Transmit but not Receive

Hi,

First off let me state that I'm a newbie in RF hardware so my apologies
if this is a dumb question. For an experiment I'm conducting I want to
be able to transmit two data streams from two WiFi(b) routers at the
same time. I realize that they have carrier sense avoidance so that if
they are right next to each other they will not transmit at the same
time. Is there a piece of hardware (filter, diode? or something?) that
i can put between the output of the routers and their respective
antennas such that this device will pass the transmit signal to the
antenna but not pass any radiation received from the antenna to the
routers? The idea then would be that the carrier sense would not come
into play as no power would be received from the antennas. Any ideas
or suggestions would be greatly appreciated. Thanks,

Kev

On 22 Aug 2006 11:39:54 -0700, (E-Mail Removed) wrote:

>Hi,
>
>First off let me state that I'm a newbie in RF hardware so my apologies
>if this is a dumb question. For an experiment I'm conducting I want to
>be able to transmit two data streams from two WiFi(b) routers at the
>same time. I realize that they have carrier sense avoidance so that if
>they are right next to each other they will not transmit at the same
>time. Is there a piece of hardware (filter, diode? or something?) that
>i can put between the output of the routers and their respective
>antennas such that this device will pass the transmit signal to the
>antenna but not pass any radiation received from the antenna to the
>routers? The idea then would be that the carrier sense would not come
>into play as no power would be received from the antennas. Any ideas
>or suggestions would be greatly appreciated. Thanks,
>
>Kev
A circulator would do the trick.

http://www.ditom.com/microwave-circulator.htm

https://ewhdbks.mugu.navy.mil/circulat.htm

Rather expensive, sometimes to be found on Ebay.

w.

Helmut Wabnig wrote:
> On 22 Aug 2006 11:39:54 -0700, (E-Mail Removed) wrote:
>
> >Hi,
> >
> >First off let me state that I'm a newbie in RF hardware so my apologies
> >if this is a dumb question. For an experiment I'm conducting I want to
> >be able to transmit two data streams from two WiFi(b) routers at the
> >same time. I realize that they have carrier sense avoidance so that if
> >they are right next to each other they will not transmit at the same
> >time. Is there a piece of hardware (filter, diode? or something?) that
> >i can put between the output of the routers and their respective
> >antennas such that this device will pass the transmit signal to the
> >antenna but not pass any radiation received from the antenna to the
> >routers? The idea then would be that the carrier sense would not come
> >into play as no power would be received from the antennas. Any ideas
> >or suggestions would be greatly appreciated. Thanks,
> >
> >Kev
> A circulator would do the trick.
>
> http://www.ditom.com/microwave-circulator.htm
>
> https://ewhdbks.mugu.navy.mil/circulat.htm
>
> Rather expensive, sometimes to be found on Ebay.
>
> w.

Thanks for the suggestion, a circulator/isolator is what i need. If
anyone has any suggestions on a bargain vendor I'd appreciate it.

Also as a follow up does anyone have a sense as to what the default
carrier sense power threshold is on WiFi routers in dBm? I've seen a
few isolators so far that range from 20dB to 40dB isolation, my router
puts out 20dBm so it would be good to know if one isolator will do the
trick. Thanks again.

Kev

In article <(E-Mail Removed)>,
Helmut Wabnig <...._.--_.-_-..._-._.._--.@.-_---_-._*_.-_-> wrote:

> On 22 Aug 2006 11:39:54 -0700, (E-Mail Removed) wrote:
>
> >Hi,
> >
> >First off let me state that I'm a newbie in RF hardware so my apologies
> >if this is a dumb question. For an experiment I'm conducting I want to
> >be able to transmit two data streams from two WiFi(b) routers at the
> >same time. I realize that they have carrier sense avoidance so that if
> >they are right next to each other they will not transmit at the same
> >time. Is there a piece of hardware (filter, diode? or something?) that
> >i can put between the output of the routers and their respective
> >antennas such that this device will pass the transmit signal to the
> >antenna but not pass any radiation received from the antenna to the
> >routers? The idea then would be that the carrier sense would not come
> >into play as no power would be received from the antennas. Any ideas
> >or suggestions would be greatly appreciated. Thanks,
> >
> >Kev
> A circulator would do the trick.
>
> http://www.ditom.com/microwave-circulator.htm
>
> https://ewhdbks.mugu.navy.mil/circulat.htm
>
> Rather expensive, sometimes to be found on Ebay.
>
> w.

