Thursday, 12 December 2013

Transmit Power Control - Wireless tips (No. 1)

You may have seen on your enterprise access points that the power level can be moved up and down. You may also see, in most cases, that the power levels are numbered 1-7 or 1-8. So what do these numbers mean?

If you are using a controller-based solution, you will see something like the above screenshot in your access point settings. If you don't use a controller then these settings should be on your interfaces of your AP. At first glance you might take a look at that and go "hang on, is 1 the highest or is 7 the highest"?

The table below shows the meanings of the power level on our access points. The reason they are numbered like this is because you can have different maximum power levels in different countries, and setting the country code determines what it is, rather than vendor having to write a different config page for each country.

The dBm and mW conversions are much easier to work out if you remember the 3's and 10's rule. This rule simply states that:
  • Adding 3dBm doubles the power in mW
  • Removing 3dBm halves the power in mW
  • Adding 10dBm adds ten times the power in mW
  • Removing 10dBm removes ten times the power in mW
So you can see now, looking at the table, that as the power level drops one 'level' the transmit power goes down 3dBm and the mW of power is reduced by half.

As another example, if you were to reduce the power level from 20dBm to 10dBm (so you have removed 10dBm), you would have gone down to one-tenth the power in mW, 100mW to 10mW. This rule is especially handy when working out antenna gain.

So why would you want to reduce power?

Where do I start? There are a whole host of reasons, which mainly involve interference. Say you've got your AP on 'full whack' for best coverage and you're leaking onto the floors below in a shared building or into a neighbor's building. Firstly, that's not good etiquette, but say they have their own wireless network, and they have a protection mechanism that will send deauth packets to rogue AP's. Now they're going to kick all your users off your AP.

Now, if you have more than one AP. Say, for example, you are having a conference of 400 people. You're probably going to need more than the 3 x 2.4GHz channels you would normally deploy but the access points are all in the same room, so you're going to want to reduce the power to prevent the AP's interfering with each other.

My third example (I could go on) is, say you have an access point in a lecture theatre turned up too high, somebody who can authenticate with your network walks past outside. They are not using wireless, but have their device switched on. It automatically associates and now every device in the lecture theatre has to share airtime. Worse still, say the device connects using 802.11b. The protection mechanisms will be switched on and you have lost 50% of throughput (Source).

I hope this has given some idea about power settings on wireless access points.

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