🚀 go-pugleaf

In article <bwilson-1301960848000001@bwilson.traveller.com> bwilson@traveller.com (Bob Wilson) writes:
>How do cable-modem systems (i.e., digital in a 6 mhz band on a cable
>system), handle channel arbitration? 
>
>Cable networks are orders of magnitude too large to support CSMA unless
>some sort of bridging / routing is used. This in turn introduces a host of
>other problems. Most of the network available information indicates
>transmission and reception is split within the 6 mhz bandwidth (i.e., full
>duplex). My guess is they use part of the bandwidth for channel
>arbitration but I've not found that in my researches. In fact, considering
>the 75 ohm cable and likly variability of cable plants, I would doubt that
>CSMA would be an optimum solution. 
> ...

There are a bunch of schemes for sharing the bandwidth.  One popular
proposal is DQDB.  DQ-WRAP (however you spell it) was also being
pushed.  At least one of the commercial cable modem vendors is using
a TDM/polling scheme driven by the box at the head end.

It's been a while since I heard from the mailing list for IEEE 802.14
(the IEEE date over cable TV standards committee).  Either I've fallen
off the list, or they finally took the suggestion of one of the
commercial participants and disbanded.  No, never mind.  Standards
committees never disband themselves.


Vernon Schryver    vjs@rhyolite.com

The current generation of RF cable modems use a modified form CSMA/CD,
allowing symmetrical,full-duplex communication up to 100 miles from the
head-end (round trip).

Zenith Electronics HomeWorks allows, 4,  500Kb/s networks  to reside on a
6 Mhz channel. We need 2,  6 Mhz channels to do this; one in the
downstream (from the head-end) working at 50 Mhz to 750Mhz and the return
from 12 to 108 Mhz. A device called a frequnecy translator is located in
the head-end. This up converts the upstream and makes the system appear
full-duplex.

The system is set up on a hybrid-fiber-coax system which has fiber going
to a neighborhood  node of 500 tp 1000 home. From the fiber node, coax is
dropped into each house. The use of fiber optics allows the reuse of
spectrial frequencies; a router at the CATV head-end feeds ports to each
fiber-node with bridging beteween nodes.

The "next' generartion of cable modems do not use CSMA/CD for
transmission.  Zenith's next modem will use CDMA  (code division
modulation) allowing speeds of 256Kb/s up to 10.5 Mb/s allowing CBR, ABR
and VBR operation (voice, video and data).

James Treuhaft: Zenith Electronics 

cablemodem@aol.com (Cablemodem) wrote:
>The current generation of RF cable modems use a modified form CSMA/CD,
>allowing symmetrical,full-duplex communication up to 100 miles from the
>head-end (round trip).
>
>Zenith Electronics HomeWorks allows, 4,  500Kb/s networks  to reside on a
>6 Mhz channel. We need 2,  6 Mhz channels to do this; one in the
>downstream (from the head-end) working at 50 Mhz to 750Mhz and the return
>from 12 to 108 Mhz. A device called a frequnecy translator is located in
>the head-end. This up converts the upstream and makes the system appear
>full-duplex.
>
>The system is set up on a hybrid-fiber-coax system which has fiber going
>to a neighborhood  node of 500 tp 1000 home. From the fiber node, coax is
>dropped into each house. The use of fiber optics allows the reuse of
>spectrial frequencies; a router at the CATV head-end feeds ports to each
>fiber-node with bridging beteween nodes.
>
>The "next' generartion of cable modems do not use CSMA/CD for
>transmission.  Zenith's next modem will use CDMA  (code division
>modulation) allowing speeds of 256Kb/s up to 10.5 Mb/s allowing CBR, ABR
>and VBR operation (voice, video and data).
>
>James Treuhaft: Zenith Electronics 

    Of the 6MHz-wide channel, conventional television/CATV uses some
for luminance (black-and-white [brightness]) some for chrominance (color)
some for audio, some for closed-captioned, etc.  Our prototype cable
modems (well, they're much more than just modems...) run full-duplex
1.25Mb/sec, using most of the video-parts (luminance/chrominace) at the
(lower end of the channel) for carrying digital signals.  In the forward
direction, our media-arbitration mechanism will involve the audio part
of the signal (towards the high end of the 6MHz) as an analogy to a
"carrier" in the "CS" portion of "CSMA/CD" (Carrier Sense - Media Access/
Collision Detect).  Since forward traffic is "pure," and the audio signal
is far enough away from the video domain, utilizing the audio portion should
reliably detect a desire for media-access.  (Plus, it'd sure be cool
to watch *and* listen to incoming network traffic on your TV!)

    I'd like to hear more about the Code-Division Modulation idea.

    Payter Versteegen.
    payter@niccolo.gsfc.nasa.gov

=;)

PS: BTW, closed-caption is at the very front of the RF signal, a bit too
    close for [cheaper] hardware to distinguish from luminance.