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

john harrison (
Thu, 18 May 1995 23:42:47 -0700

could someone send me 1 more beer:)

At 09:47 AM 5/15/95, wrote:
>[Text item: Re: use the outer edge of Eithernet or "What is bandwidth an]
> 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.
> -----------------------
>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 (
>>>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
>>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?
>DisplaySubject: Re: use the outer edge of Eithernet or "What is bandwidth an
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