Combine Power over Ethernet & Power Line Carrier for low cost & low power networking - Embedded.com

Combine Power over Ethernet & Power Line Carrier for low cost & low power networking

Today, there are different interfaces on the market that providedata and power over the same medium. By reviewing the pros and cons ofeach technology it becomes clear that Power over Ethernet (PoE) andpower line carrier (PLC) technology are a good match; combining the twobrings additional value. Power line modems take advantage of existingpower lines and use the power wire as a communication channel, whilePoE acts in a complementary fashion; distributing power over theEthernet data cable.

Applying each technology where it is most beneficial in a data andpower network can bring additional savings, flexibility andoptimization. By choosing the right technology for each part of thecommunication network some significant benefits can be realized.

PoE technology adds a power capability to classic Ethernet. Thetechnology uses the existing or new data cabling infrastructure todistribute power to devices on the Ethernet cable. The Ethernet cablecan be twisted pair CAT5, CAT5e or CAT6, and each has different loopresistance yielding slightly different performance. CAT5 has a loopresistance of 20 for a 100 meter cable, while CAT5e and CAT6 are both12.5 .

Because the data cable is unprotected and the technology uses a DCvoltage, the maximum voltage level on the cable is limited to 57V forsafety. These cable resistance and voltage level restrictions put somelimitations on the efficiency of the system, especially over longdistances.

According to the IEEE802.3af standard, the drop on the cable for amaximum power of 13W is 7V, resulting in a total loss of 7V x 350mA =2.45W. For the draft IEEE802.3at (D3.0) standard, the drop for amaximum power of 25.5W is 9V, giving a total loss of 9V x 600mA = 5.4W.However, despite these limitations, this technology does supportgigabit Ethernet over cables of 100m in length, with a marginalincremental cost for power distribution.

On the other side of the spectrum, PLC technology takes advantage ofexisting power lines and distributes the data on top of the powersignal. There are two popular applications for PLC technology today.The first one is a SOHO network where the existing power grid in ahouse or office is used to distribute data. The second is in industrialapplications, such as power metering, where the meter output data isprovided over long distances over the power cables.

But PLC is not without its own issues. The primary concern centersaround the fact that power lines are inherently a very noisyenvironment; every time a device turns on or off, it introduces a 'pop'or 'click' onto the line. The system must be designed to deal withthese natural signaling disruptions and work around them. This isespecially true in industrial environments where the error rate shouldbe low and the cable distances could be high.

In this type of configuration often the frequency shift keying (FSK)modulation technique is used. This approach guarantees good dataquality but does not allow for very high data rates. SOHO applicationsare typically over smaller distances but require higher data rates andtherefore the orthogonal frequency-division multiplexing (OFDM)modulation scheme is used. This is a multi-carrier scheme, allowinghigher data rates.

A third technology that provides power and data over the same mediumis USB. Compared with PoE and PLC, a USB interface is most commonlyapplied to PC peripherals. One drawback of this interface type is thelimited power it can bring to the application. Another disadvantage isdistance, as it is only possible to span about four meters between thepower source and the application. This fact excludes many potentialhome and industrial applications.

Figure 1 below compares allthree technologies. It is clear that the PLC excels in bridging thelongest distance for power and data. On the other hand, the PLCsolution is quite expensive due to the complex electronics required formodulating the data signal on the power signal. Therefore this is not atechnology that can be applied cost effectively to single devices, andis better suited to use in switches and hubs.

Figure1: Comparison of technologies for data transmission

A potential threat for all three could be wireless LAN technology(Wi-Fi, WiMAX, Bluetooth), since this technology also reduces theamount of cabling and can bridge long distances.
One argument againstthe use of wireless in buildings revolves around the shielding effectof metallic materials embedded in the concrete structure which canblock the wireless signal. Because of this it is usually necessary toinstall several accesspoints to ensure good coverage.

Secondly, to build up a wireless bridge there must be a wireless accesspoint in every appliance or device to enable it to receive the signal.The wireless access point combined with a traditional power converterhas a cost disadvantage against standard Ethernet with PoE. Forindustrial environments wireless is limited to non-secure, non-criticalapplications as there is always a risk that the wireless signal will beshielded or interrupted by accident.

By utilizing PoE and PLC technology where they best fit, a systemcan be optimized for cost and efficiency. If we consider a potentialhome application like a set-top box, the box is always located in aplace where power outlets are available.

In Table 1 below , apotential home application of a set-top box is explained. The networkenters the home via an ADSL line, optical ONT or coax network. Data isdistributed over the power network to every power plug in the house.

Table1: A potential home application of a set-top box.

By alternating two existing technologies for building and industrialautomation, or home and office applications, several advantages can beachieved. It is possible to optimize the cost of installation for apower and data network while still maintaining a high level of powerefficiency for the overall system.

Koen Geirnaert is PoEand PLCproduct marketingmanager at ON Semiconductor.

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