Ukraine-Russia Megathread 18

CARCOSA [mirror/your pronouns] · edit-2 3 years ago

Ukraine-Russia Megathread 18

learntocod [they/them] · 3 years ago

The cell carrier verizon is a spin out of us military tech research. At least at the time I was researching this (3G) I learned that while AT&T will give your phone a little slice of the frequency range, verizon would spread the bits for your phone out across the whole spectrum. The phone and tower have a way of determining a secret pattern of where/when to look in the spectrum. Bluetooth is similar, but simpler.

More modern tech (especially military shit) is using a much much much broader spectrum and more robust techniques to disguise it. So it’s very hard to monitor and while it’s not impossible to jam, it would require pumping huge amounts of energy across a large spectrum.

frostycakes [he/him] · edit-2 3 years ago

That was actually the 2G days with GSM and cdmaOne, UMTS (what AT&T used for 3G) is a form of wideband CDMA. China even developed a hybrid of the two for their homegrown 3G system that used time slices and spreading out data across the full spectrum within said slices.

Also fun fact, Hedy Lamarr invented the core concept behind CDMA.

learntocod [they/them] · 3 years ago

Shit, you’re right, I’m older than I thought.

Animasta [any] · edit-2 3 years ago

So this is basically what they call frequency hopping, right?

learntocod [they/them] · edit-2 3 years ago

Frequency hopping is what Bluetooth does, a moment at 2.423 GHz, the next moment at 2.457 GHz.

This is a more complex version of that. If you imagine the spectrum like a screen full of static, then it would be like the transmitter and receiver share a secret code about putting information in that static: A random set of pixels for each unit of signal on the spectrum, and the encoding is more complex as well (think of all the different ways you could describe the relationships between the randomly used pixels).

So Bluetooth would look like blips hopping around and, whereas this looks like static

Animasta [any] · 3 years ago

Wait, is the transmitter actually generating misleading static on frequencies currently not in use or it just a metaphor?

PorkrollPosadist [he/him, they/them] · edit-2 3 years ago

It's more of a metaphor. What's happening is that the signal to noise ratio across the band of spectrum is incredibly low, but if you know exactly what parts of the band to look at in that exact moment and filter out the rest, the signal is there. It's not deliberately transmitting noise.

Also to clarify, "frequency hopping" more often refers to older mechanisms like the trunking systems commonly used by police departments and utility companies, where you have a control frequency and a block of communication frequencies which are allocated on demand (often changing several times per second). These are easier to jam than spread spectrum, either by jamming the control frequency or jamming a known block of communication frequencies. They follow a much more distinct and recognizable pattern. The frequency changes, but they still put the full transmission power into narrow slices of spectrum which can be seen on any decent spectrum analyzer (expensive hardware, but hardly state exclusive).