Subject: [manet] Is it difficult to receive simultaneously over multiple channels?

From: Leon Martin <[email-masked]>
Date: Tue, 5 Oct 2004 10:50:59 -0700 (PDT)

Hi,

In 802.11 multiple channels are defined. Ususally a
node listens on only one of them. I was wondering if
it would be possible to let a node listen (scan)
multiple channels? 

Thanks.

- Leon

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---

From: [email-masked]
Date: Tue, 5 Oct 2004 20:22:54 +0200 (CEST)

Hi :=),

When a host is listening on a particular channel, it cannot hear
communication taking place on a diferent channel. Due to this, a new type
of hidden terminal problem occurs in this multi-channel environment
(multi-channel hidden terminal problem). However, protocols  are proposed
to enable hosts to utilize multiple channels by switching channels
dynamically to increasing network throughput.

Hakim.

---

From: David Young <[email-masked]>
Date: Tue, 5 Oct 2004 13:33:16 -0500

> it would be possible to let a node listen (scan)

Yes.  In the 802.11 chipsets by Realtek, ADMtek, and Atheros, there is
no firmware mediating access to the synthesizer, so you can rapidly tune
the radio from channel to channel.  I figure you can tune a new channel
in *much* less than a minimal packet transmission time, but somebody
would have to try it to see.

You cannot listen to all channels at once.  At least, not very well.
An 802.11b receiver tuned to, say, ch 6, can usually receive packets sent
on *any* channel (ch 1 - ch 11), but with high attenuation (tens of dB).

Dave

-- 
David Young             OJC Technologies
[email-masked]      Urbana, IL * (217) 278-3933

---

From: Geethapriya Thamilarasu <[email-masked]>
Date: Tue, 5 Oct 2004 14:45:05 -0400 (EDT)

hi,
I think the use of directional antennas can enable a node to scan multiple
channels simultaneously.

GP

---

From: "John Mullen" <[email-masked]>
Date: Tue, 5 Oct 2004 13:15:26 -0600

Hi Leon,

In general, if channels are established by a pattern of slots (e.g.,
every fourth slot) on a common frequency, it is possible for a receiver
to monitor all channels.  However, if channels are implemented as
distinct transmission or sub-frequencies, then it is possible for more
than one channel to be active simultaneously, and you would need a
separate receiver for each channel.  This is, of course, feasible, but
may not be practical.  Remember that the node will have to do at least
some processing of the off-channel packets and, unless they offer
something of value, that could be simply a waste of limited power.

John Mullen

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Leon Martin
Sent: Tuesday, October 05, 2004 10:51 AM
To: [email-masked]
Subject: [manet] Is it difficult to receive simultaneously over
multiplechannels?

Hi,

Thanks.

- Leon

---

From: [email-masked]
Date: Tue, 5 Oct 2004 21:14:44 +0200 (CEST)

> I think the use of directional antennas can enable a node to scan multiple

yes, you are right, The potential benefits of using directional antennas
include range extension, better spatial reuse, multipath diversity,
interference suppression, capacity increase, and data rate increase.
however, the major problem is the power consumption.

hakim

---

From: Jim Thompson <[email-masked]>
Date: Tue, 05 Oct 2004 09:47:58 -1000

> would have to try it to see.

Likely, but most of the drivers I've seen for Realtek and Atheros 
currently implement a chip reset after changing channels, which consumes 
a whole lot of time.

So, some work remains to be done.

> You cannot listen to all channels at once.  At least, not very well.

Engim has a chipset that allows this.

> on *any* channel (ch 1 - ch 11), but with high attenuation (tens of dB).

This is just a lack of selectivity in the receiver, and is nearly always 
not a good thing.  At a high enough RSSI, it is possible to decode 
packets sent on say, ch5 and ch7 when the receiver is tuned to ch6. 
(This is one of the reasons why the channel is advertised in 802.11's 
beacon frames.)

---

From: Geethapriya Thamilarasu <[email-masked]>
Date: Tue, 5 Oct 2004 16:07:12 -0400 (EDT)

> however, the major problem is the power consumption.

Hi,
Are n't directional antennas providing energy conservation,
because of directionally transmitting data instead of wasting power  
transmitting in all directions?

how do you say the major problem is power consumption?

