LAN and Telecom Cabling Program to Estimate Cat 5 Cable Length

> Dmitri(Cabling-Design.com wrote:

>> Don't know if that was true or not, but it
>> made sense to me when I read it.

> It's the first I've heard of it, and I've spent 23 years in the telcom
> industry. There are a lot of things that "make sense", but
> are in fact
> fiction.

> Also, I don't recall AT&T being much in the local lan business.
> Phone
> cables are capable of much greater distances.

> James Knott wrote:
>

> > Dmitri(Cabling-Design.com wrote:

>

> >> It just doesn't make sense: to accommodate for all possible
> >> combinations
> >> of lengths that a pair of Ethernet devices may encounter (from 0
> >> to 100
> >> meters), the devices are designed to hold the data received in the
> >> buffer
> >> memory for at least the duration of the longest trip - 100 meters.
> >> Therefore, any measurement you might be able to take this way
> >> (without
> >> arguing whether it's even possible to measure such tiny time
> >> interval with
> >> devices not specifically designed for that) will always show one
> >> length -
> >> 100 meters (295 ft)

>

> > Actually, the longest trip may be far greater than 100 M. For example,
> > on
> > 10base5 network, the maximum distance is 500M and fibre can go much
> > further
> > than that. Also even with CAT5 on a switch, you can go 200M etc.
> > Ethernet, being a level 2 protocol, will make sure the packet gets
> > placed
> > on the wire, without experiencing a collision. Once 512 bits have been
> > transmitted, it's considered collision free and the ethernet portion of
> > the
> > stack has no more concerns about the data, such as if it made it to the
> > destination intact. It then becomes the responibililty of a higher
> > layer
> > (either TCP or the application, if UDP) to ensure end to end integrity.

>

> > UDP apps might not worry about that.

>

> > The 100M CAT5 limitation is due entirely to signalling limits and has
> > nothing to do with collision detection etc.

>
> I agree with you, James.
> I was trying to make a point that any delay that actually has anything to
> do with the real cable length (no matter how small or long of a delay)
> will essentially be "normalized" to the maximum allowable limit on the
> hardware level.
>
> Also, being a cable guy I am, I have another, fully "passive physical
> level", explanation of the 100 meter cable length limit:
> As read in a source that cannot recall the name or origins of, some 25
> years ago AT&T commissioned a survey of about 1000 (OK, make it any
> statistically significant number - I don't recall the actual number
> anyways ;-)) commercial buildings in the US to find out that more than 80%
> of the useable spaces in the buildings surveyed can be reached by a cable
> that's 100 meters long from the telecom closet. Thus they decided to stick
> with the length as being a nice rounded figure, and the electrical
> characteristics have been maxing out at that length ever since. The
> electronics components manufacturers in turn had to work around the
> electrical characteristics limit to make sure the equipment will work on
> the installed base of cable. Don't know if that was true or not, but it
> made sense to me when I read it.
>
> --