在准备为您的计算机运行一些新电缆时，了解这些限制是值得的，这样您以后就不会遇到任何问题。有鉴于此，今天的超级用户问答文章为困惑的读者提供了一些有用的知识。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图为达尼·摩尔（Flickr）提供。

问题

超级用户读者Bhavin想知道一个人在一台计算机和一个交换机之间能走的最长的Cat6电缆是什么：

When I am using a “single PC to switch” setup with a VLAN function/configuration, what is the longest Cat6 cable I can use? I have read that there is a 100 meter length limitation, but I am not talking about daisy-chaining or having other users on the same line (the PC connects directly to a switch).

一个人在一台计算机和一台交换机之间能走的最长的Cat6电缆是什么？

答案

超级用户贡献者Ron Maupin为我们提供了答案：

The UTP standard, to which the various Ethernet standards adhere, is 100 meters, but that assumes 90 meters is solid-core (better performance, but more fragile) with 5 meters on each end as stranded (worse performance, but more flexible).

It has nothing to do with daisy-chaining, but with several measurements such as frequency, insertion loss, NEXT, PSNEXT, FEXT, ELFEXT, PSELFEXT, return loss, propagation delay, delay skew, balance, longitudinal conversion transfer loss, etc.

The 100 meter limit also assumes that the cable is pre-built or professionally installed with all the same rated components and checked to see if it will pass the full-test suite. Even experienced installers have problems when installing Category-6 cabling, although Category-5E cabling can do 1000BASE-T at the same distance. Category-6 cabling can do 10GBASE-T at 55 meters, but Category-6A cabling can do 10GBASE-T at a full 100 meters.

For those who are curious, there are some big equipment vendors (i.e. Cisco) that support Category-6 cabling with 10GBASE-T for 55 meters. The participants in the new NBASE-T Alliance also support 5 gigabits on Category-5 cabling at 100 meters. You can look at the documents, such as At-A-Glance – Cisco Multigigabit Ethernet Switches (this is a direct PDF download link).

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