Cabling Installation & Maintenance
测试以太网供电 (PoE) 技术

[摘要]以太网供电 (PoE) 为标准数据电缆提供了新功能，但是用户需要了解 PoE 会如何影响认证。重新认证 PoE 系统需要新的测试程序。“只需进行相对简单的修改，安装人员即可测试和认证供电布线系统。”Fluke Networks 以太网供电 (PoE) 测试一文的作者 Steve O’在文章说道。

Relatively simple modifications help ensure the validity of powered cabling systems.

Power over Ethernet (PoE) is an innovative technology that is increasingly being used to power Internet Protocol (IP) telephones, wireless LAN access points (APs), network cameras, and other network appliances. In areas where alternating current (AC) power is not readily available, PoE saves time and money by avoiding the expense of laying separate power and data cables.
 
In many cases, structured cabling contractors are now installing power as well. Companies that have implemented or are considering implementing PoE, and installers offering this technology as part of their service, should be aware of the potential impact on the network-testing process.

With endspan PoE technology (bottom), the power-sourcing equipment is located within a network switch or router and is not part of the cabling system�s permanent link. With midspan technology (top), when the power-sourcing equipment is built into a patch panel, it becomes a part of the cabling system�s permanent link. 

Recertifying the existing cabling plant.

PoE equipment on the network may require that you change test procedures to ensure accurate testing of the data network. It may also be useful to perform additional tests on the new PoE equipment to verify and document proper operation.

An up-and-coming technology

Current PoE technology has been codified by the publication of the IEEE 802.3af standard, which specifies the operation of Ethernet power-sourcing equipment (PSE) and powered devices (PDs). The specification involves delivering up to 15 watts at a nominal 48 volts direct current (DC) over UTP wiring.

PoE works with the existing cable plant-including Category 5, 5e, or 6, horizontal and patch cables, patch panels, outlets, and connecting hardware-without requiring modification.

All network appliances require data connectivity and a power supply, and PoE lets you use only one set of wires to fulfill both of these needs, reducing installation time and cost, and saving space. PoE also makes it easier to move an appliance, which can simply be plugged into a PoE-enabled network jack.

Types of PSEs

The two basic types of PSEs, endspand and midspan, vary significantly in their effect on network testing.

• Endspan systems use an Ethernet switch with an embedded power supply to deliver power and data. IEEE 802.3af permits endspan devices to supply power on either the 1-2 and 3-6 pairs, or on the 4-5 and 7-8 pairs. Endspan PSEs may be compatible with 10Base-T, 100Base-TX, or 1000Base-T data transmission.
• Midspan systems sit between legacy switches or routers and the PDs. They can be standalone devices or can be integrated into a patch panel. In the latter case, each of the midspan ports in the power insertion patch panel forms one end of a permanent link. 802.3af-compliant midspan devices are only permitted to use the 4-5 and 7-8 wire pairs to carry power, and the 4-5 and 7-8 pairs do not pass data traffic through the midspan PSE.

Because 1000Base-T traffic requires all four pairs to transmit data traffic, 802.3af midspan PSEs are limited to 10/100Base-T. Midspan PSEs exist for support of 1000Base-T, but technically, they are non-compliant with the IEEE 802.3af standard.

An important difference between power insertion patch panels and endspan PSEs is that power insertion patch panels become part of the permanent link but endspan devices do not. The connection’s performance and the termination’s quality within power insertion patch panels are major contributors to permanent link performance, so each permanent link must be recertified whenever a power insertion patch panel is installed. Endspan devices are not in the link when it is certified and have minimal effect on network testing.

How PoE systems work

To prevent damage to existing Ethernet equipment that is not equipped to receive PoE power, PSEs run a discovery process that looks for devices that comply with the PoE specification before applying power to the lines. This process works by applying a small current-limited probing voltage to the cable and checking for the presence of a 25-kW resistor in the remote device. (The probing voltage is typically less than 10 volts and can be applied as often as every two milliseconds.)

If it finds the resistor, the PSE then applies the full 48 volts specified by the PoE standard, up to a maximum of 13 watts. Power is provided as long as there is a device present that draws a minimum level of current. If the device is turned off or removed, the PSE removes the power and begins running the discovery process.

