[HN Gopher] How a cable modem works (c. 2002) ___________________________________________________________________ How a cable modem works (c. 2002) Author : jqcoffey Score : 43 points Date : 2022-07-30 15:31 UTC (1 days ago) (HTM) web link (support.usr.com) (TXT) w3m dump (support.usr.com) | teeray wrote: | > All data that is present on the downstream is encapsulated into | MPEG-2 frames. | | In other words, "you're now watching The Internet Channel(tm)" | formerly_proven wrote: | Only slightly more ridiculous than the ADSL encapsulation | stack. | password4321 wrote: | https://news.ycombinator.com/item?id=31358855#31362310 | | MoCA: _run your ethernet over the cable tv coax in your house_ | gregmac wrote: | 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 curious if | there's more details behind why this could even happen. Did this | also mean you'd be able to sniff other people's network traffic? | p_l wrote: | 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. | crancher wrote: | 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. | Aloha wrote: | I do! It was before they started blocking NetBIOS over TCP/IP. | bob1029 wrote: | I do recall this. There were so many insane networking issues | that cropped up in the early days of "always-on" internet. | grubbs wrote: | You also used to be able to plug in any standard docsis cable | modem, change DNS, and you'd have free internet. | ninkendo wrote: | 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. | [deleted] | a-dub wrote: | > 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 represents computer | information. Due to the "spectral shaping" of a data signal, | there are no video or audio signals present, and the graph looks | like Figure 2-3. | | this seems wrong. i think figure 2-2 is an analog ntsc video | channel and figure 2-3 is a digital mpeg-2 or docsis over mpeg-2 | channel. both of the digital channels should have the same | spectral envelope. | | interesting that they put mpeg-2 headers on the data frames, | probably "system" frames and done so for compatibility with | existing headend and stb equipment. | drmpeg wrote: | 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... ___________________________________________________________________ (page generated 2022-07-31 23:00 UTC)