I imagine that for any of you that are actually reading this post it’s because you have, in some way, a commitment or passion for Wi-Fi. And so, as a community we have a common experience in that somewhere in our path we were introduced to this technology and it sparked in us a curiosity, an itch, that we just had to find out more about.
We caught the bug.
What was that moment for you? What was the moment where you were like “This is some seriously cool business!!!”?
Mine was modulation.
I’ll never forget the moment where I found out about the way that 256 QAM works and I leaned over to my classmate and was like “My mind is completely blow… I can’t believe this stuff actually works”
I was taking the CWNA class by the magnanimous Rick Murphy from Wireless Training Solutions and he started to explain signaling techniques. He started by talking about that all symbols need to have meaning. And that in computing we assign meaning to bits… 1s and 0s. And different combinations of these two bits that are sent by the transmitter are interpreted by the receiver when the language is commonly understood.
Then he went on to simplify and solidify the concept by offering a visual allegory. He said that if two friends were on the top of two buildings within visual range of one another and wanted to send information they could start with one piece of cardboard. The cardboard was black on one side and white on the other. And it was commonly understood among the friends that the white side was equal to the number 1 and the black side was equal to the number 0. And so if you wanted to send your friend across the way some piece of information you could hold up the white side for the 1 and the black side for the 0.
So black, black, black, white, black, white, black, white, white is interpreted by the receiver as 0,0,0,1,0,1,0,1,1. And depending further up the level of interpretation would get you some kind of meaningful information (At the application layer).
If the data was robust, it would take a very long time to transmit the data this way so it would be much better if you could send combinations of bits. So you’d still need only cardboard, but now you would need enough colors for bit combinations of 00, 01, 10, 11. So perhaps you now have two pieces of cardboard, one with black on one side (00), and white on the other (11) and another with red on one side (01) and blue on the other (10).
So Black, white, blue, red, red, black, blue, white, black is interpreted by the receiver is 0,0,1,1,1,0,0,1,0,1,0,0,1,0,1,1,0,0. Same number of transmissions as the previous example, but twice the data.
This can go farther if we add an additional bit. 000, 001, 010, 011, 100, 110, 101, 111. If we assigned each on of those colors they would be black, gray, blue, red, green, pink, yellow, white. The problem we run into as we add colors is that the interpretation of those colors could get mixed up. Especially if something like a fog rolls in, or if the daylight starts to fade. Pink could be interpreted as red, or green as blue. Which then causes data corruption and requires retransmission.
The two modulation techniques he outlined in these examples are BPSK (Binary Phase Shift Keying) and QPSK (Quadrature Phase Shift Keying). This was cool, but it hadn’t quite blown my mind just yet. When he started getting into QAM is really where it knocked me off my feet.
Obviously all of this talk about cardboard and colors is simply an analogy for a number of complex algorithms and error correction decision making that allows transmitters to adjust their RF waves to be modulated in just the right phase to be able to send the right combination of bits and for it to be received correctly. As the number of bits per transmission increases the harder it is for the data to be received correctly.
When I saw the constellation diagram of 64 QAM for the first time and recognized how minutely each of the waves had to be modulated for both amplitude and phase and how precise those changes had to be made in order to be able to not have data retransmission my mind was officially blown. Then the ante was upped again, when we look at 802.11ac and 256 QAM where 8 symbols per bit are transmitted. I couldn’t even believe that it was possible.
And yet, this goes on silently beyond our seeing all the time in the ubiquitous space around us. Even after studying Wi-Fi even further, my mind is still blown at our ability to manipulate the air in this way.