Wireless Environment Sizing

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Mar 23rd, 2009
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Hi,


is that a way to size wireless environment (e.g. a doc containing parameters like range or something), related to AIR-LAP1242AG-E-K9, or AIR-LAP1131AG-E-K9, in order to make a proposal to our customer. I have to know how many AP I need to cover the office.


Thanks,


Alessandro

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ryhicks Sun, 03/29/2009 - 09:07
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Some vendors are relying on CAPWAP or LWAPP sizing tools alone to determine AP placement. But I would say that there is not one tool alone or a single sizing guideline to use.



A general use of thumb is listed below. But be careful. This is not a substitute for a site survey. Why? Tools and guidelines cannot take into account RF properties of a building or area or the 802.11 clients. If your customer is firm about not purchasing a site survey, you may want to cover yourself by stipulating that any area that is not adequately covered or where issues arise will have to be investigated with a site survey tool and visual inspection to determine proper AP placement for throughput and coverage for the application. (Site survey after the fact). If voice is involved, look at the manufacturer's transmitter and radio capabilities in the client itself. This may dictate much of the design if voice coverage is needed everywhere. Some voice vendors have a very weak transmitter in the voice (802.11) device and gets even weaker when the battery is at half battery power or lower. This means that while the APs can transmit adequately to the client, the client cannot transmit back in the coverage area. This can prove challenging without a site survey. Use the actual clients that your customer will use to test with and survey or use a site survey tool that allows you to adjust the transmit power.



If you must go in blind - (wouldn't recommend it - but sometimes you must because of customer requirements)



1 AP per 2000 sq feet - EXTREMELY SAFE VOICE and DATA


1 AP per 3000 sq feet - VERY SAFE FOR DATA and VOICE


1 AP per 3500 sq feet - SAFE FOR DATA and SOME VOICE


1 AP per 4000 sq feet - AGGRESSIVE DATA ONLY


1 AP per 5000 sq feet - DATA ONLY

According to Cisco:


"As a guideline and starting point for site surveys, in a voice-ready WLAN, access points should be deployed at a density of approximately one every 3000 square feet, as opposed to one every 5000 square feet used for data-only networks. So to calculate the number of access points required for deployment, divide the total number of square feet to be covered by 3000 and you will have a good starting point in your design. This level of density helps ensure that voice services have the necessary RF coverage redundancy and throughput required to provide optimal service capacity. Ideally, there will still be a site survey to maximize coverage and minimize interference."

However, your mileage may vary, as they say.  The 3000 sq. ft. rule of thumb above number may be more applicable to open-cube, carpeted environments.  When you get into locations with a significant number of walls, and depending upon the type of construction (drywall vs brick, cinderblock or concrete), as well as increased wall density (many small rooms) you will see significant differences that may require more access points.

It can be helpful to characterize the building's RF characteristics by placing an access point in the building at the intended power settings and measuring how the RF propagates into different areas.  In multi-floor buildings, it is helpful to measure RF propagation on different floors as well.

Hope this helps,
WJR

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