Gear Review: SSID OVERHEAD CALCULATOR

SSID Overhead Calculator by Revolution Wi-Fi’s Andrew von Nagy and Ryan Adzima is the IOS App adaptation of the Wi-Fi SSID Overhead Calculator in the Capacity Planner Excel Workbook.

The SSID Overhead Calculator App was released on 2/23/2016 during the Wireless Lan Professional’s Conference (WLPC) in Phoenix, Arizona. The App can be downloaded in the Apple App Store.

SSID CALC Photo1

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What the SSID Overhead Calculator does is to calculate the channel utilization from network management frames. Wireless access point (AP) stations send out Beacon management frames at a specified intervals typically 1024microseconds. These special management frames contain all the information a station needs to know to initiate the 802.11 association and authentication to the wireless network. APs can be programmed to manage multiple wireless networks. Each wireless network is called a Basic Service Set (BSS). The AP stations send out Beacons for each Basic Service Set they are programmed to broadcast and manage. If you increase the number of BSS that the AP station must manage, you also increase the amount of network overhead due to the management frames.

Generally accepted best practice is to limit the number of SSIDs (Service Set Identifier) for each BSS to four SSIDs or less. Less network overhead equals more time stations can contend for transmit opportunities and actually send radio frames containing data payloads. More transmissions of frames containing data (Data frames) means more web pages displayed, more “Tweets”, more Facebook updates, and more Snapchat.

It is important to note that this is a calculator. It calculates this information from the input data. It does not, sniff the radio frequency and measure the channel utilization. The application is a useful tool for wireless network design and troubleshooting if used in conjunction with other tools such as WiFi Explorer by Adrian Granados.

Using WiFi Explorer, I sniffed the radio frequency to see how many APs I could hear from my laptop. You can see that on channel 11 of the 2.4GHz, I can here 4 APs with 8 total SSIDs being broadcast.

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If I calculate my AP with the 4 SSIDs that I broadcast with the lowest basic rate of 5.5mbps into the SSID Overhead Calculator, I get 2.81% channel utilization due to management frames. Damn good!

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Unfortunately, my laptop could see the neighboring businesses APs as well since I work in a multi-tenant building. All AP’s transmissions that a station (including an AP station) can successfully receive and demodulate will cause the station to back off the air and count toward channel utilization during that AP’s transmission.

So lets input the other information into the SSID Overhead Calculator!

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22.4% would be very bad! That wouldn’t leave enough air time for all the clients to transmit data. However, remember my laptop could hear 4 APs transmitting 8 SSIDs? All four APs are not transmitting all 8 SSIDs. I have 1 AP transmitting 4 SSIDs, 1 AP transmitting 2 SSIDs, and 2 APs transmitting 1 SSID each for a total of 8 SSIDs transmitted.

The input into the calculator should look more like this:

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You have to know what you are trying to see with this calculator and use the correct input.

Let us look at what happens if you program 10 SSIDs on 2 APs. I have seen this in the medical vertical. Many vendor device’s best practice is to require an SSID for their device. This is not wireless engineering best practice, and the SSID Overhead Calculator will show us in a clear to see format!

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The calculator shows a 14% channel utilization just from management frame overhead! This is with only 2 APs that a station can hear. It isn’t uncommon to hear many more especially on the 2.4GHz frequency.

In summary, the SSID Overhead Calculator is a fantastic application that can help you calculate and display the channel utilization impact from the use of AP stations and the number of SSIDs broadcasted by those AP stations.

You can download the SSID Overhead Calculator in the Apple App Store. Hopefully, an Android app will be in development in the future!

 

 

 

 

 

 


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