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

Jay_Feldis@logitech.com
15 May 95 09:47


[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.

-----------------------

Michael,

There is some confusion about the word bandwidth in this discussion.
Traditionally bandwidth meant the part of the frequency spectrum which
could be supported by a particular medium. for instance, as Matthew said,
70 Ohm coaxial cable as used by CATV systems has about 700Mhz of bandwidth
practically available for television signals, but a single television
channel uses only 6Mhz. The human ear can only hear from about 20Hz to
20,000Hz while sound's transmission medium, air, is capable of supporting a
much greater frequency spectrum.

Ethernet doesn't use the whole frequency spectrum of its various possible
transmission media leaving room for other types of signals to be used, but
even if it did use the whole frequency spectrum, it would still be possible
to add additional information capability. In the case of NTSC television,
which as I said above, specified a 6Mhz channel width, a high frequency
subcarier was added later to that same 6Mhz channel to produce a color
signal which was compatable with existing black and white televisions while
still fitting into the same 6Mhz channel.

A better analogy from the world or television might be that when the FCC
divided the existing spectrum, it provided for 13 broadcast channels of
6Mhz with some guard band frequency between. Channel 1 has since been
reserved so there are effectively 12 broadcast channels available. When
additional channels were needed, the FCC allocated a UHF range and started
issuing licenses. That UHF frequency range was out of the VHF spectrum so
it didn't affect the existing television transmission scheme while adding
more channel space at other frequencies.

The real point is that the bandwidth of ethernet is different from the
bandwidth of its transmission medium, they are measured differently and
they mean different things. The bandwidth of an essentially analog
transmission medium, like fiber, coaxial cable, or even a copper wire is
expressed as a frequency range and the bandwidth of a digital protocol such
as ethernet is measured in bits per second. This seeming anomaly exists
because the concept of bandwidth existed before the advent of digital
technology and in an analog world, frequency range is bandwidth because it
is that frequency range which defines how much analog information a
particular transmission medium can handle.

Another way of saying this is that when ethernet is out of bandwidth, the
fiber or cable on which it is implemented isn't.

John (john@carey.com)

>>Well, think of it as "out-of-band" and you'll be a step ahead.
>>The 802.3 (ethernet) only uses a certain portion of the physical
>>bandwith on the cable...
>>
>>The same thing is done with these video/voice over existing
>>systems. The data is modulated onto an unused "channel" of
>>the existing ethernet cable. ie: The 802.3 only takes a fixed
>>amount of bandwith to work.. the cable has physical characteristics
>>allowing it to carry more data.

>This is news to me. I always assumed that the amount of data one could
>pull around a network was constrained by the physical limitations of the
>cable/fiber, not by some standard that decreed what the max bits/second
>are.

>If there's really "unused" space on any given network, wouldn't this be
the >first place hackers and providers would work on to get more bits in
and out >of a system?
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