Several years ago I wrote an article series on making the transition to gigabit Ethernet. Even though that series is several years old, I still get a lot of e-mail in regard to the series. One question that I have been asked twice this week already is whether the network cabling has to be upgraded, or if CAT 5 cable can safely carry gigabit traffic.
The best way that I know to explain things is that even though there is intelligence in the switch and in your network cards, the switch and the cards have no way of knowing what type of cabling you are using. If you have a gigabit switch and gigabit NICs, then the computers are going to try to talk to each other at gigabit speeds.
Now here is the tricky part… Cat 5 cable is not rated for gigabit communications. Only Cat 5E and cat 6 are certified for gigabit speeds. Having said that, it doesn’t mean that Cat 5 can’t carry a gigabit signal, only that gigabit speeds cannot be guaranteed up to a distance of 100 meters. A short span of Cat 5 cable may very well be able to support gigabit speeds, but as the length of the cable increases, the reliability decreases. Long spans of Cat 5 cable carrying gigabit traffic may suffer from attenuation or cross talk. Having said that though, there are an awful lot of articles on the Internet posted by people who have successfully used their existing CAT5 cable for gigabit Ethernet.