升级到四线对 PoE:您需要知道的内容

2018 年 5 月 2 日 / General

多年来,您一直在部署支持以太网供电 (PoE) 的线缆设备,以用于 VoIP 电话和监控摄像机等各种设备。So far, up to 30 Watts is all you’ve been requested to support, but with the plethora of devices now able to take advantage of higher levels of PoE—like the latest 802.11ac Wi-Fi access points, digital displays and even desktop computers—your customers are starting to ask for four-pair PoE.

Maybe you’ve heard some buzz surrounding performance and heat rise concerns in four-pair PoE applications, and you’re getting a little worried about making sure the cabling plant you deploy is ready to power your customer’s whole new range devices. And you should be—the higher power levels of four-pair PoE don’t come without caveats.

Let’s take a closer look.

Just Your Type

First, it helps to understand the differences between the different types of PoE. Type 1 PoE delivers up to 15.4 W, with 13 W available for the device, and Type 2 PoE (sometimes referred to as PoE Plus) delivers up to 30 W, with 25.5 W available for the device. 两种类型都使用两种方法之一 —— 方案 A 或方案 B —— 通过两个线对供电。

在方案 A 中,电力是和数据同时在线对 1-2 和 3-6 上传输的。In Alternative B, power is delivered over pairs 4-5 and 7-8.

为四线对 PoE 提议的 802.3bt 标准包含 Type 3 和 Type 4,这两种均与数据一起使用所有四个线对提供电力。Type 3 PoE 最多提供 60W,设备可用 51W,而 Type 4 最多提供 90W,设备可用 71W。

Time to Keep a Balance

对于使用方案 A 的 Type 1 和 Type 2 PoE,电力的传输方式是在两个线对上施加一个共模电压 —— 这意味着电流是在两个导体上平分的。为此, 线对中各导体的直流电阻必须平衡(相等),且其差将称为直流电阻不平衡。过多的不平衡会使数据信号失真,产生位错误、重新传输,甚至数据链路功能异常。

和 Type 1 和 Type 2 PoE 方案 A 一样,四线对 Type 3 和 Type 4 PoE 也通过共模电压供电,所以直流电阻不平衡也会造成很大影响。但是,在 Type 3 和 Type 4 中,你需要担心的不仅仅是各线对上的直流电阻不平衡。Excessive DC resistance unbalance between multiple pairs can also wreak havoc on data transmission or cause PoE to stop working.

除质量较差的线缆直流电阻不平衡的风险较高(例如导体直径和同心度【圆度】差异较大)外,端接不一致(例如导体在 IDC 内的位置不正确或不一致)也会导致直流电阻不平衡。因此,虽然您可以在供应商的线缆上看到直流电阻不平衡的规格,但现场测试确实是保证安装后直流电阻不平衡性能的唯一方法。

Thankfully, Fluke Networks’ DSX CableAnalyzer™ Series of Copper Cable Certifiers quickly and easily tests DC resistance unbalance within a pair and between pairs, so you can rest assured that the cable plant you deploy will perform in two- and four-pair PoE applications.

More to Know

遗憾的是,直流电阻不平衡不是唯一需要担心的。PoE 在双绞线对铜缆上传输时,线缆中的温度升高会导致插入损耗升高。这会导致信道无法通过插入损耗测试,或需要减少线缆长度。

多根输送 PoE 的线缆紧密捆绑在一起时,PoE 产生的热量会成为更大的问题 —— 而且功率越大,热量越多。The National Electric Code specifies the number of cables allowed in a bundle based on conductor size and temperature rating for 60W or higher PoE. UL has also introduced a Limited Power (LP) rating that verifies cables do not exceed their temperature rating for a certain current per pair, and TIA is also developing guidelines for limiting temperature rise in a bundle.

To take a deeper dive into ensuring that your cabling plant is ready for four-pair PoE, read the article in the March/April issue of ICT Today—but please excuse the mix-up on Figures 1 and 2 (as of the publication of this blog, they are reversed). They’re correct in this blog.

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