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How a cable modem works (c. 2002)

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Re: How a cable modem works (c. 2002)

#11

Post by grubbs »

gregmac wrote: post #4

Does anyone remember early cable modems allowing viewing other computers? What allowed that to happen? I didn't know enough about networking at the time, but I recall seeing this at friends houses in maybe the late 90's. You could go into "Networking" in Windows, and see basically all the PCs on the street/neighborhood. I assume this was with the PC directly connected (no router) and maybe using WINS, but I'd be curi…

You also used to be able to plug in any standard docsis cable modem, change DNS, and you'd have free internet.

Re: How a cable modem works (c. 2002)

#12

Post by crancher »

p_l wrote: post #7
gregmac wrote: post #4

Does anyone remember early cable modems allowing viewing other computers? What allowed that to happen? I didn't know enough about networking at the time, but I recall seeing this at friends houses in maybe the late 90's. You could go into "Networking" in Windows, and see basically all the PCs on the street/neighborhood. I assume this was with the PC directly connected (no router) and maybe using WINS, but I'd be curi…

Essentially you were linked into badly managed network that inter-routed clients on the head end side, without isolating clients. Often with ethernet emulation involved if not straight ethernet going on. Essentially, routing done badly by ISP.

When I moved to Santa Monica (Los Angeles) in January 2000 I made many text files on neighbor Mac desktops explaining how having sharing turned made their private files available to everyone in the 'hood via their cable modem.

Re: How a cable modem works (c. 2002)

#13

Post by drmpeg »

a-dub wrote: post #6

> That 6 MHz is used to encode MPEG-2 frames containing video, color, and audio information that your cable box or TV decodes into picture and sound. If you graphed a single channel provided by the cable operator, it would look similar to Figure 2-2. > A DOCSIS channel can be graphed in the same fashion; however, instead of video, color, and audio information inside the MPEG-2 frames, it contains a data stream that r…

It is incorrect. As you state, a video/audio channel and a DOCSIS channel would look the same on a spectrum analyzer.

Here's a sweep of all the channels (from 80 to 750 MHz) on my Comcast system. This was taken back in 2014, and there were still three NTSC channels (two of which were just a black frame and silence).

https://www.w6rz.net/span.png

A zoom of the last channel at 729 MHz.

https://www.w6rz.net/last.png

MPEG-2 Transport Streams are used for DOCSIS because it's baked into the QAM specification. It's built around 188 byte packets that start with 0x47.

https://wagtail-prod-storage.s3.amazonaws.com/documents/ANSI...

Re: How a cable modem works (c. 2002)

#14

Post by ninkendo »

gregmac wrote: post #4

Does anyone remember early cable modems allowing viewing other computers? What allowed that to happen? I didn't know enough about networking at the time, but I recall seeing this at friends houses in maybe the late 90's. You could go into "Networking" in Windows, and see basically all the PCs on the street/neighborhood. I assume this was with the PC directly connected (no router) and maybe using WINS, but I'd be curi…

IIUC, old cable modem networks were all one simple circuit, such that there was no unicast traffic. Much how Ethernet hubs used to work… everyone who transmits would be communicating with everyone else’s cable modems on the same node, not just the gateway. So it was trivially easy to spy on others’ traffic, and if you plugged your computer straight into the cable modem (and didn’t use a router of your own), it was pretty much as if you were on a LAN with everyone else on the same node (basically your whole neighborhood.)

In the beginning of cable modem rollout, consumer routers were not yet common either, so most people were plugging straight into their modem. Cable companies encouraged this, and would charge for additional cable modems if you wanted to use more than one computer.

Re: How a cable modem works (c. 2002)

#15

Post by lxe »

I didn't think it would combine the downstream cable video as well as "data that is decoded and presented as information available for computer usage (i.e., the Internet). (whatever that means)" into mpeg2 frames.

I was under the impression that internet downstream and upstream are simply operating on different multiple QAM channels in different distinct frequency bands.

Re: How a cable modem works (c. 2002)

#16

Post by userbinator »

Notice that the introduction section of the document is titled "Information for End Users, Customer Support Technicians, Field Engineers, and Network System Administrators Introduction"

It's hard to imagine that just twenty years ago, we treated users far less like idiots, and gave them plenty of documentation even if they wouldn't read or understand it all.

Re: How a cable modem works (c. 2002)

#17

Post by jakebasile »

userbinator wrote: post #16

Notice that the introduction section of the document is titled "Information for End Users , Customer Support Technicians, Field Engineers, and Network System Administrators Introduction" It's hard to imagine that just twenty years ago, we treated users far less like idiots, and gave them plenty of documentation even if they wouldn't read or understand it all.

I get what you're saying, but 20 years ago a larger percentage of end users were computer-literate than today.

Re: How a cable modem works (c. 2002)

#19

Post by Taniwha »

lxe wrote: post #15

I didn't think it would combine the downstream cable video as well as "data that is decoded and presented as information available for computer usage (i.e., the Internet). (whatever that means)" into mpeg2 frames. I was under the impression that internet downstream and upstream are simply operating on different multiple QAM channels in different distinct frequency bands.

essentially yes - the video channels and the docsis channels are distinct - and your street or close neighborhood may share the same docsis (and on-demand TV) while you city might share the same broadcast channels.

The document refers to "qam-lock" IMHO wrongly (I was a cable protocol/encryption wrangler in a previous life) - "qam-lock" is more a matter of tuning to a channel, sampling it to extract the signal (this means extracting a clock from it) then running the signal through enough of the error correction and framing logic so that you can extract the transport stream packets

That's "qam-lock" - at this point you still don't know what's in the stream - you could just look for 0x1ffe PIDs and decide you have DOCSIS - video qams will carry PAT tables in PID 0 which will point to PMTs (which can be in pid 0x1ffe) which in turn point to qams and PIDs containing video/audio/CA/etc (details vary greatly between implementations). Which is a long way of saying "just looking for PID 0x1ffe may not be enough"

Re: How a cable modem works (c. 2002)

#20

Post by azinman2 »

I was surprised to see USR still exists, and they even make modems! Doesn’t seem like they’ve done much of any innovation, and now they’re part of some weird IT corp that I can barely understand what they even do. Who makes these websites for enterprise customers? “We sell solutions for your problems”. Ok but what do you actually do?
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