GHz has many advantages over 2.4 GHz, including *many* more non-overlapping channels. In fact, 5 GHz has 24 non-overlapping channels, which make it an especially popular choice for industries with high density requirements. That being said, one of the most important considerations for 5 GHz deployments is to ensure that all your end user devices are capable of transmitting in the 5 GHz spectrum.

Douglas J. Haider is a Principal Technologist with Xirrus. He hosts a personal blog at WiFiJedi.com, and micro-blogs on Twitter @wifijedi