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I've been interested in networking for awhile and have a basic question that was never answered or brought up in any of my classes.

I was looking at the 802.11 WLAN frame format:

enter image description here

It looks like the order of the 802.11 MAC header starts with the Frame Control field (2 Bytes), which is the Version, then the Type, Subtype, and then the Frame Control Flags, in that order.

I then looked at a Wireshark capture and saw that for some reason the bits were "reversed":

enter image description here

If the 802.11 MAC header shows that the "Protocol Version" is "first", why does Wireshark show it to be the two bits on the "right" most in the Frame Control field bits?

When computers receive bits over the wire (or in this case over RF), do they really receive the bits in the order that Wireshark is showing?

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    – Ron Maupin ♦
    Commented Feb 19, 2018 at 19:39

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Many transmission systems specify the byte ordering separately from the bit ordering.

Ethernet and as far as I know all derived systems are least-significant bit first. IEEE 802.3-2012 Section One 3.3 "Order of bit transmission" says "Each octet of the MAC frame, with the exception of the FCS, is transmitted least significant bit first"

http://standards.ieee.org/getieee802/download/802.3-2012_section1.pdf

Bit ordering is normally invisible to software, you'll only see it in documentation or if you get an oscilloscope out.

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That's right. The numbering in binary system starts form left. For easier reading in the documentation you starts on the right. See the GIF bellow:

source: https://en.wikipedia.org/wiki/Binary_number source: https://en.wikipedia.org/wiki/Binary_number

A very short explanation, but I hope it helps.

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  • Thanks. I understand that the binary starts from the left, but how does the order of the bits received across the wire correlate with the frame fields? I worked out several problems, but am still confused. And within each field, how is the order of bits interpreted?
    – slantalpha
    Commented Oct 18, 2017 at 16:40
  • Not sure what you mean by the numbering starts of the left in binary systems: the whole problem is there are a lot of conventions and it can be tricky to find which applies from which author at which part of a communications system.
    – jonathanjo
    Commented Oct 18, 2017 at 18:56

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