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I am trying to understand how packets are sent from one device to another in a wireless network? (Perhaps the same question also applies to a wired network)

How does a router know in which physical direction to send a packet in order for a particular IP/MAC address to receive it?

Or is this not the case, and is the data sent out or scattered in a random direction, and all devices on a network listen to all network traffic and act only on those matching its IP/MAC address?

If the latter is true, does this not raise security vulnerabilities? And is the same is true of wired networks?

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How does a router know in which physical direction to send a packet in order for a particular IP/MAC address to receive it?

The transmitter doesn't "know" which direction to send. Radio waves, like light, radiate in all directions from the source. Directional antennas can focus energy in a particular direction somewhat, but it's not absolute. In addition, like light, radio can reflect off of surfaces.

That said, newer APs (802.1n or ac) use beam focusing that concentrates energy towards the last known direction of the wireless client. Again, it's a general direction, so there's a lot of energy going other directions as well.

Is the data sent out or scattered in a random direction, and all devices on a network listen to all network traffic and act only on those matching its IP/MAC address?

Not random, but all directions (think of a lightbulb). Clients listen for their MAC address.

Does this not raise security vulnerabilities? And is the same is true of wired networks?

Yes, which is why Wi-Fi is usually encrypted. Only the stations that have the correct key can decrypt the data.

Wired networks have similar concerns, but since they require physical access to the network, it's less of a problem. 802.1x and 802.1ae were developed to address those concerns.

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  • Thank you for your in depth answer. How does the router record the last know direction of the client? The antenna is able to detect this in receipt of information from that MAC address and record the angle with reference to some centre line? – Sam Smith Aug 1 '18 at 17:04
  • Modern access points have multiple antennas, and the phase difference in the received signals is used to determine direction. – Ron Trunk Aug 1 '18 at 17:22

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