Power over Ethernet has potential
Should you use PoE, and if so, how and where?
By Sean Bacher, Computing South Africa | Computing South Africa | Published: 00:00, 14 September 2005
When installing access points, designers often use the availability of electrical outlets to base decisions on where to install their access points. In some cases, companies only locate their access points close to electrical outlets. These situations limit the location of the access points, and, as a result, reduce the overall flexibility of the wireless network.
In the world of networking, convergence has held centre stage for some time, especially with the hype around VoIP. But another convergence, which aims to send additional traffic over existing networks, has taken place as well.
Countries with some level of development typically have many networks interconnecting various services - road, rail and electricity grids are among them. While some of these networks cannot be harnessed for data transmission - sending data over the road in a literal sense presents serious problems - it is quite possible to establish Internet connectivity using the power grid, and send power over the networks that connect computers, a technology we now know as power over Ethernet (PoE).
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According to Pierre Holtzhausen, a network specialist at DCC, the uptake of PoE devices has been very slow. "At present companies are not too keen to change their current network infrastructure. However, as companies implement wireless networks and VOIP the need will start increasing," he says. According to Gartner research vice-president, Neil Rickard, 30 percent of all enterprise gear shipped at the moment is POE-enabled.
PoE integrates data, voice and power on a standard Ethernet infrastructure, providing new options for power distribution. This allows IP telephones, wireless LAN access points, surveillance cameras, and other embedded appliances, to receive power as well as data over standard CAT5 Ethernet cabling.
In 2003 the IEEE approved PoE as an international standard, and called it IEEE802.3af, opening up new approaches to installing appliances and computer systems.
The IEEE802.3af standard defines the amount of power that needs to be delivered to each device as well as the process that needs to be followed in order to determine if a device is capable of accepting power over Ethernet cabling. The standard also defines how power should be delivered to devices using the 10Base-T, 100Base-T and 1000Base-T networks.
How it works
In order to have a working PoE network, companies only need two major pieces of hardware, namely a Powered Device (PD) - this is the actual device that will receive the power - and the Power Sourcing Equipment (PSE), which will provide the power to the PD.
Two variants of the PSE are available, midspan and endspan, each designed with a specific network design in mind.
"Midspan devices are commonly known as power injectors," says Holtzhausen. "These power injectors would typically be used when introducing PoE into an already existing network infrastructure," he adds. The midspan hub would be installed in between the Ethernet hub and the device requiring power.
"An endspan device is a PoE-capable hub," continues Holtzhausen. Basically an endspan device would replace an already existent hub or switch, thus being able to provide the receiving device with both data and power.
According to Holtzhausen, in the case of a 10Base-T network, the power is supplied over the unused cable pairs, but, in the case of 1000Base-T, where all the cables are used, the power is merely sent over the same cables that carry the data.
So should I install a PoE network?
"PoE equipment is considerably more expensive than standard Ethernet equipment," comments Rickard. He says that if a company already has a working network in place it should not upgrade to PoE unless it will really have need for it in the next twelve months. "Companies that are planning on implementing a wireless network, and maybe even a VOIP system, may want to start looking PoE, but if a company has no plans to implement either of these technologies, it is really not worth it," he says.
On the other hand however, he says that companies that are just starting up, or companies that are putting a new network infrastructure in place, might want to go the PoE route. "Granted, wireless routers and VoIP phones are the only devices that are really driving the technology at the moment, but I feel that in the not too distant future more devices will be IEEE802.3af compliant," continues Rickard.
Rickard believes that notebooks, PDAs, smart phones, and perhaps even PCs, will soon be able to use a single cable for both power and data.
PoE sounds like a great technology, however, as with all good things in life, there are a couple of drawbacks.
The first is that when power is carried over data cables, one has to be very careful with the engineering around it, as it causes crosstalk, or interference with the data. This requires taking a step back and looking at the reasons for wanting to power devices through the data cable.
If the device is a phone, perhaps, it would be feasible, but with advances in ‘soft’ technology (such as soft phones), the need for power delivery through a cable becomes restricted to niche applications.
The second drawback is that the PSEs themselves generate heat. "The PSE has to get rid of the excess power flowing through it," says Holtzhauzen. "This is usually given off in the form of heat," he adds.
According to Holtzhauzen, administrators already have an ongoing problem with properly cooling their data centres, and introducing PSEs is just going to make the problem even worse. He advises that companies wanting to introduce PoE should make sure that their data centres are already properly cooled, and can handle the excess heat.