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

Koen Geirnaert

May 05, 2009

Koen GeirnaertMay 05, 2009

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

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

PoE technology adds a power capability to classic Ethernet. The technology uses the existing or new data cabling infrastructure to distribute power to devices on the Ethernet cable. The Ethernet cable can be twisted pair CAT5, CAT5e or CAT6, and each has different loop resistance yielding slightly different performance. CAT5 has a loop resistance of 20 for a 100 meter cable, while CAT5e and CAT6 are both 12.5 .

Because the data cable is unprotected and the technology uses a DC voltage, the maximum voltage level on the cable is limited to 57V for safety. These cable resistance and voltage level restrictions put some limitations on the efficiency of the system, especially over long distances.

According to the IEEE802.3af standard, the drop on the cable for a maximum 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 a maximum power of 25.5W is 9V, giving a total loss of 9V x 600mA = 5.4W. However, despite these limitations, this technology does support gigabit Ethernet over cables of 100m in length, with a marginal incremental cost for power distribution.

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

But PLC is not without its own issues. The primary concern centers around the fact that power lines are inherently a very noisy environment; every time a device turns on or off, it introduces a 'pop' or 'click' onto the line. The system must be designed to deal with these natural signaling disruptions and work around them. This is especially true in industrial environments where the error rate should be 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 data quality but does not allow for very high data rates. SOHO applications are typically over smaller distances but require higher data rates and therefore the orthogonal frequency-division multiplexing (OFDM) modulation scheme is used. This is a multi-carrier scheme, allowing higher data rates.

A third technology that provides power and data over the same medium is USB. Compared with PoE and PLC, a USB interface is most commonly applied to PC peripherals. One drawback of this interface type is the limited power it can bring to the application. Another disadvantage is distance, as it is only possible to span about four meters between the power source and the application. This fact excludes many potential home and industrial applications.

Figure 1 below compares all three technologies. It is clear that the PLC excels in bridging the longest distance for power and data. On the other hand, the PLC solution is quite expensive due to the complex electronics required for modulating the data signal on the power signal. Therefore this is not a technology that can be applied cost effectively to single devices, and is better suited to use in switches and hubs.

Figure 1: 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 the amount of cabling and can bridge long distances.
One argument against the use of wireless in buildings revolves around the shielding effect of metallic materials embedded in the concrete structure which can block the wireless signal. Because of this it is usually necessary to install several access points to ensure good coverage.

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

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

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

Table 1: A potential home application of a set-top box.

By alternating two existing technologies for building and industrial automation, or home and office applications, several advantages can be achieved. It is possible to optimize the cost of installation for a power and data network while still maintaining a high level of power efficiency for the overall system.

Koen Geirnaert is PoE and PLC product marketing manager at ON Semiconductor.

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