Wireless Access

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.

Hello !

Thanks for you reply

remenber that in the beggining i said this is just to calculate a aproximate number of APS that you need, which will help you to have an idea around how many aps you need in there

Answering you

Yes this takes

"'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"

Yes it takes in mind that they are all using the minimum BW and im agree with you that this will hardly happen but  it considers the worts case scenario.

"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"

I already made a point of this on the conclusions

I have been using this for a a time and it is working pretty good, i mean for a starting point it is.   But its true there are other things you need to take in consideration but for a starting point it does helps a lot.  Like i said in the begginng this does not take in consideration thigns like RF issues or other things. :)

Cheers

Carlos

Ha yes, sorry I should have begun by saying that I agree your post was a good starting point. 

Dont worry everything is cool!

I always apreciate  good comments

Cheers

Carlos

Thanks you all for the kudos!

Lets keep them coming :)

Cheers

Carlos

Formula 1

(MBW/RBW)*100 = AP

Formula 2

(AP*ND)/80 = Total of Radios

Hi,

I m not really understand on the formula. Why is that /80 ?

Could you please explain more on how the formula work?

Thanks in advance.

the 80 is a 80% of airtime utilization

Im dividing it with 80% becasue is a more realistic number.

In the real world a single channel would be maxed out at 80% of channel utilization not really on 100% so thats why that number

With the first formula you are calculating the Aritime percentage using the minimum BW needed for each device and the real max BW of the device(depending in what you using for example a tablet normally does not go over 65mbs... but a 2x2:2 on 5ghz can go up to 300Mbs)

With the second formula you calculate the total radios needed when i say radio i mean:  for an AP 105 for example it has a radio of 2.4ghz and 5ghz working at the same time so that would be one AP but 2 radios...

Now the second formula  to do that use the airtime porcentage and dividing it with the number of the devices... Remenber all those devices are sharing the same media...

does this asnwer your question??? im not that great explaining :/

definitely thanks!!!

Hi 

I have question about formula as  below. 

AP = (MBW/RBW)*100 ( what is 100 ? ) 

Let’s substitute the values

AP = (3Mbps/50Mbps)*100

AP = 6% ( how to calculate to   6%   )

and 

Why , you using ap 135 ? 

Yongyuth