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

John Carey (john@carey.com)
Mon, 15 May 1995 11:06:52


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.