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...