Why would there be a significant problem in the first place....
these aren't CW devices, they are Packetized Spread Spectrum
and two devices next to each other, even on the same channel, would
not run into each other, due to different spreading code roots
anyway..... Hmmmmm, non problem, from where I sit.....

In article <(E-Mail Removed) om>,
<(E-Mail Removed)> wrote:

>> A circulator would do the trick.
>>
>> http://www.ditom.com/microwave-circulator.htm
>>
>> https://ewhdbks.mugu.navy.mil/circulat.htm
>>
>> Rather expensive, sometimes to be found on Ebay.
>>
>> w.
>
>Thanks for the suggestion, a circulator/isolator is what i need. If
>anyone has any suggestions on a bargain vendor I'd appreciate it.

Ummm... realize that a circulator/isolator hooked between the antenna,
and the antenna jack of a typical 802.11b router, is going to dump
essentially _all_ of the incoming power from the antenna into its
dummy load.

This will include any power generated by the other AP's transmitter
and reflected back from the antenna.

It'll also include any power picked up by the antenna.

In short, the AP's receiver will "go deaf". It won't be overwhelmed
by transmitted signal from the other radio, but neither will it "hear"
the signals it's supposed to respond to.

That's fine if all you're doing is broadcasting/multicasting packets,
and don't need to accept packets or acknowledgements in return. It
won't work, however, with a typical 802.11b AP-and-client system, in
which the client device has to transmit to the AP in order to
associate with it.

I really don't know if an 802.11 AP is going to try to broadcast or
multicast packets if it "thinks" that there aren't any client devices
associated with it.

--
Dave Platt <(E-Mail Removed)> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

You wrote:
> In article <(E-Mail Removed)>,
> Helmut Wabnig <...._.--_.-_-..._-._.._--.@.-_---_-._*_.-_-> wrote:
>
> > On 22 Aug 2006 11:39:54 -0700, (E-Mail Removed) wrote:
> >
> > >Hi,
> > >
> > >First off let me state that I'm a newbie in RF hardware so my apologies
> > >if this is a dumb question. For an experiment I'm conducting I want to
> > >be able to transmit two data streams from two WiFi(b) routers at the
> > >same time. I realize that they have carrier sense avoidance so that if
> > >they are right next to each other they will not transmit at the same
> > >time. Is there a piece of hardware (filter, diode? or something?) that
> > >i can put between the output of the routers and their respective
> > >antennas such that this device will pass the transmit signal to the
> > >antenna but not pass any radiation received from the antenna to the
> > >routers? The idea then would be that the carrier sense would not come
> > >into play as no power would be received from the antennas. Any ideas
> > >or suggestions would be greatly appreciated. Thanks,
> > >
> > >Kev
> > A circulator would do the trick.
> >
> > http://www.ditom.com/microwave-circulator.htm
> >
> > https://ewhdbks.mugu.navy.mil/circulat.htm
> >
> > Rather expensive, sometimes to be found on Ebay.
> >
> > w.
>
> Why would there be a significant problem in the first place....
> these aren't CW devices, they are Packetized Spread Spectrum
> and two devices next to each other, even on the same channel, would
> not run into each other, due to different spreading code roots
> anyway..... Hmmmmm, non problem, from where I sit.....

I'm running 802.11b @ 2.4Ghz. As I understand it at 11Mbps (which I'm
using) the signal is "spread" using the CCK code, and each packet/user
uses the same spreading function. So a transmission only goes forward
if the channel is "empty", i.e. the received power at the transmitter
is below some carrier sense threshold (dBm)......

