Source
r/networking•
petester

How many people have access to a single coaxial bus?

I've done a little bit of googling, but I’m not able to really understand what’s going on with the coaxial tap in my wall. If somebody here has a little bit of expertise and can explain/clarify things it would be great.

I’m trying to understand what kinds of signals anybody would be able to see on a typical coaxial tap. Since most of the infrastructure was originally intended for one-way broadcast of television, I assume the cabling is designed to support pretty high bandwidth. If a television signal in the coaxial is the same as over the air, then 6 MHz per channel multiplied by ~100 available channels would eat up about 600 MHz total. This way, multiple houses would access the same tap / bus to watch TV. I’ve also read that RG-58 cable can support up to 1 GHz of bandwidth, which according to this back of the envelope thinking would leave a bunch of bandwidth left over (depending on all kinds of things). I’m assuming that televisions don’t send a request to a switch about which station they want to listen to, so I could obviously be wrong here.

Broadband internet started to be offered over the same infrastructure, which means that time or freq division would be used to keep the internet connections from different houses separate. Again, this is all pure conjecture and I’m hoping somebody can clarify how things work. The way I’m thinking, I should be able to hook up a frequency analyzer to the cable tap and see the frequency bands being used by one of my neighbors while they’re watching Netflix or whatever. Obviously everything would be encrypted and therefore there would be no way to capture and decode their traffic, but I’m wondering if this is really the way it works because this would imply that a bad guy could jam their neighbor’s internet. Is this an active area for security research?

Thanks for reading if you got this far. I’m really hoping somebody can give me more information.

Posted Feb 8, 2016, 7:38 PM UTC

111
gusgizmoCCNA•

Yes, since you are on a shared coax loop you can do all sorts of nasty stuff like spoofing or intercepting frames intended for another user.

This definitely falls into felony territory.

3
noukthx•

Most DOCSIS modems have some encryption (albeit fairly basic/low level) to protect against some of that.

3
petesterOP•

I didn't know that's what it was called, thanks!

1
gusgizmoCCNA•

I wonder if DOCSIS version 3 moved past DES encryption?

Seems like many providers could still be using legacy or even more lackluster security than DES as well.

1
kenuffff•

it has AES

1
petesterOP•

I'm definitely not 'cool' enough to do something like that. I just started thinking about it a lot while doing some SDR stuff on digital television. The data isn't really sent on that line in the clear, though, is it? It seems like that would be a pretty big security issue...

1
cocaina_rhinoplasty•

No, data is not sent in the clear. Unless someone messed up.

Traffic between the modem and the CMTS that the modem terminates on is encrypted using BPI/SEC.

DOCSIS 1.1 & 2.0 is typically DES, and 3.0 is AES128.

https://en.m.wikipedia.org/wiki/DOCSIS

2
kenuffff•

even if it was sent in the clear, how many sites do you go to with sensitive info are not SSL?

1
CBRjackCCNA RS/W, BCNP, BCvRE•

I work for a Cable ISP and can shed some light on how it all works out.

The cable transport multiple frequencies, usually from about 5MHz to 1000MHz. Those frequencies get allocated, here's an example on how they can be distributed :

5-40 MHz : DOCSIS (data)
54-88 MHz : TV channels
88-108 MHz : FM Radio
108-426 MHz : TV channels
426-498 MHz : HDTV channels
498-660 MHz : VoD and DOCSIS
660-852 MHz : HDTV channels
852-1000 MHz : DOCSIS

Data is transmitted by cable modems on the DOCSIS frequency, at the same time that it is received on different frequencies. MoCA is using 1000-1600 MHz to be able to transmit data at the same time as everything else. Look up FDM, TDM and DOCSIS, it will give you lots of info on how coax data is transmitted.

On the wire, with a frequency analyser, you would see each channel on the 54-88 band since they are the old analog ones. You would also see the FM radio channels. However, everything else uses QAM, quadrature amplitude modulation. A standard QAM setup uses the same bandwidth as an old 6MHz TV signal, but can carry between 3 (HD) and 19 (SD) channels. VoD is also sent on QAMs, a good part of DOCSIS data is also sent on QAM.

You would see blocks of 6MHz using these QAM and without proper decoding equipment (like your set top box or your modem) you cam't see the actual data.

Also, what comes out of your modem is encrypted (albeit weakly...) and isn't sent "in the clear". With proper equipment, you could possibly manage to "see" what all the modems in your cell (125-1000 houses) are sending and try to decrypt it.

I'd be glad to answer any questions you might have. :-)

Hope this helps.

3
kenuffff•

you can put the carriers wherever you want .

1
petesterOP•

I'm honestly a little upset that I didn't see this response until now. Knowing that a cell has 125-1000 cells forces me to seriously rethink how I thought these networks were put together.

Yesterday I saw a guy from the SBE, and he talked about some of this stuff. The only thing there that I would be genuinely interested in, without knowning anything else upfront, is the 8VSB cert. Do you have any idea what these certs are, or do you have an idea of their value in starting a career?

1