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

John Carey (john@carey.com)
Sun, 14 May 1995 12:06:06


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?