Dave Platt wrote:
> In article <(E-Mail Removed) om>,
> <(E-Mail Removed)> wrote:
>
> >> A circulator would do the trick.
> >>
> >> http://www.ditom.com/microwave-circulator.htm
> >>
> >> https://ewhdbks.mugu.navy.mil/circulat.htm
> >>
> >> Rather expensive, sometimes to be found on Ebay.
> >>
> >> w.
> >
> >Thanks for the suggestion, a circulator/isolator is what i need. If
> >anyone has any suggestions on a bargain vendor I'd appreciate it.
>
> Ummm... realize that a circulator/isolator hooked between the antenna,
> and the antenna jack of a typical 802.11b router, is going to dump
> essentially _all_ of the incoming power from the antenna into its
> dummy load.
>
> This will include any power generated by the other AP's transmitter
> and reflected back from the antenna.
>
> It'll also include any power picked up by the antenna.
>
> In short, the AP's receiver will "go deaf". It won't be overwhelmed
> by transmitted signal from the other radio, but neither will it "hear"
> the signals it's supposed to respond to.
>
> That's fine if all you're doing is broadcasting/multicasting packets,
> and don't need to accept packets or acknowledgements in return. It
> won't work, however, with a typical 802.11b AP-and-client system, in
> which the client device has to transmit to the AP in order to
> associate with it.
>
> I really don't know if an 802.11 AP is going to try to broadcast or
> multicast packets if it "thinks" that there aren't any client devices
> associated with it.
>
> --
> Dave Platt <(E-Mail Removed)> AE6EO
> Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
> I do _not_ wish to receive unsolicited commercial email, and I will
> boycott any company which has the gall to send me such ads!

Yah let me clarify that, I have a hacked up router for running
experiments, all I'm using it for is to transmit broadcast packets, so
no receive information is necessary for my app....I just want to get
rid of carrier sense so that I can have more than one co-located tx
running at once...and as i see it the easiest way is a hardware fix.....

On 22 Aug 2006 16:28:09 -0700, (E-Mail Removed) wrote:

>> Why would there be a significant problem in the first place....
>> these aren't CW devices, they are Packetized Spread Spectrum
>> and two devices next to each other, even on the same channel, would
>> not run into each other, due to different spreading code roots
>> anyway..... Hmmmmm, non problem, from where I sit.....
>
>I'm running 802.11b @ 2.4Ghz. As I understand it at 11Mbps (which I'm
>using) the signal is "spread" using the CCK code, and each packet/user
>uses the same spreading function. So a transmission only goes forward
>if the channel is "empty", i.e. the received power at the transmitter
>is below some carrier sense threshold (dBm)......

Hi All,

Spread Spectrum is merely the 0 level of the protocol stack and has
nothing to do with "going forward." That is a software issue at a
much higher level that deals with packet collisions. The hardware may
augment collision detection through carrier sense (which would only
occur if both share the same code). In fact, thresholds may be well
below the noise floor (dBm) and still work with notably lower
throughput, but this is an even more remote issue, except you may be
fighting poor isolation from even the best of isolators.

73's
Richard Clark, KB7QHC

(E-Mail Removed) hath wroth:

>For an experiment I'm conducting I want to
>be able to transmit two data streams from two WiFi(b) routers at the
>same time.

Through the same antenna or seperate antennas? If one antenna, use a
Wilkinson combiner to isolate the xmitters so there's no intermod.

>I realize that they have carrier sense avoidance so that if
>they are right next to each other they will not transmit at the same
>time.

Sorta. 802.11 is CSMA/CA or collision avoidance, not collision
detection. It will do it's best to prevent simultaneous
transmissions, but it still can happen. The receiver will listen for
data, but will only wait until the end of the random contention time
period. Allegedly, it will wait 3 contention periods before
transmitting. Unfortunately, I've found that it will transmit anyway
after these 3 periods. This is to prevent broadband noise from
creating the ultimate denial of service attack and shut down all
wireless transmissions by transmitting continuous noise.
http://www.zytrax.com/tech/wireless/802_mac.htm

>Is there a piece of hardware (filter, diode? or something?) that
>i can put between the output of the routers and their respective
>antennas such that this device will pass the transmit signal to the
>antenna but not pass any radiation received from the antenna to the
>routers?

Others have suggested a circulator or isolator. That will work.
So will a hybrid ring.

>The idea then would be that the carrier sense would not come
>into play as no power would be received from the antennas.

It might be easier to simply disable the receiver section of your
unspecified wireless device. Many Wi-Fi router use seperate RF power
amplifiers in the TX path. That means there's an exposed T/R swich
somewhere on the board. Find it, chop the RX input line, ground or
terminate the RF input pin, and you're done. If you can identify the
radio chip, you should be able to easily find the RX pin.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558