Jeff Duntemanns Drive-By Wi-Fi Guide

2017-07-07 02:10:07

Power Over Ethernet (PoE)

Here's a problem that you might encounter in setting up your Wi-Fi system: You have a access point that you want to install in the clear and up in the air. Perhaps you want the access point to act as a wireless bridge to another similar access point some distance away, on top of the next building-or in the next town. Getting power up to the access point can be a real problem and a significant expense. Fortunately, a new technical standard for power transmission over Ethernet cables will help. The IEEE standard for Power Over Ethernet (PoE) will reduce the number of wires needed to get the job done. The IEEE task group is 802.3af, and that's the number to watch in coming months. This new standard will let you string a single Ethernet cable to an access point and provide it with both data and power through the same cable.

In a nutshell, PoE allows the same Ethernet category 5 (CAT 5) cable to carry both data and electrical power to an access point or other Ethernet device. There's nothing magical about it. CAT5 cables have more conductors than they need to carry Ethernet data, so PoE uses two of the otherwise idle conductors to carry DC power through the cable.

The trick, of course, lies in knowing which two idle conductors. There are four possibilities, and the primary benefit of the standard will be industry-wide agreement on the conductors that carry power. Some vendors already offer PoE adapters, but absent a standard they have defined their own schemes. If you decide to go with PoE pre-standard, keep in mind that you may have to scrap existing adapters once the IEEE 802.3af standard is adopted. Also, once you choose a PoE vendor, use only adapters from that vendor. Adapters from other vendors may use other pairs of wires and simply not work together.

Why PoE?

Why is PoE important in the 802.11b world? Simple: A need to keep the access points as close to their antennas as possible. If you want an antenna way high up, you have to mount the AP way high up as well, or you'll simply dissipate the AP's signals (both transmitted and received) in the coaxial cable between the AP and the antenna.

This is important when creating a point-to-point bridge (see Chapter 16) because such bridges are often 'rooftop to rooftop' with the endpoints as high as possible to avoid intervening obstructions. People implementing community hotspots may want an AP in a weatherproof housing up on a 30-foot pole with an omnidirectional antenna attached directly to the AP.

The key is keeping the cable length between the AP and its antenna to an absolute minimum. In my experience, coaxial antenna cables longer than three or four feet are poison, absent a very expensive, low-loss variety of cable. (See Chapter 8 for a detailed discussion of this problem.) It's easier and cheaper to send Ethernet data over long runs of cables than microwave signals. PoE allows you to send power to the AP right through the same CAT 5 cable that carries Ethernet data to the AP.

How PoE Works

Power is applied to a run of CAT 5 cable through a small device called an injector. At the other end of the CAT 5 cable, power is taken from the cable using another small device called a tap. (Sometimes a tap is called a splitter, but I consider that term confusing for various reasons and won't use it here.) The injector is typically in the server closet, or wherever Ethernet data must be sent to a device like a wireless access point. The tap is near the device being powered. The tap has a power jack on it, and the AP (or other device) plugs its power cable into the tap (see Figure 6.7).

Figure 6.7: How Power over Ethernet (PoE) Works.

Note that the injector and the tap both pass the Ethernet conductors through without any kind of interruption.

Passive and Active Devices

Injectors and taps come in two varieties: passive and active. Passive injectors and taps simply provide a connection for power to a chosen pair of the 'spare' conductors inside the CAT 5 cable. Passive injectors and taps can be made from small networking junction boxes, if you have some skill and experience handling wire and simple hand tools.

Active injectors and taps provide additional electrical services:

An active injector typically provides short-circuit protection, which is something like a circuit breaker: It stops current flow if a short-circuit occurs in the power circuitry somewhere. This prevents the cable from overheating and possibly causing fires and damage to the remote device or devices being powered.

An active injector may also provide overcurrent protection. This limits current from the power supply in case the device at the other end of the CAT 5 cable attempts to draw excessive current for some reason. (This often happens when multiple devices are powered through the same CAT5 cable. Too many devices may demand more current than the power supply can provide without damage.)

An active tap usually includes a voltage regulator, which takes DC power from the CAT 5 cable and converts it to the specific voltage required by the device being powered. This way, voltage changes in the cable will not affect the device being powered.

Commercial active injectors typically put either 24V or 48V DC into the CAT 5 cable. For each device powered through the cable, an active tap is chosen with an appropriate voltage regulator inside. A LinkSys access point, for example, may require 5V DC. An active tap with a 5V regulator would be used to convert the 24V in the cable down to 5V.