Regards
GP

---

From: [email-masked]
Date: Tue, 5 Oct 2004 22:36:36 +0200 (CEST)

Sorry, lot of energy is wasted when nodes equipped omni-directional
antennas and not directional antannas.

directional transmissions cause some serious problems in an ad-hoc
environment. These problems are the increase of the instances of hidden
terminal problem, the problem of deafness and the problem of the
determination of neighbors’ location.

Hakim

---

From: "John Mullen" <[email-masked]>
Date: Wed, 6 Oct 2004 09:12:37 -0600

Hi,

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Jim Thompson
Sent: Tuesday, October 05, 2004 12:48 PM
To: David Young
Cc: [email-masked]
Subject: Re: [manet] Is it difficult to receive simultaneously over
multiplechannels?

David Young wrote:
>> <snip>

> An 802.11b receiver tuned to, say, ch 6, can usually receive packets 
> sent on *any* channel (ch 1 - ch 11), but with high attenuation (tens 
> of dB).

------------------

Actually, it is the other way around.  High selectivity would reduce the
number of channels that can be simultaneously received.

If you want to receive on two or more channels that are simultaneously
active, you must have dedicated circuitry for each channel.  Otherwise,
the RF signals will mix and interfere with each other, causing data
collisions, rather than distinct data streams.  

John Mullen

---

From: "John Mullen" <[email-masked]>
Date: Wed, 6 Oct 2004 09:25:24 -0600

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Geethapriya Thamilarasu
Sent: Tuesday, October 05, 2004 1:07 PM
To: [email-masked]
Cc: [email-masked]
Subject: [manet] power consumption

> yes, you are right, The potential benefits of using directional 
> antennas include range extension, better spatial reuse, multipath 
> diversity, interference suppression, capacity increase, and data rate 
> increase. however, the major problem is the power consumption.

Hi,
Are n't directional antennas providing energy conservation, because of
directionally transmitting data instead of wasting power  
transmitting in all directions?

Regards
GP

--------------------------------

The main advantage of directional antennas is in reception, because the
volume of interference and noise can be sharply reduced.  In
transmission, the benefits are mixed, especially in an urban
environment, where it is not obvious in which direction the transmitter
needs to be pointed.  Directional transmission can be very effective in
point-to-point links, but when things are in motion, the problem is
significantly more complicated.

The problem in ad hoc nets is that things are in motion and any node
might need to communicate with any other.  Unless receiving a particular
data stream, therefore, a node would need to listen for a signal from
any direction.  Even when receiving a stream, motion might require
adjustments in transmitter and receiver antenna settings.  Also, until a
transmitter locates a receiver, it will have to broadcast.  Thus, the
directional antennas need to be adjustable, not just in direction, but
also in degree of directional selectivity.  This implies some sort of
electronic circuitry that will require power to operate.  Also, some
directional antennas are more lossy than their omnidirectional
counterparts.

So, the impact on power depends on the fraction of time and degree to
which selective transmission can be employed in the transmitter vs. the
energy consumed in operating the directional circuitry and antenna in
both the transmitter and receiver.

John Mullen

---

From: "Christian Kuhtz" <[email-masked]>
Date: Wed, 06 Oct 2004 14:18:26 -0400

> collisions, rather than distinct data streams.

Seems that's a fairly easy to solve problem.  Anyone could cobble this
together today, and there's no real reason why you couldn't do that in a
more integrated fashion if there is market demand.

Thanks,
Christian

*****
"The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential, proprietary, and/or privileged material.  Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited.  If you received this in error, please contact the sender and delete the material from all computers."  118

---

From: Jim Thompson <[email-masked]>
Date: Wed, 06 Oct 2004 09:59:00 -1000

> more integrated fashion if there is market demand.

Obviously, you should file the patents now.  You'll shortly be proven 
wrong, or become a very rich, and very famous man.

---

From: "John Mullen" <[email-masked]>
Date: Wed, 6 Oct 2004 14:12:22 -0600

-----Original Message-----
From: Jim Thompson [mailto:[email-masked]] 
Sent: Wednesday, October 06, 2004 12:59 PM
To: Christian Kuhtz
Cc: John Mullen; 'David Young'; IETF manet
Subject: Re: [manet] Is it difficult to receive simultaneously over
multiplechannels?

>>active, you must have dedicated circuitry for each channel.  
>>Otherwise, the RF signals will mix and interfere with each other, 
>>causing data collisions, rather than distinct data streams.
> 
> 

> together today, and there's no real reason why you couldn't do that in

> a more integrated fashion if there is market demand.

