Friday, October 4, 2013

SSID Overhead - How Many Wi-Fi SSIDs Are Too Many?

One of the most commonly cited best practices among Wi-Fi professionals is to the limit the number of SSIDs you have configured on your WLAN in order to reduce the amount of overhead on the network and to maintain high performance. But there is not a lot of public data out there to really drive home this point when explaining it to another engineer, management, or a customer. Simply telling someone that they shouldn't create more than 'X' number of SSIDs isn't very convincing.

Therefore, I've created a visual tool to help you explain WHY too many SSIDs is a bad thing:

The Wi-Fi SSID Overhead Calculator
(Click Image to Download)
Wi-Fi SSID Overhead Calculator

This tool calculates the percentage of airtime used by 802.11 beacon frames based on the following variables:
  1. Beacon Data Rate - beacon frames are sent at the lowest Basic / Mandatory data rate configured on the WLAN. Beacons must be sent at a "legacy" data rate, meaning only 802.11a/b/g rates. Select the beacon data rate from the drop-down menu within the tool.
  2. Beacon Frame Size - beacon frames can vary in size based on the version of the 802.11 standard implemented (802.11a/b/g/n/ac) and features enabled on the WLAN (such as 802.1X authentication, CCX, 802.11r fast roaming, and 802.11u Hostpost 2.0). I recommend using a wireless sniffer to capture a beacon frame from your WLAN for use within the tool. Enter a beacon frame size that represents the total size of the MAC header and data payload.
  3. Beacon Interval - beacon frames are sent at a default interval of 102.4ms, but this may be modified in most enterprise WLAN products. Note that beacons are always sent at a multiple of the Time Units (TUs), where one TU equals 2^10 Kilomicroseconds (or 1.024 milliseconds). Therefore, 100 TUs equals 102.4ms. Enter the time interval between beacons, in milliseconds.
The calculation includes the inter-frame spacing (using WMM), physical layer preamble and header, MAC layer header, and data payload. It calculates the amount of time required for modulation of the bits over the air, but does not account for collisions or retransmissions. Technically, you wouldn't reach 100% airtime utilization on a Wi-Fi network because medium contention due to collisions and retransmission backoff result in a maximum airtime utilization of around 80-90%. But for SSID overhead planning purposes this level of detail is not required because the network will be equally degraded if we represent it with or without the collisions.

The tool also takes into consideration the number of co-channel APs within the physical area. All access points, either from your WLAN or a neighboring WLAN, contribute to the overhead on the channel. Remember, Wi-Fi operates in unlicensed spectrum and everyone shares the airtime!

I have also included a subjective rating of the amount of overhead into the following categories:
  • 0-10% = Low Overhead
  • 10-20% = Medium Overhead
  • 20-50% = High Overhead
  • >50% = Very High Overhead

You should ALWAYS attempt to keep your WLAN at low overhead (0-10%). 

If you have an existing deployment that falls within the medium overhead range (10-20%) you might consider methods to consolidate SSIDs and reduce the amount of overhead as your WLAN needs evolve over time. 

If you have an existing deployment that falls within the high overhead range (20-50%) you are likely experiencing significant performance problems on your WLAN already and should investigate immediate methods to consolidate and eliminate SSIDs at the earliest possible time.

If you have an existing deployment that falls within the very high overhead range (>50%) it is likely that you are in a highly congested area and will need to coordinate the WLAN configuration with your neighbors in order to reduce the amount of overhead to a reasonable level. This is common in dense urban / downtown areas and in multi-tenant buildings. 

I hope this tool proves useful. Enjoy!



  1. wow, that looks really familiar .........

  2. Andrew, imagine you have a static scenario, I mean, some stationary clients (no roaming) connected to an AP. If I set beacon interval to the highest value, I would have the best performance, true?. Because beacons are useful for a client to know SSIDs and properties at the moment to associate, but this is not my case.
    Nice blog!! Very interesting! One of my favorites.

  3. Alberto,
    Beacon interval impacts not only AP discovery for association and roaming, but also broadcast and multicast traffic delivery. This could be a big impact to Bonjour service discovery, IPv6 router advertisements and neighbor discovery, IPv4 ARP resolution (depending on if the WLAN solution implements proxy ARP), or any multicast application. This is due to power-save clients and the DTIM interval.

    I typically do not recommend adjusting the beacon interval. If you do, make sure you perform adequate testing to cover all of your client devices and applications before rolling out the change to production.


  4. Isn't the probe, probe response and ACK between STA and AP going to have a large impact on the overhead of the network and should also be commented if we are discussing overhead generated by number of SSIDs. Illustrating just beacons only really tells half the story and probably less than half the overhead if we have multiple clients and several SSIDs.