Re: use the outer edge of Eithernet or "What is bandwidth anyway"?

Dan Magorian (magorian@ni.umd.edu)
Mon, 15 May 1995 18:12:02 -0400


Yes, a number of desktop video vendors have this type of system. I've seen
one recently at a Sun booth by somebody like Starlight Systems, streaming
real-time full-motion video from a Sun server to Macs and PCs with extra
transceiver to the same RG-58 cable running baseband ethernet. The
problems are:

Doesn't get past routers and bridges as mentioned, probably not even repeaters.

I doubt if the systems interoperate in their frequency allocations. Who
wants to buy into some proprietary system? For interoperability, it's back
to good-old-broadband and all the fun of trying to run ethernets over that.
So who uses coax cabling these days anyway? This type of approach is
trying to squeeze life out of a dying technology.

I thought this was a CUSeeMe list, implying "low-cost
you-get-what-you-get-over-your-existing-net-connection" approaches rather
than expensive
"buy-into-proprietary-technology-and-create-high-performance-video-ghetto"
approaches.

Dan

>The "Catches" to this technique is the fact that it requires additional
>hardware on each video or audio sending or receiving station on the network in
>order for it to work. There are some other limitations as well, but they are
>beyond the scope of this discussion and are theoretically irrelevant anyway.
>
>Since routers typically route data packets and treat any additional analog
>signal as noise, you are correct when you assume that for the most part, video
>and audio won't make it past a router.
>
>You seem to be inferring that this "extra bandwidth" is some sort of margin or
>overhead within the ethernet specification. That is not the case. This is
>another type of analog signal modulated on part of the frequency spectrum not
>used by ethernet. There is extra bandwidth on all cables and it is guaranteed
>by the laws of physics, not any manufacturer.
>
>Regarding the emissions issue, I really can't say with authority since I am
>not familiar with the analog signaling scheme that is being used, but I will
>say that to a greater of lesser degree depending upon the nature of that
>scheme, that the same emissions protections that are used to make the analog
>transmissions of the digital ethernet packets along the cable will also make
>other analog signals more or less compliant. It is the analog part of the
>transmission that creates RF, so the same shielding and grounding techniques
>that suppress the ethernet emissions should surpress other analog signals as
>well.
>
>Having said that, I would check it carefully before attempting to add
>additional analog signals to my network infrastructure.
>
>In response to your comment about the designers of ethernet throwing bandwidth
>away, that again misses the point. The designers of ethernet selected a
>portion of an analog spectrum to use to transmit digital data. This selection
>was made for a variety of reasons involving efficiency, reliability and
>practicality. They weren't throwing anything away, they were creating
>something.
>
>John (john@carey.com)
>
>>[Text item: Re: use the outer edge of Eithernet or "What is bandwidth an]
>>OriginalPath:
>> I have seen these techniques for using "ordinary cables" to transmit high
>> bandwidth video in the video market for a while, but I have not seen them
>> become widely adopted. I suspect there are some catches to this
>> technique.
>
>> 1) What happens when goes through a bridge or router or some other piece
>> of equipment? I t could lose this "extra" bandwidth at these points
>> because the BW exists only in the cable.
>
>> 2) I wonder if the extra BW is gauranteed to be available on all cables.
>> Poor cable quality, poor connections, and long runs of cable can all
>> reduce this extra BW. The designers of ethernet probably took this into
>> account and left margin in the design.
>
>> 3) There may be FCC (and EN550022) emmissions implications to running
>> these higher frequncy signals over the ethernet cables. This could
>> potentially cause interference with other equipment or violate FCC
>> requirememnts.
>
>> I doubt that the ethernet designers just threw this BW away.