QAM
/ QAM -> 'kwam' or said as letters Q-A-M /
Modulation lets you vary a carrier's amplitude or its phase. QAM asks the obvious next question: why not vary both at once? By choosing among several amplitude levels and several phase angles together, each symbol can stand for one of many distinct combinations — and the more distinct combinations there are, the more bits one symbol carries. It is like a signalling lamp that can be at several brightnesses and several swing-timings simultaneously, multiplying the messages a single flash can send.
Engineers picture QAM as a constellation: a grid of dots, where each dot is one allowed combination of amplitude and phase, and each dot is assigned a unique bit pattern. 16-QAM has 16 dots, so each symbol carries log2(16) = 4 bits; 256-QAM has 256 dots and carries log2(256) = 8 bits per symbol; modern Wi-Fi reaches 1024-QAM, 10 bits per symbol. The transmitter sends the carrier set to one dot's amplitude and phase; the receiver measures what arrived, finds the nearest dot in the constellation, and reads off its bits. More dots packed into the same space means more bits per symbol — and more speed at the same symbol rate.
But there is no free lunch, and QAM makes the Shannon trade-off vivid. Pack more dots into the constellation and they sit closer together, so it takes only a little noise to push a received point nearer to the wrong dot, causing a bit error. That is why a device uses dense QAM (say 1024-QAM) only when the signal-to-noise ratio is high — close to the router — and drops to sparse QAM (16-QAM or even simpler) as you walk away and noise starts winning. The constellation you can use is set directly by your SNR, which is Shannon's capacity formula in visible, physical form.
16-QAM lays out 16 points in a 4-by-4 grid; the symbol carrying bits 1011 is one specific point. Noise nudges the received point a bit, but as long as it stays closest to that same dot, the receiver still reads 1011. Push to 256-QAM and the dots are 16 times denser — far less noise margin per dot.
More constellation points = more bits per symbol, but each point tolerates less noise.
High QAM numbers in an ad ('up to 1024-QAM!') are only achievable with a strong, clean signal. In real homes with walls and interference, devices spend most of their time at much lower QAM, so the headline speed is rarely sustained.