Mobility Hero Tutorials

Calculating Number of APs for High Density Scenarios - Mar 2014

The tutorial was by: 

I have seem soo many times people asking regarding in how many APs they need for certain sceanrios.  And the only answers they get " it depends" but they never get a concrete asnwer...

This tutorial will show you how to calculate the numbers of AP you need based in information you can get by calculating the Airtime Needed for those devices.

Also to prove that the formula is working properly i took one of the Aruba success history and applied the formula!

 

This Tutorial will just calculate the aproximate number of APS needed, it does not take in consideration RF issues in the auditorium deployment or other similar things

 

How to calculate number of APS for High Density

 

Introduction

In this tutorial I will guide you in how and what you need to calculate the total number of APS in a High Density scenario, such as an Auditorium, Conference Room etc

 

Information needed

In order to calculate this you need to get some information of your client which is the following:

  1. Total of devices expected to be  connected
  2. Minimum BW needed for each device
  3. Type of Devices expected(Laptops, Smartphones, Tablets)

 

Formula components

MBW = Minimum BW Needed for each device

RBW = Real BW of the Device

ND = Number of Devices

AP = Airtime Percentage

Notes:

Real BW of the Devices is the real TCP/IP Throughput which is around haft of the total raw speed

Example 1  

A laptop with a wireless card of 3x3:3 the raw speed of it is 450Mbps

The TCP/IP Throughput would be haft of it which is 225Mbps

Example 2

A smartphone with a wireless card of 1x1 the raw speed normally is = 65Mbps

The TCP/IP Throughput would be haft of it which is 30Mbps

 

Formula 1

(MBW/RBW)*100 = AP

Formula 2

(AP*ND)/80 = Total of Radios

Note:

Remember that most of Aruba APS are dual radio which mean that each AP got 2 radios!(one 2.4ghz and the 5ghz Radio)

Example with real Aruba Deployment

http://technologycourses.wiki.mtnbrook.k12.al.us/file/view/Designing+WiFi+for+Multimedia+in+the+Age+of+the+iPad.pdf

Let take this scenario and fill the information we need by reading the document above:

Filling basic information

  1. Total of devices expected to be  connected = 100
  2. Minimum BW needed for each device = 3 Mbps(2Mbps for apps and 1Mbps for Video)
  3. Type of Devices expected(Laptops, Smartphones, Tablets) = IPADS with a maximum raw speed of 100Mbps and TCP/IP Throughput of 50Mbps(In the document implies that the IPAD provides 100Mbps Wireless speed Connection)

 

Formula 1

AP = (MBW/RBW)*100

Let’s substitute the values

AP = (3Mbps/50Mbps)*100

AP = 6%

Formula 2

(AP*ND)/80 = Total of radios

Let’s substitute the values

Total of Radios = (6*100)/80

Total of Radios = 7.5 Radios

Result:

Total Amount of Aps 4( In this case 4 AP 135)

As you see the total APS is the exact number used on the Aruba Deployment.

 

Conclusion

With this formula you will be able to calculate the approximate number of APS needed in a High Density deployment such as an auditorium.  

Remenber also that this will give you just the total of radios needed,  as in the auditoriums you sometimes or at least i do, turn off the 2.4ghz band of some of the AP because of the overlapping.  So for example 10 radios could be  7 APS,  3 of them with 2.4ghz and 5ghz on, and  4 on just 5ghz which give a total of 10 Radios.

 

 I'd add a few more design considerations:-

 

You may get away with fewer APs because...

 

I'd suggest that it is worth considering the real-world utilization of any particular device or user. In your Auditorium, do you expect 100% of users to be using the bandwidth at once? If not you could consider contending the bandwith provisioning at more than 1:1

If it's possible, can you get any usage statistics to quantify likely usage? 

 

You may on the other hand need more because...

 

It is important to consider overhead from SSID beaconing. Although an approximation, the examples given do not account for bandwidth lost to beaconing by APs. (Good design and consideration to increasing basic rates can minimise this.)

 

You probably don't want to re-use any 2.4 channels. Even given a highly engineered design with respect to APs, the client broadcasts are going to be omni-directional. 

 

The approximation assumes that all cients will associate at top rate. In reality a good proportion will rate shift down and send data at lower rates. Aruba's ClientMatch can help  to some extent with this by shifting clients to the most suitable AP.

 

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Last update:
‎04-14-2014 11:13 AM
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