Does UDP broadcast translate into 802.11/Ethernet broadcast?

1

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.

ToniAz

Posted 2018-07-02T15:43:42.773

Reputation: 145

2This 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 – 2018-07-02T15:46:51.357

@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 – 2018-07-02T16:13:46.803

1Either; 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 – 2018-07-02T17:01:45.990

@grawity Thank you. What should have been part of the question? – ToniAz – 2018-07-02T18:18:18.053

@ToniAz if that is your actual question it should be in your question, currently it isn't. – Seth – 2018-07-03T06:13:24.530

Answers

2

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.

dirkt

Posted 2018-07-02T15:43:42.773

Reputation: 11 627

Asked: 2018-07-02T15:43:42.773

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Active: 2018-07-03T06:16:12.937