Limit SSIDs & Data Rates to Maintain Network Performance

To maintain proper Wi-Fi network performance, limit the number of active SSIDs to reduce channel overhead used by network management frames, especially beacons. Most enterprise Wi-Fi solutions on the market allow administrators to configure multiple virtual SSIDs on a single access point, each creating a distinct Basic Service Set (BSS). Therefore, each BSS sends out beacon frames at the Target Beacon Transmission Time (TBTT), which typically defaults to every 100ms.

Enabling too many concurrent SSIDs results in more network overhead from beacons. This can cause performance degradation by utilizing a large amount of available airtime on the channel.

Tips for maintaining high network performance:

1. Limit active SSIDs to 6 at maximum
   This is a general rule-of-thumb, and should be adjusted based on your environment and network design and performance requirements. Lower this value even further if you plan on deploying voice over Wi-Fi, perhaps down to 3 or 4 SSIDs max.
   
   
2. Disable Low Data Rates
   Since beacons are sent at the lowest "basic rate" of the BSS, disabling lower data rates forces beacons to use higher data rates and reduces network overhead. Be sure to test changing data rates prior to implementation to ensure adequate coverage still exists and that no impact to clients in your environment results from this change. A good starting point is to disable the 1-2 Mbps 802.11b data rates!

Here is an example. Assuming 6 SSIDs per-AP, 100ms TBTT, and 3 co-channel APs in the same area (within signal range to induce CCA medium busy backoff amongst each other):

Note 2013-08-16: This assumes a 180 byte beacon frame, which is probably too low for modern WLANs which have added many features that result in more Information Elements. For example, I typically measure beacons at around 370-400 bytes today. 802.11u and Hotspot 2.0 capabilities will increase this further. So, these calculations on bandwidth utilization are probably about half of what they are today. Double these numbers :) I'll try to post another blog updating these numbers accordingly.

   Beacon Data Rate     Resulting Channel Bandwidth Utilization

   1 Mbps               25.92%

   2 Mbps               12.96%

   5.5 Mbps             4.71%

   11 Mbps              2.36%

   6 Mbps (802.11a/g)   4.32%

   12 Mbps (802.11a/g)  2.16%

Same network example, now with only 3 SSIDs per-AP:

   Beacon Data Rate     Resulting Channel Bandwidth Utilization

   1 Mbps               12.96%

   2 Mbps               6.48%

   5.5 Mbps             2.36%

   11 Mbps              1.18%

   6 Mbps (802.11a/g)   2.16%

   12 Mbps (802.11a/g)  1.08%

Limiting active SSIDs and disabling lower data rates can make a HUGE difference!

Additional Reading Materials / Tools:

Cisco's Beacon Bandwidth Estimator (Only available from Cisco Advanced Services)

Wireless LAN Professionals - Episode 10 (@KeithRParsons)

Computerworld Article by Douglas Haider (@wifijedi)

Aruba Airheads Articles (3 in total)

Cheers,

-Andrew