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Looking at the Vocera Infrastructure Planning Guide, there are some recommended power levels for WLAN coverage.

The guidelines include:

* Make sure the signal strength is always greater than -65 dBm

* Make sure AP transmit power is set to a level comparable to the typical transmit power of the Vocera badge, 14.5 dBm (28 mW).

I'm fine with the -65dBm figure, but I don't understand the 28mW transmit power for an AP. There is talk in the manual about avoiding power asymmetry, but surely this will also depend on antenna gain?

If I put a 3dB antenna on, I'm effectively pumping out 56mW, so I'm going further. Won't I get power asymmetry anyhow, as the Vocera badge is still limited to 28mW?

Nigel.

nbowden Escribi?3:

Looking at the Vocera Infrastructure Planning Guide, there are some recommended power levels for WLAN coverage.

The guidelines include:

* Make sure the signal strength is always greater than -65 dBm

* Make sure AP transmit power is set to a level comparable to the typical transmit power of the Vocera badge, 14.5 dBm (28 mW).

I'm fine with the -65dBm figure, but I don't understand the 28mW transmit power for an AP. There is talk in the manual about avoiding power asymmetry, but surely this will also depend on antenna gain?

If I put a 3dB antenna on, I'm effectively pumping out 56mW, so I'm going further. Won't I get power asymmetry anyhow, as the Vocera badge is still limited to 28mW?

Nigel.

Nigel,

From my experience I have used 25mW on the radio and 3 dBi gain antenna on a number of Vocera deployments and havent had any issues.

What I will say, Vocera is one of the most tricky surveys you will have to do. I would highly recommend the following:

1. Get a vocera badge and use it during your survey. Wear it around your neck how it is intended to be used. Also double check the badge readings all the time.
2. Enter all rooms and close the door behind you. Go to the far corners of the rooms and check signal on the badge.

I have seen a number of sour surveys done which cause the vocera product to fail right after deployment.

Nigel,

GStefanick brings up some good points in order to more fully test Vocera in your environment. We also recommend "tweaking" your survey tool to more closely match the Vocera badge. This can be done by lowering power on your survey client and maybe turning off diversity (if allowed).

With regard to antenna reciprocity, you will need to ensure that the input powers to the each antennae are closely matched (25mw AP > 28mw Badge). The Rayleigh Helmholtz reciprocity theorem states:

If an electromagnetic force of some particular magnitude is applied to the terminals of antenna "A" and the received current is measured at some other antenna "B" then an equal current (in both amplitude and phase) will be obtained at the terminals of antenna "A" if the same electromagnetic force is applied to the terminals of antenna "B".

In more simple terms this means given the same input power on both ends (antenna A and B) "if you can hear me, then I can hear you!" and this is irrespective to antenna gain.

In my experience when looking to expand coverage horizontally, it is best to use a slightly higher gain antenna (either omni or patch depending on RF environment you are deploying in) and keeping TX power on AP's within proper range (i.e. 25-30mw).

More on time symmetry and reciprocity theorem can be found at the below link (WARNING: this can be mind numbing RF reading :)

RF Engineering Perspective: "I'm Going To Let My Chauffeur Answer That"

http://www.connect802.com/download/techpubs/2008/my_chauffeur_BD0414.pdf

Hope this helps!

-Chris

Chris,

Thanks for the input. Much appreciated.

Is the Vocera badge 28mW ERP, or is that the transmit power in to the antenna (which also maybe has some gain?).

The deployment I am looking at needs to cover some outside areas, so I am looking at some higher gain antennas. I am looking at more of a margin of error when using higher gain antennas, so just want to be sure I am matching my AP ERP with the Vocera badge ERP.

Thanks

Nigel.

Chris,

After reading your answer more carefully, I realise that you have already answered my question.
:)
Thanks again.

Nige

GStefanick,

Thanks very much for your input, much appreciated.

I'd be interested in your thoughts on predictive surveying with Vocera (though I think I know what they will be). My company insist on using predicitive surveying for econmomic reasons, which has worked fine with Cisco data & voice, but we have not really tried it for Vocera.


