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This is a very strange question--I'd go so far as to say it's a stupid question.

I'm being told that it is possible to, to describe it briefly, use a cable to connect an access point and a receiver directly to one another. This means that I would unscrew the access point's antenna, and attach one end of a cable to the port. Then, on the wireless receiver, I would also unscrew the antenna and plug in the other side of the cable. I'm being told the connection would work after this, just as a normal Wifi connection would.

Bonus mini-question: if this works, would it still work if a splitter were attached to the access point and multiple receivers plugged in to the network?

What would happen if I do this? Based on my surprisingly deficient knowledge of radio transmission, I don't think it would work. I would like some help knowing why it won't (or will) though, if possible.

This is a somewhat hypothetical question--I realize that Ethernet does this exact job very handily, and I could just throw in a switch instead of the splitter. I simply feel that I should understand this scenario. Thanks for any help you can offer.

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2 Answers

up vote 5 down vote accepted

Yes, this works. In fact it's how a lot of testing of 802.11 gear gets done.

There's one critical thing you're missing from your described setup, and that's sufficient RF signal attenuation so your receivers aren't completely overloaded by your transmitters. You generally need about 60 dB of attenuation between transmitter and receiver. If you have the 3-leg (via splitter) scenario you described, you should put 30 dB of attenuation on each leg, so that all 3 paths end up with 60 dB of attenuation end to end.

I should elaborate on my "60 dB attenuation" rule of thumb.

That's based on the assumptions that most Wi-Fi gear nowadays uses +20 dBm transmitters, and that -40 dBm RSSI is a plenty-strong signal that should allow you to get maximum data rates without overloading your receiver. I've seen poorly-designed Wi-Fi cards that overload not much above -40 dBm RSSI, but I'm told that well-designed radios should be good up to -20 dBm RSSI. -10 dBm RSSI is probably pushing it.

If I were designing an RF cable rig for Wi-Fi testing, I'd shoot for max RSSI values below -20 dBm but above -70 dBm, maybe above -65 dBm. Somewhere around -70 dBm RSSI is where most clients start to be unable to maintain their maximum data rates. If you're designing something with a programmable attenuator or step attenuator to simulate different signal strength conditions, and want to be able to simulate being on the hairy edge of the network and losing signal, make sure you have a way to get to -95 dBm RSSI or less (so with, say, the maximum FCC-allowable +30 dBm (1 Watt) transmitter, you'd need 125 dB or more of path loss. You won't be able to get that much path loss just by putting attenuators in the RF cables. To achieve that much separation, you'll need to put the devices in separate shielded boxes, because the cables and the antenna connectors and things leak a fair amount of signal.

Update 2: Getting tired of trying to make this fit in the Comment box. :-)
@31eee384 In your comment where you mention your 1-to-4 and 1-to-8 splitters, you're close to getting the math right, but it sounds like you didn't take the insertion loss of the splitters into account. You may need a brief lesson in RF power splitters.

In, say, a 1-to-2 splitter, the "Sum" port (the "1" in "1-to-2") is different from the other two ports (let's call these other two ports "A" and "B"). The goal of most splitter designs is to get as much signal as possible from Sum to A and from Sum to B, and as little signal as possible between A and B.

"Isolation" is the attenuation between A and B.
"Insertion loss" is the attenuation between Sum and A, or between Sum and B. Because this is a passive splitter, power from the Sum port is split roughly evenly between A and B. Remember that in dB, adding 3 dB is a doubling of the power, and subtracting (or "attenuating by") 3 dB is a "halving" (cutting-in-half) of the power. So if you take a given input signal on the Sum port, the A port only sees half of that. We say that there is a 3 dB a "insertion loss" introduced by inserting the splitter into your RF path.

Splitters aren't perfect, so there will be a little more insertion loss than just the unavoidable loss inherent in splitting up a signal n ways. So the insertion loss of a specific splitter might be 3.2 dB, which is sometimes quoted as "0.2 above 3 dB", which calls out the fact that even a perfectly ideal 1-to-2 splitter will have 3 dB of insertion loss, and the 0.2 is in a way a measurement of how much (or little) they've varied from the ideal.

A 1-to-4 splitter splits up the signal 4 ways, so each port gets about a quarter, so an ideal one has 6 dB of insertion loss. A 1-to-8 splitter splits up the signal 8 ways, so each port gets about an eighth, so an ideal one has 12 dB of insertion loss.

So in your example, the two clients on the same 1-to-8 splitter will see at least 30 + 22 + 30 = 82 dB of attenuation between them. Good.

