The thought occurred to me that I have a an old DOCSIS 2.0 Motorola SB5100 modem in a bin in the garage, that was my home modem several years ago, but long since replaced with a series of DOCSIS 3.0 modems. However, it SHOULD still work (which is why I kept it - as a backup). I figured even though it wasn't authorized on Comcast's network, it should still try to connect and fail the authentication.... and I was right.
I tried it first at home to make sure it would work. Since it's an older design with individual lights, it's easy to tell when it's connected to the local node: The "Receive" and "Send" lights will come on in sequence, followed by a blinking "Online" light, meaning it is talking to the node, but there's no Internet:
That's really all you need to do if you just want to check that the line is working, but I went a little farther to check the signal levels to make sure we didn't have a bad connection or cable.
So I logged into the modem from my laptop connected directly to it (the default IP for all Motorola modems at that time was 192.168.100.1). Then I brought up the Status page which confirmed that the modem was connected to the node, but was offline (this page will show nothing "Done" if it's not seeing the node, and Acquire Downstream Channel will say "in progress"):
Also, if you check the Logs page, you'll see a list of everything connecting, but the modem will be "unauthorized".
Next I went to the Signal page and this is where you find out how good of a connection you have:
I have, in the past, been told by Comcast engineers, and through research online, that these are the ranges of the numbers you want to see, and my numbers above are all in the green EXCEPT Downstream Power level is a little low, but I fixed that by removing the splitter. She's only going to use a single modem, so I connected it as a direct run from the box.
Downstream Signal/Noise Ratio: >30dB
Downstream Power Level: -15dbmV to +15dbmV
Upstream Power Level: 37dbmV to 55dbmV (the lower this number is, the better)
