Basic Understanding

I'm trying to understand the fundamentals of Wi-Fi. So far I have
ascertained that the 2.4 GHz band is divided up into 11 channels in Europe,
and because several routers can work on the same channel in the same area, I
have come to the conclusion that the rf carrier is switched on and off every
time a packet is transmitted, as in packet radio. I can access my router and
change its channel, hence its frequency, but I have yet to find an option
using Windows 7 that allows me to change my adapter channel. Whilst a
wireless adapter can see several routers within range and therefore is
capable of receiving a band of frequencies/channels, what determines the
channel it transmits on. Does it automatically switch to the same channel as
the router it attempts to connect to?

Fred

"Fred Burnett Preston UK" <fred.burnett-btinternet.com> wrote in message
news:suSdnRVCDcJt5QjQnZ2dnUVZ8iKdnZ2d@bt.com...
> I'm trying to understand the fundamentals of Wi-Fi. So far I have
> ascertained that the 2.4 GHz band is divided up into 11 channels in
> Europe, and because several routers can work on the same channel in the
> same area, I have come to the conclusion that the rf carrier is switched
> on and off every time a packet is transmitted, as in packet radio. I can
> access my router and change its channel, hence its frequency, but I have
> yet to find an option using Windows 7 that allows me to change my adapter
> channel. Whilst a wireless adapter can see several routers within range
> and therefore is capable of receiving a band of frequencies/channels, what
> determines the channel it transmits on. Does it automatically switch to
> the same channel as the router it attempts to connect to?

The channel is only set at the wireless router (or wireless access point),
you have no control over it at the PC end. As far as I know (and I'm no
expert in this field) the PC always replies back on the same channel as the
router is transmitting on.

So if you can see say three different wireless networks each on different
channels (or perhaps all using the same channel) your PC will sort out what
channel it needs to be using when it connects to the network.

If you are setting up a wireless router then its worth doing a scan to see
what channels might already be used and going for a free one. If you have
trouble connecting then its worth changing channels at the router.

Hope this helps.
--
Brian Cryer
http://www.cryer.co.uk/

In article <in485n$8at$1@dont-email.me>,
"Brian Cryer" <not.here@localhost.invalid> wrote:

> If you are setting up a wireless router then its worth doing a scan to see
> what channels might already be used and going for a free one. If you have
> trouble connecting then its worth changing channels at the router.

Fred & Brian-

In addition, The channels are broader than the channel spacing. Each
channel overlaps 2 adjacent channels on each side. To avoid
interference, your router needs to be 5 channels away. That is one
reason you often find routers on channels 1, 6 and 11.

One way to do a scan of nearby routers, is to use a "stumbler" program.
(I use iStumbler for the Macintosh.)

Fred

On Fri, 1 Apr 2011 08:04:16 +0100, "Fred Burnett Preston UK"
<fred.burnett-btinternet.com> wrote:

>I'm trying to understand the fundamentals of Wi-Fi.

There are numerous web sites that explain how wi-fi works. This is
kinda scraping bottom, but it's a fair start:
<http://www.howstuffworks.com/wireless-network.htm>

>So far I have
>ascertained that the 2.4 GHz band is divided up into 11 channels in Europe,

The number of channels varies by country. Some countries have 14
channels, most have 11 channels. There are also differences in
allowed power levels and EIRP (radiated power).

>and because several routers can work on the same channel in the same area, I
>have come to the conclusion that the rf carrier is switched on and off every
>time a packet is transmitted, as in packet radio.

Correct. It's a simplex system. The radio is either in transmit or
receive at any given time. Never both.

Also, like packet radio, there are long periods of time between
transmissions. This is to allow other users of the channel to find
some "air time" to operate. In other words, a single system cannot
monopolize a channel.

>I can access my router and
>change its channel, hence its frequency, but I have yet to find an option
>using Windows 7 that allows me to change my adapter channel.

You won't find one. In Infrastructure mode (that's the one using an
access point or wireless router), the operating channel is controlled
by the access point. The client radios just follow. If you change
the access point channel, the clients will scan for a suitable access
point, find the new channel, and reconnect.

However, in ad-hoc mode (also known as peer to peer mode), there is no
central controlling access point. The channel must therefore be set
individually by each client radio. If you want to change channels,
you have to change the configuration in every client radio in the
network.

>Whilst a
>wireless adapter can see several routers within range and therefore is
>capable of receiving a band of frequencies/channels, what determines the
>channel it transmits on.

The client is scanning *ALL* the channels before making a connection.
When you see a list of available access points, they're on any of the
11 channels. Once the client connects to a given access point, it
stops scanning, and uses just that channel.

>Does it automatically switch to the same channel as
>the router it attempts to connect to?

Yes.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Fred Burnett Preston UK" wrote in message
news:suSdnRVCDcJt5QjQnZ2dnUVZ8iKdnZ2d@bt.com...

I'm trying to understand the fundamentals of Wi-Fi..................

Many thanks for all your input, and confirming the conclusion I had reached.
I have read that it is possible to set the adapter channel using something
like backtrack running under Linux, but that's not under normal use. I was
also aware of the channel spread and have seen a spectrum analysis of it.
Whilst I've read in several places that one should choose a channel away
from other router channels, it strikes me that if you are using a laptop or
other device to identify channels in use, it's pointless doing it any
distance from the router as you may be moving away from the strongest one.
If you are moving towards a stronger one, the router might not even detect
it, although that could cause problems for the laptop on reception. I guess
what I'm trying to say that unless you are using the adapter in the same
place all the time, changing the channel might be a pointless operation.
Also if you are significantly close to your own router, then you are likely
to swamp any other adapters in the vicinity using the same channel and
interference isn't a problem at all. I just wonder if many people following
this advice can end up making matters worse?

Fred

Meanwhile, at the alt.internet.wireless Job Justification Hearings, chose
the tried and tested strategy of:

> I just wonder if many people following this advice can end up making
> matters worse?

Given that you've only got a small number of channels to play with, I
wouldn't think it would take too long to work out which the best is for you.
Therefore I wouldn't worry about the methodology.

--
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