Bandwidth, Ethernet, ISDN

Ian Carr-de Avelon (avelon@phys.uva.nl)
Mon, 15 May 1995 16:42:47


Hi All,

>>>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.

The limit on ethernet is the packet card, it works at a rate
which was chosen as "sensible" for the technology at the time.
It gives reliable communication even if the cables reach the
maximum permissable length. If the cables were shorter or
better, a card could be designed which works faster. Assuming
that you are at the maximum cable length, any signal at higher
frequencies will not be reliable. Errors in an uploaded
program would be a real pain, but a few splutters on a voice
channel are what you come to expect.

>My understanding is that bandwidth effectively limits the
>signal rate. That does not mean that a particular signal will
>necessarily exploit the available bandwidth but why >anyone
would leave available bandwidth lying around
>unexploited is a mystery to me.

Analog bandwidth is the range of frequencies which pass
reasonably well through the system. Technically, have
less than 3dB loss. Digital bandwidth is the bits per
second which the modem/packet card will reliably send
and receive. The reason that the bandwidth is left lying
around is that the system as a whole has to be reliable
for people using it at the limits. This is why ISDN is
mostly on offer within 6 miles of the exchange. The
Telephone system has cable with a bandwidth sufficient
for sound even at long distance. If the wire is short,
it has a much higher bandwidth.
There are ever more applications being developed and
the electronics becomes ever cheaper, but the cable
does not. So one solution is to use the bandwidth lying
around NB. on some systems. They can become "information
haves". People who live out of town can remain "have nots".

>
>>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?

No, because this bandwidth (for voice on ethernet) is not
decoded by the packet card, so anything at those frequencies
is ignored. That is unless you have another system exploiting
them of course.

Yours
Ian

*************************************************
* Ian Carr-de Avelon avelon@phys.uva.nl *
* http://pc-ia184.phys.uva.nl/avelon.htm *
* *
* dept. of Physics Education *
* University of Amsterdam *
* Netherlands *
*************************************************