Introducing the blend

With power over Ethernet technology now ratified into the IEEE802.3af Ethernet standard, users can finally benefit from ultimate flexibility for the installation of powered equipment, says David Moss.

Picture the scene; you've toiled long and hard working out how best to install networked devices in some hard-to-reach location, only to discover that it is either too expensive or too burdensome to add a power outlet.

How many times must that have happened?.

Well, it need be a problem no longer, because with power over Ethernet (PoE), the same wires that carry the data to and from the connected equipment can also carry the power.

The emergence of PoE means both suppliers and users of industrial Ethernet products have a beneficial new technology for a multitude of different application environments such as utilities, substations, mines and quarries, transportation systems, factory facilities, and warehouses.

Concentrating for a moment on the "power" aspect of PoE, this technology represents the only truly universal power standard.

Although there are differences between mains plugs in the UK, Europe and the USA, the RJ45 connection and -48V voltage of PoE are standard the world over.

Indeed, when you think about the multitude of electronic devices that require a DC input but consume less than 15W of power, it is not difficult to foresee a time when PoE is the world's most popular source of power.

The potential for PoE, then, is enormous, and need not be limited to the harsh environments of the industrial marketplace.

One electronics company has manufactured a PoE electric razor.

Obviously it does not require data transfer, but it gets the power it needs, and will certainly help to drive acceptance of PoE as a de facto standard.

Today there are nearly 200 types of device which are PoE enabled including: wireless access points (airports, warehouses etc); VoIP phones; industrial devices (sensors, controllers, meters etc); access control and help-points (intercoms, entry cards, keyless entry etc); lighting controllers; remote point of sale (POS) kiosks; electric razors; clocks; and security cameras.

This is by no means an exhaustive list.

And over the next few years it is likely that PoE will rise to become the power source of choice for devices such as smoke/fire detectors, RFID scanners, audio/video jukeboxes, vending/gaming machines and even chargers/docking stations for PDAs and mobile phones.

Thus, power over Ethernet has become one of the hottest topics in 21st century technology.

As trends in VoIP, Wi-Fi (802.1) wireless networks and security accelerate, the numerous benefits provided by PoE can help turn good ideas into practical applications.

Easier deployment - access to hard-to-reach locations or places with a lack of space for power deployment, such as roadside, external walls and internal ceilings, becomes much simpler and easier to maintain.

Lower cost - costs can quickly escalate when the installation of separate power outlets are factored into a project.

PoE switches save both time and money by avoiding the need for separate installation of power outlets.

Multi-location - wherever there is an Ethernet connection, a powered device can be used.

Increased uptime - if an UPS is used in the power supply for the PoE switch, reliability rises towards the optimum "five nines" due to reliance not being placed on the mains AC voltage supply.

Improved safety - 48V DC levels are much safer than AC mains voltage.

Plug and play - equipping applications with PoE switches is simple and enables interoperability with a growing variety of devices.

The PoE standardisation project began in 1999, and in 2003 the IEEE task force ratified the technology into the Ethernet standard 802.3af.

The IEEE802.3af standard - also referred to as data terminal equipment (DTE) power via media dependent interface (MDI) - is the first international standard to define the transmission of power over Ethernet infrastructure.

The mechanism for delivering power via PoE is similar the way in which the PSTN (public switched telephone network) carries enough power to every public telephone handset.

PoE delivers DC power of up to a theoretical maximum of about 15W per cable.

In practice, however, a maximum power of 12.95W is available, limited by degradation of power over the cable - typically standard Ethernet cabling of Cat5 or better.

A Cat5 cable has four twisted pairs, but only two of the pairs are used for data.

The IEEE802.3af specification allows either the spare pairs to be used (pins 4 and 5, or 7 and 8) or the data pairs (pins 1 and 2, or 3 and 6) to carry the power.

Typically a PoE system consists of both power sourcing equipment (PSE) and a powered device (PD).

The PSE may either be an endspan (L2 Ethernet switch supporting PoE) or a midspan (PoE hub).

The PD is a PoE-enabled terminal such as an IP phone.

PoE systems are deployed in a "star topology", so each PD is connected to a separate channel of the central PSE.

Users need to be cautious when using midspan products because they are not 802.3af compliant and therefore do not verify that the end device they are powering is a PD.

The 802.3af standard requires that a PSE probes the end device to determine whether or not it is a PD and, if so, what its capability is.

The 802.3af standard ensures the safety of PDs by making a 25kohm resistor in the PD mandatory.

Power is applied only when the "discovery process" running from the PSE detects the resistor.

This prevents damage from occurring to non-PD devices.

In addition, the PoE standard calls for overcurrent protection, undercurrent detection and fault protection of PDs from shorts, power fluctuation or failure.

GarrettCom has built the first hardened PoE switch specifically targeted at the industrial market.

By integrating Ethernet switching, PoE power sourcing and industrial-hardened components, GarrettCom has delivered single a unit that is ready and able to support industrial strength PDs.

The arrival of PoE power sourcing Ethernet switches for industrial Ethernet applications changes the network design possibilities.

The four-port PS14P PoE convenient switch will save money and space, as well as increase reliability for a wide spectrum of applications.

As an example, Dataradio, based in Montreal Canada, is deploying PS14P switches to power its spread-spectrum, licence-free HiPR900 wireless radio system that is used for Scada information in power utility and industrial environments.

The driving factors in Dataradio's decision to use PoE were the lack of availability and space for alternate power sourcing.

Dataradio chose GarrettCom's PS14P both for its "plug and play" installation simplicity and its ruggedness.

With its introduction of the first industrial PoE switch less than two years after the IEEE802.3af standard was solidified, GarrettCom has made a major contribution to speeding the adoption of PoE technology.

In the past, it has taken 10 years or longer for enterprise-based Ethernet technologies to be adapted for use in industrial applications.

Not only can PoE allow a whole host of network devices to be powered in remote locations where access to power is unavailable, but it opens the door to a whole new world of exciting possibilities for consumer and business electrical products.

It is almost certain that PoE will herald major changes for industrial applications.

Despite the available power in a PoE connected device being limited to about 13W, the advantages of blending signal and power in one Ethernet cable connection will contribute to the already rapid transition to Ethernet based industrial control systems.

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