The Voltage Drop Problem

Why not just put 5V into the cable and avoid the cost of active devices? There is an unavoidable voltage drop across significant lengths of cable (more than twenty feet or so) that must be taken into account. This voltage drop depends on two things, according to a principle of physics called Ohm's Law:

The electrical resistance in the length of cable between the power source and the devices using the power

The total current being drawn through the cable

If either factor (or, worse, both) goes up, the voltage available at the other end of the cable goes down. At some point, the voltage will drop so far that the devices drawing power from the cable will no longer function.

Each conductor in a CAT 5 cable has a resistance of about three ohms per one hundred feet of cable. If you use a long enough cable (and power enough devices through it) the voltage available to the devices under power will drop so low that the devices will no longer work. One device, in fact, may be enough. The governing equation is this:

Voltage drop across the cable = Current in amperes X resistance in ohms

EXAMPLE: Dealing with Voltage Drops

Say you're powering a wireless access point (AP) through one hundred twenty feet of CAT5 cable. At 3 ohms per hundred feet of cable, the resistance in that length of cable would be 1.2 x 3 ohms, or 3.6 ohms. If the AP draws 1.4 amperes of power through the cable, you have:

Voltage drop = 1.4 amperes x 3.6 ohms = 5.04 volts

How bad is this? Well, that depends on what voltage your AP requires. The LinkSys line of access points use a 5 Volt power source, so if you drop 5.04 volts through the cable while putting only 5 volts into the opposite end, you have… no voltage at all.

Not good. Fortunately, there are two ways out:

Use active injectors and taps that provide a high enough voltage level through the cable that modest voltage drops may be absorbed without problems. Even if your cable drops 5V, if you begin with 24V, you'll still have 19V at the other end of the cable, which an active tap can regulate down to 12V or 5V as needed.

Use passive injectors and taps and calculate (or measure) the voltage drop through the cable, and choose a power supply that provides precisely enough voltage to compensate for the measured loss in the cable. In the example shown above, you would need to use a 10V power supply, so that losing 5V in the cable would still leave 5V to power the access point.

Using passive injectors and taps is less expensive, but tricky, and unless you have some skill with electrical calculation and test equipment, you should stick with commercial active PoE products. Recent PoE product introductions from companies like D-Link have taken the cost of injectors and active taps down to $39 for a set containing one of both, so there's less and less reason to make your own as time goes on.

If you decide you're willing and able to construct PoE injectors and taps on your own, perform a search on the Web to see some of the sites posted by others who have done this. The process involves some Ohm's Law math and some bench electrical work, plus some test equipment. A variable voltage power supply is extremely helpful-and egad, a soldering iron!

The Power Connector Problem

Another unexpected problem in using PoE with some Wi-Fi gear is that certain units take power through non-standard power connectors on their back panels. The Cisco Aironet 340 access point runs on 5V-but it uses a connector you won't find on a rack at Radio Shack or anywhere else, except perhaps direct from Cisco. Many Linksys products, on the other hand, use a standard 'coaxial' or 'barrel' DC power connector which you can buy at almost any electronic parts retailer. If you're designing a system that will require PoE, take a look at the power connectors on the devices that will have to draw power through the Ethernet cable. Make sure you can buy or cobble together a jumper cable to run from the tap to the Ethernet device.

Commercial PoE Products

You can buy readymade PoE injectors and taps from a number of companies. Until very recently they were outrageously expensive for what they are, which is what has prompted many people to construct their own. My own experience leads me to recommend the recent D-Link PoE product, the DWL-P100, which lists for about $40 (see Figure 6.8).

Figure 6.8: The D-Link DWL-P100: Power over Ethernet System.

The DWL-P100 is a kit containing an injector, a tap, and a fairly gutsy power supply, plus all the associated cables you'd need to power one of the D-Link access points remotely through its Category 5 cable. In Figure 6.8, the injector is at the bottom, and the tap is at the top. The modules are 3" long, 1 3/4' wide, and 1/2' thick.

One caution: The D-Link PoE system works beautifully-with D-Link products. I still haven't seen many 802.3af PoE products on the market, and D-Link's remains a proprietary system. To use the D-Link PoE devices with other vendors' access points requires that you know enough about electricity to know whether the voltage and available current at the tap are correct for the device you want to power. Tread carefully here-burning out an AP by feeding it excessive voltage will void the warranty.