Effects on network certification testing

While the DC voltage on the link is isolated from the high-frequency data signal, it can affect test instruments. Nearly all testers use DC signals to perform basic connectivity testing, such as wiremap and resistance tests. In addition, many testers contain features designed to protect sensitive measurement circuits from other active communications devices, such as Integrated Services Digital Network (ISDN) and Plain Old Telephone Service (POTS) that might use DC voltage on differential pairs, and might accidentally be connected to the tester.

When the cable is tested, there is no powered device on the line, so the PSE is in the discovery mode. The DC voltages sent as part of the discovery process will have no effect on data transmission tests, such as return loss, insertion loss, and near-end crosstalk. But they can interfere with DC measurements that are used by most network testers during the certification process to measure the link resistance and generate a wiremap.

The DC voltage generated by the PSE as part of the discovery process might accidentally trigger protective features in the network test equipment. For this reason, DC power should be turned off for any line that is being tested. The simplest way is to remove power from the PSE device, but this will have the often-unwanted effect of removing power from all links-not just the one being tested.

Fortunately, most PSEs come with a software configuration utility that allows power for individual ports to be turned on and off. With power shut off to the port being tested, you can then test the data transmission properties of PoE-enabled links in the same way as conventional links.

Testing the PSEs

There are several possible approaches to testing the PSE, ranging from complex and highly accurate to simple go/no-go-and some points in between.

You can test PSEs with laboratory equipment designed to evaluate the performance of power supplies by configuring these high-end devices to evaluate the performance of PSEs both in the discovery and power modes. The problem is that these devices are expensive, bulky, and complicated to use, since they are intended for equipment manufacturers and not network technicians.

Simple devices that basically consist of a resistor, light-emitting diode (LED), and 8-pin modular (RJ-45) plug provide a much simpler and less-expensive alternative. These devices can be plugged into the network jack and appear as a resistance to the PSE. The PSE then sends 48 volts of power, which lights up the LED.

While these devices provide a quick and simple way of testing PSEs, they don’t provide any information about the PSE’s voltage levels and power capacity, nor a method to document the availability of PoE power. So, these devices should be considered a quick go/no-go solution.

The ideal solution will arrive soon, when test-equipment manufacturers develop modules for cable analyzers that will provide more-extensive PoE testing as part of the same suite of tests that are currently used to certify the link. Modules of this type will provide the ability to completely evaluate PSE performance without adding a significant amount of time to the certification process. Another advantage of integrating the PoE test into the link certification is that the results of the PoE test can be documented and archived as part of the certification testing process.

A new standard

Up to this point, we have addressed how to test power and data on all types of PoE-enabled links complying with the 802.3af standard. But the standard does not support Gigabit Ethernet, which uses all four pairs for data transmission. Consequently, a number of manufacturers have introduced Gigabit Ethernet PoE equipment that does not conform to any existing standard. Additionally, some PSEs may have power greater than 15 watts to support devices that require more power.

This equipment has already attained a considerable market presence, and has prompted the IEEE to create a task force chartered to address these testing issues in an update to the standard. The 802.3at task force is charged with extending the capabilities of the 802.3af standard. When drafted and published, the new standard is expected to provide at least 30 watts of power, which is needed particularly for high-data-rate APs and motor-driven devices, such as cameras with pan/tilt/zoom controls. Additionally, the task force’s objectives include researching the operation of PSEs for Gigabit and 10-Gigabit Ethernet.

Testing newer PSEs

When all four pairs are used for data transmission, the challenge is preventing DC voltage from accidentally being channeled back to the network switch or router where it could do serious damage. Manufacturers that have introduced Gigabit midspan devices have overcome this problem by introducing capacitors that block DC currents from flowing back into the switch or router.

But this creates an additional complication for the network-testing process: The capacitors will not affect the high-frequency signals used for insertion-loss, near-end crosstalk, and return-loss measurements. Even when the power is turned off, these devices will appear as “opens” to a network tester using DC current to check connectivity. So, the link will fail certification.

You can overcome this problem by configuring the tester to avoid using a separate wiremap measurement. Other parameters, such as insertion loss, near-end crosstalk, and return loss, will only pass if the wiremap is correct. Because of the growing popularity of PoE, test-equipment manufacturers likely will upgrade testers in the near future so they will automatically detect the presence of PoE and alter the test settings accordingly.

Each variety of PoE equipment has varying effects. By adjusting their testing procedures accordingly, network technicians can easily validate the performance of PoE systems along with the underlying Ethernet infrastructure.

Cabling Installation & Maintenance February, 2006
Author(s) :   Steve O'Hara