• United States

The Cat’s out of the mailbag

Feb 13, 20033 mins

* Readers write about Category 6 cabling

Last time, I started relaying messages from those who responded to a couple of articles on cabling issues. This time, I’d like to continue with some responses regarding Category 6 cabling.

Several readers reminded me that Cat 6 is specified to handle higher speeds, and so would be a more logical choice than Cat 5 for something as powerful as 10 Gigabit Ethernet.

But a couple of readers are so far not impressed with Cat 6. Cat 6 is considered the next generation of copper, as Cat 5 is widely installed and Cat 5E is becoming increasingly popular.

One reader writes:

“Proponents of Cat 6 (a cable specification that is not quoted or required by… Fast and Gigabit Ethernet) often cite that a better cable such as Cat 6 would lower the number of retransmissions. Several months ago I asked the participants of a mailing list dedicated to cabling issues to report their experience. As I described, it is very easy to get the number of retransmissions reported from a Windows PC after a day’s work on a LAN. Just run ‘netstat.exe -s -p tcp’. It would therefore be easy to show at least anecdotal evidence of frequent retransmissions on non-Cat-6 LANs. None were reported so far…

“My own measurements invariably show that there are about 0.5% retransmissions. Now even with error-free equipment some retransmissions are expected, since TCP’s congestion avoidance mechanism uses dropped (and subsequently retransmitted) packets as its congestion signal. Half a percent retransmissions therefore is absolutely reasonable. Even if ALL of these were attributable to poor LAN transmissions supposedly caused by marginal cabling, the performance penalty would be marginal. But again, there is in fact no indication that transmission quality in Cat 5E based Fast and Gigabit Ethernets results in retransmissions.”

The reader says he would like to hear others’ experiences with retransmissions. Let me know if you have any data on this topic.

Another reader had other issues with Cat 6:

“I install network cabling for schools. Cat 6 cabling is extremely difficult to install because of the core that separates the four twisted pairs. The entire length of the run must be replaced if the cable kinks, and the cable is very intolerant to installation mistakes. This makes the installation work expensive. We also must pay careful attention to electromagnetic interference from power, alarm systems, induction motors, ballast transformers, coil-actuated dampers, old telephone cable, and other stuff jammed into the nearby space. Optical fiber has an edge on bandwidth potential and is impervious to EMI. Its cost is coming down to about 10% more than copper. I recommend optical fiber connections at least for the backbone.”

Yet another reader agrees that the choice comes down to Cat 6 or fiber-optic lines, and that maybe we should focus on fiber optics:

“If engineers can pull it off, and it’s not more error prone, and it’s much cheaper, fine. But I have difficulty with the statements about the huge expense of fiber, especially in a computer room. When you’re talking about connecting 5,000 servers, and the technical support required to deal with media problems, fiber seems cheap to us, even at gigabit… I’d personally probably rather see the industry focus on driving the costs of the optical components down.”

Big thanks to everyone who wrote in.