Well, filing patents is no guarantee.

http://www.inventions.org/culture/female/lamarr.html

:-)

John Mullen

---

From: Jim Thompson <[email-masked]>
Date: Wed, 06 Oct 2004 10:41:51 -1000

> number of channels that can be simultaneously received.

What I described, and the statement I made are accurate.

> collisions, rather than distinct data streams.  

I've already mentioned Engim.  Note that GNUradio can receive two FM 
stations simultaneously.

http://www.gnu.org/software/gnuradio/examples.html

Extending this to 802.11 is left as an exercise for the reader.

---

From: "John Mullen" <[email-masked]>
Date: Wed, 6 Oct 2004 15:09:18 -0600

Hi Jim,

Well, not exactly.  The code you refer to acts on the IF signal, which
is extracted from the media by means of a RF front end.  

http://comsec.com/wiki?RfFrontEnd 

This would mean that for this code to work, the two radio channels would
have to share a common RF frequency channel.  This is a common technique
in FM, allowing the transmission of stereo as a sum and a difference
signal, but is essentially the result of combining the signals at the
transmitter for this purpose.  I assume that the signals being decoded
in the ad hoc net would be from two different transmitters, in which
case, such a technique would not work.

There are sophisticated techniques that involve FFTs and such, but the
radios that use them are not cheap at all.

I was not able to find the detail in the link you supplied, but from its
description, it is referring to two channels in a cable modem.  Such
channels are generally multiplexed and the problem of pulling out two or
more channels from a multiplexed signal is completely different than
that of receiving two different radio channels that originate from two
different transmitters.

John Mullen

-----Original Message-----
From: Jim Thompson [mailto:[email-masked]] 
Sent: Wednesday, October 06, 2004 1:42 PM
To: John Mullen
Cc: 'David Young'; [email-masked]
Subject: Re: [manet] Is it difficult to receive simultaneously over
multiplechannels?

> Actually, it is the other way around.  High selectivity would reduce 
> the number of channels that can be simultaneously received.

---

From: "Christian Kuhtz" <[email-masked]>
Date: Fri, 08 Oct 2004 13:47:49 -0400

Why?  There's nothing preventing you from assembling a system, with several
802.11b or 802.11a cards, for example, and using them simultaneously (as
long as you use channels which don't interfere).

I think it all depends on the specifics of what is attempted.

But thanks for the flame anyway.

Cheers,
Christian

*****

---

From: Jim Thompson <[email-masked]>
Date: Fri, 08 Oct 2004 08:02:23 -1000

> I think it all depends on the specifics of what is attempted.

OK, just for the sake of discussion.  Lets put two 802.11b or 802.11g 
cards in the system, fit them both with the antenna(s) of your choosing, 
and locate those antennas 1m apart.

Please let me know which two channels you pick.  Be sure to also specify 
the transmit power of the cards, your intended modulation rate, etc.

Jim

---

From: "Christian Kuhtz" <[email-masked]>
Date: Fri, 08 Oct 2004 14:15:25 -0400

Jim,

Assuming it's 802.11b and you're in the U.S., why couldn't you have three
separate WLANs on channels 1, 6, and 11?

Thanks,
Christian

On 10/8/04 2:02 PM, "Jim Thompson" <[email-masked]> wrote:

>> "The information transmitted is intended only for the person or entity to
>> which it is addressed and may contain confidential, proprietary, and/or
>> privileged material.  Any review, retransmission, dissemination or other use
>> of, or taking of any action in reliance upon, this information by persons or
>> entities other than the intended recipient is prohibited.  If you received
>> this in error, please contact the sender and delete the material from all
>> computers."  118
> 

*****
The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential, proprietary, and/or privileged material.  Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited.  If you received this in error, please contact the sender and delete the material from all computers. 113

---

From: Jim Thompson <[email-masked]>
Date: Fri, 08 Oct 2004 08:37:11 -1000

First, I wasn't flaming, though I admit it was a "trick question".

Its true that the transmit masks of the "non-overlapping channels" (1, 
6, 11) don't overlap.  But none of the 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, 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.

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.

Thus, 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.

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.   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 modes.

It varies by modulation rate, but the minimum ACR for 802.11g 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.

11a isn't immune either.

Jim