# How does data rate change when Wifi channels overlap?

Let's say there are two wifi access points close to each other, both uses the same channel and the same protocol, with the maximal theoretical data rate of 54Mbit/s. I understand that in case both of them want to stream data they will be able to do that by using the CSMA/CA mechanism.

My question is, how much will the throuput be if both of them want to stream a big amount of data? Will it be 27Mbit/s?

Thank you very much in advance!

• google for "airtime fairness". Practically, I'm now in a heavily congested 2.4GHz environment and my 300Mbps 802.11n router 3 meters away actually gives me 15Mbps. – user3528438 May 3 '17 at 17:46

27Mbps each? You wish!

For starters, bear in mind that routers don't simply "stream" data like a TV transmitter, they just provide a path for other devices to carry out two way communication using higher levels of protocol such as FTP or HTTP that are in turn layered on top of TCP or UDP on top of the wireless frames. That all requires various levels of protocol management traffic to flow from the receiving interfaces and applications to the sending interfaces and applications, and with Wi-fi being half duplex that all eats into the maximum data rate. Add on the various protocol overheads and some hardware-dependent overheads for encryption and it's generally reckoned that you can't expect much more than about 20Mbps of actual data throughput if you have exclusive use of a channel.

Now add a second pair of devices on the same channel and factor in the random delays added by the CSMA/CA algorithms to avoid collisions. Although it's impossible to give a definitive answer when there are so many factors involved, I don't think you could reasonably expect more than about 6 or 7Mbps of data throughput for each pair.

It really depends on the standard the wifi access points are using, I assume you talking about 802.11g when you say 54Mbit/s. If you have two routers next to each other, depending on the router, will use OFDM to decide which one gets to talk on the shared channel. The total bandwidth cannot exceed that of the channel\spec you are using.

Access points use CSMA/CA which means that if one AP is talking the other one will wait until it is done before transmitting.

This means that if you have two AP's next to each other they cannot transmit more than the bandwidth of the channel (in the case of G its 54Mbit/s). If you add up the total transmitted bandwidth it will probably be even less than the total because the AP's will need some time to figure out who is talking.

It will be up to the Access points implementation of the CSMA/CA algorithm, the amount of data that is being transmitted by both routers and the local interference that determines how much actually gets transmitted. The only thing we know is between the two, you'll never get more than the total bandwidth.

• Can you go a bit more into detail on how they'll be using OFDM to share the channel? The link you provided doesn't really explain that, or at least I can't see it. I myself am not sure if WiFi devices are smart enough to try using something like water filling algorithm to distribute power along the carriers. – AndrejaKo May 3 '17 at 23:09
• It really depends on the implementation of OFDM in the router, if one is talking then the other one should sense it and stop and give its neighbor some time. All we know is in perfect conditions your going to get full bandwidth. – Voltage Spike May 3 '17 at 23:11
• I still don't see what does OFDM have to do with it at all, other than being the modulation method used. – AndrejaKo May 3 '17 at 23:18