at the risk of talking about cisco gear on a freebsd list, they're
full of <it>.
In 802.11 the client gets to 'pick' the AP it associates to. Scanning
(which is a client activity)
off channel takes more time than scanning on-channel. Thus, a client
could pick an AP 'on-channel'
more easily than an off-channel one.
"too much overlap" is laughable. Perhaps you refer to this:
http://www.cisco.com/en/US/docs/wireless/technology/channel/deployment/guide/Channel.html
Note how they *ONLY* speak to transmit masks?
Note how they *never mention* adjacent channel rejection or adjacent
channel interference?
This paper from TI touches on the topic.
http://focus.ti.com/pdfs/bcg/80211_acr_wp.pdf
Its true that the transmit masks of the "non-overlapping channels" (1,
6, 11) don't overlap. But none of the 802.11b radios on the market
have enough selectivity to
eliminate in-band signals from an adjacent (25Mhz away) or alternate
(50MHz away) channel.
Minimum IEEE specs for a 11b receiver is 35dB of adjacent (25MHz away)
channel rejection. IEEE doesn't publish specs for alternate channel
rejection on 802.11b, but I can tell you that the
best designs (in terms of ACR) are the "old" super-het receivers.
Intersil (now Connextant)'s Prism2/2.5 is good for about 41dB of ACR.
Alternate channel rejection is perhaps 20dB more with this chipset.
That means that if you're operating (your Prism 2/2.5 chipset radio)
on channel 11, a signal on channel 1 will be 60dB down from where it
would be if your radio was on channel 1.
We run into a lot of (sales) situations where "the competition" likes
to claim that they can co-locate APs. You can't. Here's why:
Free Space path loss in the first meter @ 2.4GHz is 41dB. Lets say
you've got a garden-variety radio that puts up 32mW (15dBm) of tx
power, and ignore antenna gain for now (so 0 dBi antennas on both
radios).
15dBm - 41dB - 60dB = -86dBm This is the in-channel 'noise power' of
the alternate channel radio. Notice that it is at least 15dB above the
thermal noise floor. Translated: you've lowered your SINR.
Moreover, this signal level is 10dB higher than the CCA threshold, so
you'll quiesce the entire cell (no matter which radio was trying to
operate) whenever one transmits. (Its worse than this, if one
radio was receiving, you've destroyed the incoming packet(s).)
Thus, in 802.11b, even if you pick "1 and 11", you're going to end up
with significant in-channel power from any operate on the alternate
channel by a close-by transmitter. For this reason, the minimum
AP separation with 2dBi antennas on channels 1 and 11 is around 10m.
Any closer and you're raising the noise floor in any AP or client that
is receiving a packet.
Now, if you add any antenna gain to the equation, then, unless you
manage to find really special antennas, (or use channel filters, etc)
then the gain of both antennas affects the above in a negative way.
The situation is (much) worse with 11g/11a. Due to their (quite
common, but non universal) direct conversion archectures, and some
interesting properties of OFDM, most 802.11g receivers can't
muster even the 35dB of ACR that the IEEE specifies when operating in
the 802.11g/a modes.
It varies by modulation rate, but the minimum ACR for 802.11g/a at
6Mbps is 16dB (alternate is 16dB more, for 32dB). This goes down to
-1dB (yes, -1dB) ACR @ 54Mbps (with alternate channel rejection at
54Mbps down to 15dB.)
If you try the obvious experiment, be sure to make sure that you've
got the same "range" from both cards operating as from one card
operating.
1) establish range for first card with no second card operations
2) enable 2nd card
3) re-check range of 1st card while 2nd card is under full operation
(passing a lot of traffic)
You'll find it appears to work if the clients are at close range.
But the thing you really have to understand is this: In wireless, it
is a sin to throw a received packet on the floor. You really can't
afford to have a near-by AP think the air is clear (because its off-
channel) and then stomp on
the reception of some near-by AP or client. In co-channel
operations, there is a function called Clear Channel Assessment (CCA)
that holds any given radio from transmitting if either
a) there is a strong signal using the wireless medium, or
b) there is a somewhat weaker signal, which is clearly an 802.11
signal (that is, "this" radio has decoded the pre-amble).
We did a bit of work at Vivato to make N radios (13 in the 11b
product, 6 in the 11g product) synchronize via the CCA signal, such
that if any of them was receiving a packet, none of the others would
transmit. The issue at Vivato
was that the 'antennas' for these radios were pointed in different
directions, so the actual signal level might not be high enough to set
CCA, but any of them transmitting would ruin the reception at any other.
Jim
On Jul 17, 2008, at 1:51 PM, Ron Lockard wrote:
They most likely are getting it from Cisco of all places. Cisco
talks about
using 3 non-overlapping channels when establishing a multi-AP roaming
wireless network with their APs, and they explain the reasoning
behind it in
their documentation. Bottom line, if there is too much overlap on
the same
channel the device may have issues with "hearing" the AP it is
trying to
connect to. I'm not going to get into the right or wrong here as
what works
with one may not with another AP/device, but their method does work
at least
with their own APs. I've implemented a multi-AP wireless network in
a large
sub-zero freezer warehouse of all places using their APs and every
AP is on a
different channel than an adjacent AP. The hand-offs are clean and
quick as
the signal strength drops from one AP and they see the next one on a
different channel. The users run around on forklifts with wireless
scanners
so their movements are fairly quick also.
Ron
On Thursday 17 July 2008 15:01, Jim Thompson wrote:
Assuming you want continiois coverage, same channel is actually best,
unless you can go cross-band, which impacts roaming.
The number of people who don't understand this, and instead want to
talk about "3 non-overlapping channels" and other cr*p is amazing.
Same ESSID is what you want, too.
Jim
On Jul 17, 2008, at 4:58 AM, Rainer Duffner <[EMAIL PROTECTED]>
wrote:
RB wrote:
I'm not very familiar with building large-scale WLANs, but AFAIK,
it's a
little more than just buying enough APs and placing them in the
right
spots...
I am, and it actually is just that. If you already have UTP ports
within 300' the AP locations, it's by far the most effective
route -
then you only have to worry about channelization and overlap.
That's what I was thinking: isn't it a problem to have to APs with
same SSID (and maybe the same channel) in reach of each other?
Don't the clients get confused? Or are the drivers usually smart
enough not to flap between the two?
Sorry, I'm just curious...
Regards,
Rainer
