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WLAN-Clients know when to send because they are listening before talking. In many descriptions I've read that they "hear into the router", if some other client is already transmitting his data.

Then I stumbled upon the "hidden-node-problem", which happens when the clients A & C are connected with the router B, but can't "hear" each other due to the distance between them (f.e. each of them is on the other side), so they are "listening" and don't hear anything (Also if the other client is sending) and therefore just send their data with a collision as a result.

Well, but if they just "hear into the router", they would hear, that some other client is transmitting his data and the distance between them wouldn't be a problem.

Question: So are all clients sending "broadcasts" (Sending the data in every direction)? Because then the other clients listen to the other clients (and not to the router) if the router is free to send something to it and then the description of the hidden-node-problem makes sense. (Because there they can't hear each other in that problem due to the distance they are appart).

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  • The term broadcast has a specific meaning in IEEE LAN protocols, and it is not the same thing as you mean. – Ron Maupin♦ Oct 15 '17 at 13:48
  • Therefore I put the apostrophe around "Broadcast". What I menat was if the signal gets spread in every direction by the client, or just to the router – asparagus Oct 15 '17 at 14:10
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    When you send a radio signal through an antenna, it goes in all directions. You can try to direct it with a directional antenna, but some of the signal will still go in all directions. You can get more information on that in Electrical Engineering. – Ron Maupin♦ Oct 15 '17 at 14:25
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So are all clients sending "broadcasts" (Sending the data in every direction)?

You clarified in your comments that you don't actually mean "broadcasts" but rather that you are asking if clients transmit omnidirectionally.

The short answer is that most 802.11 client devices transmit in a pattern that covers a complete 360 degrees horizontally around the device because most are mobile and won't have a fixed facing towards the access point. They do this by using dipole antennas.

The characteristics of how any RF device transmits is not determined by the client, but by the antenna used to transmit. Changing the antenna can have a significant impact on how the device transmits a signal.

Generally speaking, most antennas will transmit some signal in most directions, but depending on the actual characteristics of the antenna in question, that signal transmitted in a certain direction can be very low to the point of being entirely useless. The majority of antennas will have a main "lobe" along which the signal will be strongest, a number of side or back lobes with weaker signal, and a number of areas where very little to no signal is generated.

Additionally, with beamforming in use, a transmitter can also influence the nature of the propagated signal by transmitting specially formulated and timed transmissions from multiple antennas to strengthen the signal in certain directions and/or weaken it in others.

Once a RF signal is transmitted, it is then subject to the physics of RF signals, which can bend and/or be reflected so signal will often cover areas where the antenna didn't transmit initially.

WLAN-Clients know when to send because they are listening before talking. In many descriptions I've read that they "hear into the router", if some other client is already transmitting his data.

This is the first time I have come across this description of a client ability to "hear into the router" but wherever you may have come across this, it is highly inaccurate. Clients can only hear what is transmitted in the air.

Now clients do negotiate timing with the access point and the 802.11 network can use a number of control mechanisms to help manage who talks when, but this does not give the client the ability to "hear into the router" in any way.

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The radio transmission (layer 1)inherently broadcasts at all times. It's similar to early coax Ethernet with the addition of the hidden-station-problem.

The MAC layer defines how the transmissions are spread (unicast/multicast/broadcast).

Arbitration of the shared medium has several mechanisms. The one most often used is RTS/CTS (request to send/clear to send) where a station requests a transmission slot and the access point explicitly tells it to send for a specific time.

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  • So they don't "hear into the router" to check if it's free to send. They listen to the other clients if someone (which broadcasts it's signal (radio transmission) is sending it's data, right? – asparagus Oct 15 '17 at 12:34
  • The "hear into the router" might refer to the RTS/CTS mechanism. In addition, nodes listen for carriers before sending as of CSMA/CA. – Zac67 Oct 15 '17 at 15:42
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    Since CSMA/CA is always present in 802.11 and RTS/CTS or CTS-to-self are only used in particular circumstances, CSMA/CA is clearly the most often used method to arbitrate the shared medium. – YLearn♦ Oct 15 '17 at 18:17

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