Nigel.

nbowden Escribi?3:

GStefanick,

Thanks very much for your input, much appreciated.

I'd be interested in your thoughts on predictive surveying with Vocera (though I think I know what they will be). My company insist on using predicitive surveying for econmomic reasons, which has worked fine with Cisco data & voice, but we have not really tried it for Vocera.


Nigel.

Nigel,

Specific to perdictive and Vocera. I would not, Vocera is a different beast and way to many factors come into play. Floor plans often wont shows cabinets and other obstructions.

No matter what client you are implementing it is always best to follow any predictive survey with a passive/active (passive mandatory) manual survey. This way you can ensure your predictions were correct ;-)

With that said, our B2000 badge is more tolerant to variances in site surveys. Nonetheless, it is highly recommended to do a manual site survey along with using the badge.

-Chris

ChrisO Escribi?3:

No matter what client you are implementing it is always best to follow any predictive survey with a passive/active (passive mandatory) manual survey. This way you can ensure your predictions were correct ;-)

With that said, our B2000 badge is more tolerant to variances in site surveys. Nonetheless, it is highly recommended to do a manual site survey along with using the badge.

-Chris

I would agree the B2000 is much better then the B1000. In fact it is nice to SNR and not cq. What is the dwell time of the badge, just wondering.

The document that Chris mentioned is one of the best RF documents that I have ever seen. RF theory can be extremely complex. I have been involved with RF for over 25 years, and every year that goes by, I realize that there is so much more to understand.

At first glance, many are likely to go ???¡é?¡é?????¡­?¡°Yuck !! ???¡é?¡é???????|???¡é?¡é???????? and never look at it again, due to the math and terminology. However, that would be doing yourself a great disservice. If you take two pages a day and glance over them [ you don???¡é?¡é?????¡é???¡ét have to understand the math] and just read what you can understand, this will provide a ???¡é?¡é?????¡­?¡°framework???¡é?¡é???????? so that you can later on read other tutorials on the Internet that may help you understand what is going on. If you do this, in a few months you will become familiar with terms that you will see in other books and white papers such as ???¡é?¡é?????¡­?¡°Directivity???¡é?¡é????????.

Many people say ???¡é?¡é?????¡­?¡°I don???¡é?¡é?????¡é???¡ét need that !@#$ I???¡é?¡é?????¡é???¡ém a PRACTICAL guy???¡é?¡é???????|???¡é?¡é????????

Well, both are needed: theoretical and practical skills. I???¡é?¡é?????¡é???¡éve lost track of the number of times that I???¡é?¡é?????¡é???¡éve been called to a site where a ???¡é?¡é?????¡­?¡°practical???¡é?¡é???????? guy has been unable to resolve a complex problem due to a lack of understanding of polarization, VSWR, antenna characteristics etc.

This does NOT mean that people need to understand calculus or advanced math, but rather just be familiar with some of the terms in the document like ???¡é?¡é?????¡­?¡°directivity???¡é?¡é????????.

For example, if you look at the math for Fresnel zones, it???¡é?¡é?????¡é???¡és pretty horrible to anyone except an RF designer. But you do need to know some of the basic equations for clearance, even if it???¡é?¡é?????¡é???¡és just to ???¡é?¡é?????¡­?¡°plug and play???¡é?¡é???????? the numbers.

Once you see a term, you can highlight it and then look on the Internet to see if there is a less ???¡é?¡é?????¡­?¡°math intensive???¡é?¡é???????? document relating to it. If math is just too horrible [ as it was for me for many years ], just read the words and this will help make things ???¡é?¡é?????¡­?¡°click???¡é?¡é????????.


I mentioned in a previous posting that ???¡é?¡é?????¡­?¡°there is nothing new under the sun???¡é?¡é????????. This chap was working on the formulas that allow cell-phone and 802.11 RF designers to do their jobs properly when horse-drawn carriages were still around !!
http://en.wikipedia.org/wiki/James_Clerk_Maxwell

Dave