Does a router have a receiving range?

Question

So my dad bought a TP-Link router (Model No. TL-WA7510N) which apparently has a transmitting range of 1km; and he believes that it also has a receiving range of 1km.

So he's arguing with me that the router (which is a trans-receiver) can communicate with any device in the range of 1km whether or not that device has a transmitting range of 1km.

To put it graphically:

+----+                       1km                        +----+
|    |------------------------------------------------->|    |
| TR |                                                  | TR |
|    |                                             <----|    |
+----+                                              100m+----+

So here's the problem:

1. The two devices are 1km apart.
2. The first device has a transmitting range of 1km.
3. The second device only has a transmitting range of 100m.

According to my dad the two devices can talk to each other. He says that the first device has a transmitting and a receiving range of 1km which means that it can both send data to devices 1km away and receive data from devices 1km away.

To me this makes no sense. If the second device can only send data to devices 100m away then how can the first device catch the transmission?

He further argues that for bidirectional communication both the sender and the reciver should have overlapping areas of transmission:

According to him if two devices have an overlapping area of transmission then they can communicate. Here neither device has enough transmission power to reach the other. However they have enough receiving power to capture the transmission.

Obviously this makes absolutely no sense to me. How can a device sense a transmission which hasn't even reached it yet and go out, capture it and bring it back it. To me a trans-receiver only has a transmission power. It has zero receiving power.

Hence for two devices to be able to communicate bidirectionally, the diagram should look like:

Hence, from my point of view, both the devices should have a transmission range far enough to reach the other for bidirectional communication to be possible; but no matter how much I try to explain to my dad he adamantly disagrees.

So, to put an end to this debate once and for all, who is correct? Is there even such a thing as a receiving range? Can a device fetch a transmission that would otherwise never reach it? I would like a canonical answer on this.

Answer 1

Firstly, there's no such thing as a simple "transmitting range". It really does depend on a lot of factors.

At the end of the day, what determines whether you can establish two-way communications is if each end can receive a clearly distinguishable signal from the other. This is often expressed as a "signal to noise ratio" - how much of what it's receiving is a meaningful signal, rather than random or unrecognisable noise? Without interference, there's no sharp drop-off - the signal gets gradually weaker, until the ratio is too low. This is important later.

This signal to noise (SNR) ratio depends on many things. In this case of wireless communication over a long range, you can have atmospheric effects (rain? lightning?), refraction and reflection off buildings (especially skyscrapers, especially metal) and just plain distance.

You usually can't change the environment, you can't demolish skyscrapers, and your device needs to work in cities. So you're left with changing the transmitter and receiver.

Now, one of the ways you can boost the SNR is by increasing transmitter power, or directionality (rather than transmitting most of the power off into space, send most of it in the direction of the receiver). This is probably the closest to the "transmission range" you mention, and perhaps they measure their "range" by how far away their signal is detectable (maybe only in optimal conditions?). This is a change on the transmission side.

On the receiver end, you can also make improvements. Some receivers are better than others at picking out a signal when there's a lot of noise. Some are better at giving you higher usable bandwidth when the SNR is high. Some are just terrible at everything. However, the transmission power and directionality tends to have a far greater effect than any changes on the receiver side.

So, to summarise, yes different receivers can pick up the same transmitter (and environments) from different max distances. However, it is impossible to express this difference as a simple "range", and also impossible to completely disregard the other end - the transmitter on the other end must be able to supply enough power. You aren't "catching the signal" from a further distance, you're only better at filtering what little signal you can get from the noise.

---

For a simple analogy, consider sound. If someone is shouting at you across a busy street, there's going to be quite a bit of noise. You can have two different people listen to it, and one might be better at picking out the speech from the noise. Perhaps one of their hearing is better - that doesn't necessarily mean they can hear from further away, it could be that the one with worse hearing just hears the sounds all blended together. Maybe they can tell someone is shouting something (they can pick up a signal), but they can't understand what they're shouting (too much noise to decode).

And if the shouter decides to whisper instead, neither will hear them no matter what. If the shouter uses a megaphone, they're more likely to be audible - and understandable.

Answer 2

If we were in the days of the Titanic when nothing existed in those frequencies then basic laws apply. But even then were looking at many factors.

Each radio is being transmitted thru a very cheap diplexer in most cases to utilize 1 antenna situation. This alone cuts a lot of the range. But we will instead discard all that and bse ourselves into a perfect world situation or laboratory shielded metal box conditions.

Discarding variations in receiver design, and assuming all receivers arr identical in all aspects and same for the transmitters then its quite simple. Then your graphs are based on 1km units get just that 1km ranges and the third 100m unit can talk to either of the other 2 units at a range of 100m.

Now lets face reality. First off you need at least 25% overlap because the signals arr fm digital multi frequency band hopping mullti quadrature.. Meaning to even get perfect reception you need about 50% oveelapping. Next given that at any time frame there could be packet collision you can expect data loss of unknown amounts. Then factoring in that as well the 100m unit becomes at a loss by that factor because it cannot compete as much for the lost data packets and so effectively it loses even more.

Now ww fast forward to 2014 remove the metal box and the Titanic rf free conditions. Throw us into an rf jungle like an area in the middle of the city of LA or New York, where every digital channel is completely full and a variety of buildings and other interference is present. Well in that normal scenario the perfect world goes all away. You would be lucky to get it to work 1m away for any unit.

So of course in reality there is no such thing as 1km or 100m ranges. And you cannot change the weather, move buildings or even account for any moving objects, i.e. people cars planes and trains. So manufacturers are really just tossing out a Titanic world figure to make you feel good and convince you to have to own one.

It all boils down to this. If your happy and it has enough range without dead spots then all those facts and figures go right into the rrash.

Rewind a bit to another similar experience I once had. Back in the late 1980's I used dialup modems at speeds up to roughlly 19kbps. One time I had trouble with my phone line. The phone co guy fixed the problem which was rrlated to water. Anyhow he asked me what I usedthe line for and I said data. He stated that I could never go above 1200 baud speed. My reply was Ok but don't tell my modem that please and I then showed him that indeed it did go as fast as averaging 16kbps or higher. It blew his mind.

Answer 3

Consider this analogy: If he doesn't understand that he is beyond help.

You have a megaphone, he doesn't.
You both go out in the countryside a kilometer apart.
When you yell something at him through the megaphone he can probably just about hear you.
Whether or not is is understandable is another matter (background noise, distortion, etc.)
When he yells a reply, are you going to hear him?
Not likely.
You may just about catch some sound if the wind is coming from where he is standing but that would be just about it. Not usable for 2-way communication.

It works exactly the same way for radio-signals. Your logic with the pictures is spot on.
(And that is in an ideal theoretical setup without things like buildings and EMF noise getting in the way.)
See the other answers about that too.