Calculating SSID Overhead
As part of a project with the goal of transitioning a European company from two different service providers into one I took part as the wireless architect leading the design and implementation of this new environment. Wireless was to be delivered as a transparent service.
The old solution was a build on split-MAC architecture separate from each other in two countries. One country running Cisco WISM v.1 boards the other Cisco 5508 with a mixture of APs most equally as old. Access points connected through CAPWAP switching data either local or central servicing a total of no more than 23 service set identifiers. As a wireless engineer you know that this has a great impact on the overall performance of the WLAN and working with this technology is all about optimizing your cells and BSSIDs. CSMA/CD may be forgotten in cable networks but CSMA/CA is very much alive in 802.11.
Thinking about SSID overhead it is driven by our concern with the amount of airtime APs use to transmit management and control frames for each virtual BSSID hence taking up time from actual data frame transmissions. A main goal of the wireless infrastructure is to handle STA transmissions as quick as possible making that harder with more 802.11 overhead. Several factors play a role in the equation for calculating the airtime used.
There are several behaviors to look at regarding control frames as well that takes up airtime but for this article I will be looking at the beacon only. The bigger the frame the longer it will take to transmit. Therefor knowing the size of your SSID beacons is needed. The size varies according to what standards and features your different SSIDs are supporting so keep this in mind. I did a sniff to determine one of the beacons and found it to be 322 bytes. These 322 bytes is not the total PPDU sent because you do not see the preamble and PHY header in the capture but close enough.
Next looking at the WLCs configuration the beacon interval was set to 100 time units which actually correspond to a target beacon transmission time of 102.4ms. TBTT meaning as close to as possible given other STAs might be transmitting before medium is available. Therefor the number of beacons sent in 1000ms amounts to approximately 9.77.
Data rates as low as 1Mbps were enabled as basic enabling legacy clause 15 802.11b clients using DSSS encoding and DBPSK modulation with long preamble. When the PLCP long preamble is in use it and the PHY header are both sent 1Mbps. The long preamble is 144 bits and the PHY header is 48 bits which comes to a total of 2768 bits.
So to calculate the airtime it takes to send one beacon.
1Mbps = 1048576 bits
1048576/1000 = 1048.576 bits per. ms
2768 bits / 1048.576 bits pr. ms = 2.64ms per. Beacon + DIFS 0,05 ms
Now we calculate the total amount of airtime
9.77 beacons x 2.69 ms per. beacon = (25.99ms / 1000ms) x 100 ≈ 2.63% airtime
This is why we recommend 2-3 BSSIDs per radio.
Another thing to mention is that the calculation is based on DCF which is why I included the DIFS cause of the non-QoS frame. The value of a DIFS depends on the slot time and SIFS which is different from each of the transmission standards. And this of course changes when HCF with EDCA is in use.
But when applying these calculations to the different SSIDs of the WLAN one has to take CCI into account from the number of neighboring APs. If an AP on the same channel is in de-modulation range this sums to more overhead given the shared medium.
CCI due to close APs (APs in de-modulation range)
So what options do we have to minimize the beacon overhead. Well as part of the first phase we should work on identifying the absolute minimum of SSIDs needed by an organization to support their business needs. Next we should optimize their data rates by disabling the lower data rates for 802.11b to get management frames transferred faster. Then we should keep SSID features to a minimum by trimming the beacon frame size. And to top it off most vendors support AP groups on their controllers enabling your to control which AP broadcasts which SSID and not just all. Some even opts for specific radio settings per AP group which I am hoping to see a lot more to in the future.