Typically within very high density WiFi deployments we recommend a having a low number of SSIDs particularly in the 2.4 GHz spectrum. We may want to have fewer SSIDs in the 2.4 GHz spectrum than in the 5 GHz spectrum. This is due to the available airtime slots within each specific spectrum. Within this blog try to explain over a few parts in detail the cause and effect of the performance impact of multiple SSIDs within a high density WiFi environment. It will also help to illustrate the importance of a WLAN Policy within the high density WLANs to manage rogue devices and interferers.
In any WLAN environment we have management traffic and control traffic as well as our client data. This management traffic creates an overhead that has an impact on the amount of airtime that we actually have available to send data which is what we built the WLAN for in the first place. The lower our management and control overhead the more airtime we actually have to transmit data. The trade-off is that without the RF management and control traffic we would not have a WLAN to transmit data in the first place but we can take steps to limit this and claw back some of that valuable airtime. If we do not control it we can see very high channel utilisation and saturation with just management traffic and severely degraded WLAN performance.
Generally speaking the most prevalent management and control traffic will be beacons, probes and probe responses which is where we focus this discussion for having a low number of SSIDs particularly in 2.4GHz and tools which we can use to mitigate the issues within the Cisco portfolio
Equally the reason we focus on 2.4GHz is that it is by far the most difficult use case with the prevalence of interferers, the lower number of channels and its higher signal propagation.
In the 2.4 GHz spectrum we have only 3 channels as compared to typically 16 to 20 in 5GHz (depending on your regulatory domain etc), the reason we discuss 12 channels in 5GHz particularly. Therefore we have significantly less available airtime if we were to have an equal number of devices in 2.4GHz as we would in 5GHz.
Typically we are seeing a shift of devices to the 5 GHz spectrum which alleviates a lot of the pain as we have approximately only a fifth of the capacity in 2.4 GHz as we do in 5 GHz or less now with the improved efficiency of 802.11ac.