Setting 1: 1 master access point and 2 slave stations with 802.11 chipsets.

Setting 2: 1 master PC connected to 2 slave PCs through a switch and Ethernet cables.

Scenario: An application running at master wants to broadcast some data to the applications running at the 2 slaves, so it sends a UDP datagram with a broadcast address.

Question 1: Given that all 3 devices communicate via 802.11, does a UDP broadcast datagram translate into 802.11 MAC broadcast/multicast frames? If not, when is the broadcast/multicast used apart from sending beacons?

Question 2: Same as Question 1, but with Ethernet instead of 802.11.

  • 2
    This sounds suspiciously like a homework question... if not, what is the application you are trying to make work, because broadcast definitely works over WiFi. – acejavelin Jul 2 '18 at 15:46
  • @acejavelin I like how vigilant you are! It's not a homework question. I want to know if the MAC overhead (in number of frames) is proportional to the transport overhead (in number of datagrams), i.e. does one UDP broadcast frame = 1 802.11 frame, or is it as many as the number of stations? – ToniAz Jul 2 '18 at 16:13
  • 1
    Either; it varies depending on AP settings (the latter can be more efficient due to the way WiFi broadcasts work). That really should have been part of the question... – user1686 Jul 2 '18 at 17:01
  • @grawity Thank you. What should have been part of the question? – ToniAz Jul 2 '18 at 18:18
  • @ToniAz if that is your actual question it should be in your question, currently it isn't. – Seth Jul 3 '18 at 6:13

Yes, level 3 broadcasts (TCP/UDP) translate into level 2 broadcasts (LAN/WLAN), so the MAC overhead is proportional to transport overhead.

However, the situation on WLAN is a bit more complicated when encryption is used, because special keys for the broadcast are needed. Unicast uses different keys, so other members of the WLAN segment can't eavesdrop on AP <-> STA communication. So a few frames are necessary initially to negotiate these broadcast keys, just like a few frames are necessary initially to negotiate unicast keys. But that's a constant overhead.

You can read details in the standard.

BTW, beacon frames and broadcast/multicast frames are completely different.

|improve this answer|||||

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.