Now what about the attenuation between a client on one 1-to-8 splitter and a client on another one? It would be 30 + 12 + 22 + 12 + 30, or 106 dB. You happened to get close with 104, but I think you got that answer the wrong way.

Okay, so what about attenuation between a client and the AP? It would be 30 + 12 + 6 + 30, or 78 dB. Even if you only had wimpy +15 dBm (32mW) transmitters in your AP and clients, that would still come out to an expected RSSI of -63 dBm, which should be good enough to maintain maximum data rates. And if you had full 1 Watt (+30dBm) transmitters, you'd be at -48 dBm RSSI, which is fine. It sounds like you'd even have room in this design to reduce or completely remove the attenuator between the AP and the first splitter.

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Fantastic! I actually need many more splitters than one, so this is perfect. I'm aiming for about 32 receivers connected to the AP, so I think that the insertion loss might be enough to cover the 60 dB. (Correct me if I'm wrong.) If not, I'll just put in extra attenuators. Thanks a lot for this answer. –  31eee384 Jun 22 '11 at 17:31
Ah, I just realized the point of putting equal attenuators on each cable out from the splitter, and I realize my last statement about insertion loss is probably incorrect--two cables sharing the same splitter would only have the insertion loss of that one splitter, while there would be a much greater loss when the path is through many splitters. If I'm thinking about this right, I'm best off finding low-loss splitters and putting attenuators adjacent to every endpoint. –  31eee384 Jun 22 '11 at 18:10
@31eee384, Yes, in your second comment, you got it. Unfortunately, I highly doubt you'll be able to find a way to connect 32 client stations via simple splitters and inline attenuators such that they can all reach the AP simultaneously without blowing each other out. And I'm not sure their collision avoidance mechanisms will be enough to keep the clients from causing problems for each other without sufficient inter-client attenuation. –  Spiff Jun 22 '11 at 19:46
Hmm, the receivers blowing each other out would indeed be unfortunate. The number of clients I'm really going for is around 25, but I'm sure that has the same issue. Forgive my lack of knowledge about how RF works in cables, but why would this happen? If every path is attenuated 60dB (or more), what makes them blow each other out? I just noticed a stat on these splitters called "isolation" rated in dB--would high-dB isolation help the clients not destroy each other? –  31eee384 Jun 22 '11 at 20:12
Thanks for the update, I was wondering where the 60dB came from. I think I'll still need to look up exactly what some of the terminology means, but I think I at least get your math for getting the 60dB (after remembering that dB is a logarithmic unit). I'm not doing Wifi testing, though, so while your second paragraph is very interesting, it doesn't help me much. I'm still not sure why a bunch of clients (thanks for the word, btw, rather than the fallacious "receiver" I used) would overload each other. Could you address that in more detail? Thanks so much for your responses. –  31eee384 Jun 22 '11 at 20:39
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WiFi uses CISMA CA (collision avoidance) so there would be no need for a switch in such a configuration. The WiFi devices would be listening to the spectrum before sending data.... I think the only issues would be related to unanticipated resistance & power going through the wires that may reduce quality / cause issues.

I would be interested to see such a configuration...

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+1 for "I would be interested to see such a configuration". –  Shinrai Jun 22 '11 at 17:21
Hmm, thanks. I mentioned a switch as a part of the Ethernet network I'd rather do, though, not as a part of the Wifi wired network. The wires would be very short, so not much resistance until I try to put in the splitters to make it multi-receiver. Interesting that there's no fundamental property of antennas that makes this impossible though. –  31eee384 Jun 22 '11 at 17:24
A cable is a length of metal / wire that is shielded and has a relatively low-powered transmitter. An antenna is a piece of metal / wire that is not shielded and has a relatively high-powered transmitter. They are working off very similar properties except the protocols in place on these technologies are designed for their targeted use - don't get me wrong though - I would only do your proposed setup for the sake of trying it... it would not be something you would want to use every day, that's what an ethernet network is for. The cable going between WiFi cards will just be big antennas. –  Dustin G. Jun 22 '11 at 21:59
Afaik, antennas work through change in the current passing through them, so I thought that the receiver of the signal might expect a 90 degree shift in the result signal. Also, this is obviously not an ordinary situation--not even for home everyday use. I've already mentioned that I do want to use Ethernet for this, it's just a matter of what is required. I also realize that the cable between the router and the client is basically a big antenna, but that seemed way oversimplified when I first thought about it. Thanks for your answer, though; it was the first, and it did say that it works. –  31eee384 Jun 23 '11 at 4:44
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