A blog with tips, tricks and tutorials to help you prepare your CCIE Wireless lab exam.

Saturday, July 25, 2009

TPC vs DTPC vs World mode

Have you heard about TPC (Transmit Power Control), DTPC (Dynamic Transmit Power Control), and World Mode? They look the same, but do not actually do the same things... let's have a quick look at each of them:
- World Mode is probably the oldest one. It is a feature you can configure on the Autonomous (OIS) access points, and by which a client in World Mode receives its " radio parameters" from the access point. If you look a bit closer, you might read that "parameters" are actually channels and power levels. But don't take it wrong. "Channels" has an "s". It is not the channel on which the client should be! To hear the access point, the client has ANYWAY to be on the right channel. So what World Mode is about is "the list of allowed channels in this country" and "the power level ranges allowed in this country".
World mode is actually a Cisco implementation of the 802.11d protocol... but wait, why do we need that stuff? You are already on the right channel anyway! Well, for 2 reasons:
. Your client is going to scan for other APs offering the same SSID. You do not want your client to scan channel 13 if there is no AP on that channel because this channel is not allowed in your country... and you do not want your client to send a probe request on that channel if using it is forbidden...
. Your client power level might be too high for the country, thus creating issues for the other clients around it... what issues? Well, if you client is too powerful, it is going to associate at high speeds in areas where it should not even be able to associate, thus creating interference and hidden nodes issues for the other clients...
So the 802.11d protocol allows you to send the regulatory domain information to you client, which is going to adapt to them. Cool isn't it? The 802.11d protocol was integrated into the 802.11-2007 standard, thus allowing the possibility to send this information, without stating exactly how it could be sent. In a Cisco network, this can be contained in the Vendor Specific fields of the beacons. So where is that feature in the Autonomous APs? In the radio (802.11b / 802.11a) configuration page.
And in the Unified solution? Nowhere... what? did they forget it? Naahh, can't believe that... read further....

-TPC, Transmit Power Control, is actually a feature of 802.11h... You know 802.11h? This protocol that prevents your APs working outdoor on the 5 Ghz spectrum from interfering with airport radars... this protocol has two sides:
. DFS, Dynamic Frequency Selection, that makes that if your AP hears a radar blast on its current frequency, it sends a "changing channel" 802.11h message and jumps to another channel.
. TPC, Transmit Power Control, by which 2 devices initiating a communication in the 5 Ghz spectrum will negotiate so that their respective power level is as low as possible, just loud enough to hear each other (so that your noisy 50 mW access point does not disturb the poor 60 WATTS radar sitting next door!!).
Where do you configure TPC? Well, you don't really configure it. It is part of 802.11h, and your 802.11a device has to be compliant with it, and implements it automatically.

-DTPC, that's Dynamic Transmit Power Control, looks close to TPC hey? But this is Cisco stuff, not 802.11 something anymore. With DTPC, your Cisco access point transmits to your Cisco CCX compliant client information about which power level to use... looks somehow close to World mode, don't you think?
Yes, it's close... but do you know what caused the death of World mode and why you don't find this feature in a controller? Think about it... what does it exactly do? If (yes, "if"), you enable it, your clients will receive a pack of allowed channels and power levels from your AP. This information format is proprietary, so your client needs to be a CCX guy to understand it. And what does it do with that information? You don't know... your client may use it, if it is also configured for World mode. It may understand it but not use it, if it is CCX but not configured for World mode, it may not be able to use it if its drivers is not per-set for this regulatory domain, or it may not even understand it if the client CCX version is too old or if the client is not CCX... Whoa, what a result... so let's think about another approach.
If your client is CCX, you can actually do more: influence it. very often, your AP has a good 9 dBi patch antenna and your client has a poor rubber duck 2.2 dBi antenna. Your client hears the AP well, but the client signal is lost in the surrounding noise and your AP does not hear it well. Your client should increase its power level, but it does not know that the AP does not hear it well... all it knows is that it (the client) hears the AP well, and from this received signal deduces its own power level. If you client is CCX, the AP can tell to your client "I don't hear you well, increase your power to 20 mW", or "hey no need to shout! reduce your power to 5 mW, that will save your battery". In this information, the AP can communicate maximums ("increase your power again, but don't go beyond 50 mW"). Isn't that better than World mode? That's what DTPC is about. You can enable it in your controller from the Main radio menu (Wireless > 802.11a > Network, in the General field).
But what about the channels? Well, there is also another CCX feature by which an AP can tell a CCX client "don't scan, you are in my cell in a comfort zone, stay quiet and save battery". Or "your signal strength is decreasing, you should start scanning", and there are a lot of variation from there, from "scan channels 1,6,11" or "check channel 6, Ap aa:bb:cc:dd:ee:ff should be there"... isn't that even better?
Yes, but what about the non-CCX clients? Well, they would not have understood the World mode messages anyway, this is why CCX is cool...

3 comments:

  1. This was very helpful. Thank you

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  2. FYI: There is an article on the web which states 802.11d has been banned by the FCC...this appears to be in accurate....http://www.silexamerica.com/unwired/fcc-update-802-11d-world-wide-roaming/

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  3. This was a great read very helpful thank you !

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