[HN Gopher] How a cable modem works (c. 2002)
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       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...
        
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