Impedances and transmission line effects...

[Mark Hennessy]

Yes, there is just no assurance that 100m of Cat5 will perform as well as Cat5e.

Honestly, for Ethernet, that's simply not true.

Gb Ethernet requires Cat 5 or better. Not Cat 5e or better.

On shorter runs it makes no difference, and you might get more than 100m on Cat5e at 1Gbps and probably at 100Mbps

On longer runs, it also makes no difference.

Maximum segment lengths are 100 metres, irrespective of cable type and network speed. Perhaps you'd get away with longer lengths if you push it, but that goes against the standards and would fail certification.

Aside from the marketing, the only difference between Cat 5 and Cat 5e is a 3dB difference in NEXT and ACR (crosstalk). That might matter in some non-standard applications, but for Gb Ethernet, there is no material difference. Properly installed Cat 5 works perfectly at Gb speeds.

Both are specified to 100MHz, and installations are certified using a test at 155MHz.

Also, don't forget that the symbol rate is the same for 100Mb and 1Gb Ethernet - the speed increase is from the use of all 4 pairs and different modulation. So it if works at 100Mb/s and all 4 pairs are present and correct, then it'll work at Gb.

Back in the late 1990s, there was a massive project to flood-wire all our sites using Cat 5, and today, most devices in use are running at Gb speeds with no issues whatsoever. Of course, newer sites will have been done using the current flavour-of-the-month (Cat 5e or Cat 6 or Cat 6A), but the remaining original Cat 5 installations are just fine, some 20 years on, and have many years ahead of them. An excellent ROI.

I repeat: Cat 5 is fine for Gb. If you find an old box of it in good condition, there is no reason not to use it unless you think you'll be using 10Gb networking one day...

OK, the argument is perhaps moot, given how little Cat 5 is out there, but let's at least get the facts right