Cutting the cable count with PoE

When networking vendors first started selling Power over Ethernet equipment, many IT departments were sceptical.

It was 2003 and many businesses were confusing the promising new technology with a raft of failed developments in Powerline Communications which had, at the time, unsuccessfully tried to pump data into people's homes over the electricity infrastructure.

Power over Ethernet (PoE) is the opposite technology — rather than carrying data over the electricity infrastructure it uses data cables to carry power to support devices on the LAN. After the early scepticism wore away, the technology became mainstream.

The biggest advantage of PoE is that simple LAN devices, such as IP phones and wireless access points, can draw their power over the network, meaning they do not need a separate mains electricity supply.

Because mains electricity sockets have to be installed by qualified electricians, PoE can prove a real boon if businesses want to locate LAN devices in areas without electricity.

It is also especially useful where providing mains electricity would be difficult. For example, wireless access points are usually located on the ceiling, where they provide better radio-wave propogation. Providing mains electricity to the ceiling is difficult, but running a LAN cable there is likely to prove far easier.

And because the electricity is provided over standard Cat 5 cabling — the same cabling used in most LANs — only one cable is needed per device to supply both data and electricity. That allows all power to be supplied from a central data centre, simplifying IT operations.

Only one UPS (uninterruptible power supply) is then needed to back up both the data network and the devices themselves.

Bob Tarzey, an analyst with Quocirca, says there has been a significant take-up of PoE in the past five years. "It's definitely being used. The most obvious areas are IP phones and the ease of putting in access points. You've got to put in the data cables anyway. You can just use one set of cables."

Tarzey's assertions on the popularity of PoE were backed up by VDC, a US analyst firm which closely follows PoE technology. VDC says that over 30 million switch ports have been sold which can provide PoE.

Needless to say, companies that make PoE equipment agree. One of the largest PoE vendors is Microsemi, which last year bought PowerDSine, widely acknowledged as the pioneer of PoE. Microsemi supplies PoE chips to some of the largest LAN switch vendors, including Nortel, HP and 3Com. It claims its chip revenues rose 50 percent from 2006 to 2007.

How does PoE work?
PoE is reliant on two sets of equipment, which vendors often call the PSE (power sourcing equipment) and the PD (powered device). The PSE is either a switch containing PoE circuitry (sometimes called an Endspan) or a power injector (often called a Midspan), which sits downstream from a non-PoE switch. The job of the PSE is to inject the power into the network.

The PD is the device that receives the power, which could be an IP phone or wireless access point. On a 10/100Mbps LAN, data is sent across the Cat 5 cable using two of the four twisted pairs, while the power is sent across the remaining two pairs.

Gigabit LANs use all four pairs for the transmission of data, meaning power has to be sent over the same pairs. This is achieved by a technique known as phantom power.

PoE was standardised by the IEEE in 2003, a standard that permits a theoretical 15.4 watts to be sent across the network. Losses in the cable take this down to approximately 12.95 watts. This level of power is enough to support most IP phones and wireless access points, plus some other low-power devices such as IP CCTV cameras. The standard is known as IEEE 802.3af.

More powerful PoE
Not content with supporting only low-power devices, vendors are working on pushing more power across the LAN.

A taskforce at the IEEE is currently developing another standard, 802.3at or PoE Plus, which will allow up to 30 watts to be sent, by using a higher voltage and current. PoE Plus will allow additional devices to be supported such as the latest generation of access points based on the 802.11n specification.

PoE Plus will also support IP phones with large screens, some of which are used for videoconferencing, as well as thin clients. Support for thin clients will mean that retail outlets will be able to connect kiosks and other point of sale venues without having to worry about supplying electricity to them. When the standard is finalised — the estimated date has been pushed back to March 2009 — older PoE equipment will still be supported. But in the rush to develop PoE Plus, some vendors are developing their own proprietary technologies, some of which have compatibility issues.

