Tue 26 Jul 2011 17:15
Gigabit ethernet, and Category 5, 5e cabling.
There seems to be lots of folklore that says that Category 5 (Cat5)
cabling can't run gigabit ethernet. Contrary to widespread belief,
that's mostly false. Here's the situation. Cat5 has the bandwidth to
run 1000baseT. But early experience with 1000baseT showed that 1000baseT
was pickier about certain cabling issues that weren't specified in the
Cat5 standard, such as crosstalk and delay skew, so the Cat5 standard was
enhanced for 1000baseT to enforce limits on these. This enhanced standard
is called Cat5e. But the fact is that most Cat5 installations already
perform to the Cat5e spec.
If someone tells you to rip out a Cat5 installation because it can't support 1000baseT, you're being prompted to do something that is expensive and probably unnecessary. All you generally need is test the existing cables to the Cat5e standard (using a Cat5e cable tester) and replace the ones that fail. Often, most if not all the cables will be fine. Or just use the cables for 1000baseT and replace any that exhibit problems.
Cat6 and Cat6a are a different matter. Cat6 supports a spectral bandwidth of 250MHz, up from Cat5/Cat5e's 100Mhz, while Cat6a supports 500Mhz. Cat6 cabling will run ten gigabit ethernet (10GbaseT) to 37-55m, while Cat6a will run 10GbaseT to 100m. So it's worth choosing Cat6 or Cat6a over Cat5e for new cabling, if the cost increment isn't too high, so that the cabling can support 10GbaseT, even if it's not needed today.