We have recently been load testing WiFi access points for use with WiFi audio streaming. We had a lot of questions on the results because the specifications of enterprise grade WiFi do not really address our special case. One one hand, they talk about bandwidth, but we have very small packets and are not really bandwidth limited. For instance, an 802.11ac access point with an 80 MHz wide channel could theoretically support 3000 phone. That is way too high.
And to maximize the quality of the audio, we set the bandwidth to 20 MHz. This minimizes interference with other WiFi devices as well as with other radios in the band. We do this because if one has high levels of interference, one hears the lost packets. Unlike a system like Sonos, we tune our system for minimum latency and best lip sync, which means we have no time to retransmit lost packets. In our system, one does not hear a single lost packet, but if there are several in a row, one does hear that.
On the other hand, if one looks at the specifications for the number of VoIP (Voice over IP) calls an access point can support, one gets a very small number, e.g., 30 or 40. That is too low. VoIP traffic is not synchronized and people can talk at any time. In our system, there is only one transmitter and it is handing out packets sequentially to many phones. In an ideal scenario, there are actually no collisions. So VoIP specs are too low.
We set up a measurement to emulate the traffic in our system. We had a source that handed out packets just like our ExXtractor venue server and programmed a second computer to emulate the smart phone receivers, each with a different IP address. Then we set up two access points in mesh mode and blasted packets from one to the other, with the source wired to one and the sink wired to the other.
What we found was:
- Ruckus ZoneFlex 7982 worked well with 250 clients, but at 300 we heard drop outs. 200 clients seemed safe.
- Ruckus r600 worked well with 300 clients, but at 350 we heard drop outs. 250 clients seemed safe.
- Aruba IAP-205 worked well with 300 clients, but at 350 we heard drop outs. 250 clients seemed safe.
All of this was in an environment without any other traffic, but it was only using one radio. In practice one would use both bands but use band steering to send most traffic that can support it to the 5 GHz band. Interestingly, these numbers are about half the theoretical capacity of a 20 MHz link, so we are in the right ball park. We used gigabit Ethernet.
All of the devices we tested were Wave I type. Wave II could add additional capacity on 5 GHz to the extent that Wave II phones, such as modern Samsung phones, were used as clients. Thus a Ruckus r510 or r610 could be helpful in this regard.
Of course, there are all sorts of things that impact a real-world deployment, but this should give one a pretty good indication of what is possible.
Check out our blog on balancing the overall system bandwidth, here.