Cisco said last week that it had developed a technique called enhanced PoE, which will provide 18.5 watts, enough to support 802.11n access points. Enhanced PoE is based on 802.3af, but it contains a number of...

...Cisco-developed additions and relies on some of Cisco's proprietary protocols, which means it only works with Cisco equipment.

Cisco's move is a reflection of its history with the technology. Before PoE became standardised, Cisco offered its own technique called Inline Power. As its competitors' products became standardised, Cisco gave in to the competition and starting including 802.3af on its switches and IP phones instead.

HP is pursuing its own flavour of PoE, but argues that its implementation is based on IEEE standards.

The problem with the original standard, according to HP, is that it wastes a lot of power. 802.3af can only supply 3.84 watts, 6.49 watts or 12.95 watts to each device. So if an IP phone requires seven watts to run, the remaining 5.95 watts are wasted.

HP says it is countering this by including in its switches an IEEE standard called Link Layer Discovery Protocol. LLDP is not included in the PoE standard, but it will be included in PoE Plus.

LLDP allows what HP calls "fine grain power management", which provides exactly the right amount of power to the powered device. LLDP will be included in HP's 3500, 5400 and 8200 switches from next week. Existing customers can upgrade to LLDP by implementing firmware which will be downloadable from HP's website.

Michael McCormack, chairman of the IEEE's PoE Plus taskforce, says that IT professionals should expect more proprietary offerings before the standard is finalised: "I would expect to see a number of companies releasing pre-standard stuff in the next quarter or two. It's good for people to put out products that prove the technology."

However, there is no guarantee that pre-standard products will work with new products once the standard is finalised. Daniel Feldman, senior product line manager for PoE at Microsemi, expressed concerns over Cisco's enhanced PoE. "What they are doing is OK. I'm sure people will buy it," he said. "But this is not really future-proofing the network."

Gijs Zandvoort, product manager for HP's networking division, HP ProCurve, also had concerns. He said it was currently difficult to talk about interoperability with the final standard because there are still parts of the standard to be decided. It is "very likely" that interface modules will have to be added to pre-standard equipment, he said.

Heating up
One major consideration when implementing PoE is the requirement to provide sufficient power to the PSEs and, correspondingly, sufficient cooling. Because PoE centralises the provision of power, it increases the amount of heat in both the data centre and the comms room. Advancements in PoE will only increase those requirements.

These demands are on top of the extra power required by IP phones, which tend to require between 30 and 40 percent more electricity than analogue handsets.

Feldman argued that IT professionals should plan their power and cooling requirements in advance of PoE implementation. "Understand what you want to power, and could want to power," says Feldman. "Once you power everything, that dissipates a lot of power."

But there are a range of other concerns. VDC says businesses are sometimes surprised by hidden costs brought about by a need to perform a forklift upgrade on the network to support PoE.

VDC's research found that the most serious inhibitor to rolling out PoE is the reluctance to have to change LAN infrastructure. Businesses are also concerned by maintenance costs and implementation costs, the analyst firm has found.

PoE switches versus Midspans
One way of keeping control of the costs of PoE is by choosing the right infrastructure, of which a key decision is deciding whether to supply the power through switches or Midspans.

The simplest topological choice is to use PoE switches, which do away with the need for Midspans. Vendors tend to sell switches with varying numbers of PoE ports, allowing IT professionals to choose, to some extent, the number of PoE ports they need.

"It is the most elegant way of doing it," says Zantvoort. But switches are expensive, and the expenditure on new models might not be justified purely to enable the rollout of PoE.

If a company's switches are not in need of an upgrade, then purchasing Midspans may be a more sensible option. "If you need to power something now, use a Midspan," says Feldman. "They come at much lower cost than a switch. And to replace a switch, you have to take the network down."

Whichever option businesses choose, they should be certain that the network's power requirements are met and that standard Cat 5 cabling is used. "Good quality cables are a must, as is a good supply of air into the closet [comms room]," says McCormack. But, he adds, as long as those conditions are met, "IT managers can sleep well at night when adding PoE to their installation".