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Fledgling market

At the moment, there are not many devices that get their power from the network. But recently, the number of product announcements at websites such as www.poweroverethernet.com has been growing.

The obviously most common application of PoE is wireless access points. Once they get power from the network, they can be put just about anywhere, which greatly lowers costs if you consider the hourly earnings of an electrician. Entry level devices rarely support 802.3af, though the added value from PoE seems to justify the higher price of professional units.

PoE-powered surveillance cameras like Sony's SNC-CS50P with its high price tag are intended for professional users. Axis' 210A is more affordable. A number of embedded control and measuring devices require a network connection anyway and can easily make do with 12 to 15 W from PoE. Such devices will increasingly get their power from the LAN in the foreseeable future.

There is also room for innovation at home: hard drives could get their power via Ethernet cables, and streaming clients could offer music throughout the house from any LAN connection. Even devices that do not require a data connection could get power via PoE if they have a valid signature.

In the middle

Often, offices have more PCs that need to be connected to the LAN than network sockets to serve them. The 3CNJ105 and 3CNJ225 five-port switches allow them to be connected directly via one cable channel. Because they can get their energy from the data cable, no additional power supply is required. The switches merely pass on the power supply for additional consumers at one of the ports. And if a power supply is nonetheless connected, it itself works as a PoE power source, though only at one port and only with an output of 7 W. This procedure does not violate the standard because end devices that draw more power are switched off. Both switches support authentication via 802.1X and prioritisation. The manageable 225 also supports remote management (SNMP), VLANs (802.1q) and rate limiting.

Power suppliers

Multi-port injectors can be used to provide power to large installations, such as wirelesss with a number of access points. For each port, they need two RJ-45 sockets (output and input) because they do not have the capabilities of an Ethernet switch.

For instance, D-Link's DWL-P1012 supplies power to consumers via 12 ports and can be monitored and configured by means of a special network interface via SNMP and Web interface. Not only the power limiters, which can be set for each individual port, are interesting, but also the option of switching consumers on and off remotely. Because it provides up to 185W of power (12 ports x 15.4 W), it requires an active cooling system. While the AT-GS900/8POE, Allied Telesis' eight-port gigabit switch, does provide power to consumers at all eight outputs, it has to manage its output carefully.

The 3C17205 Ethernet switch can output up to 150 watts. Via SNMP, the administrator can distribute the available power across end devices.

The 3C17205 from 3Com's Superstack series cannot provide the power required at all of its 24 ports. Via SNMP or the management console, the administrator can specify how the available power -- 150 W -- is to be distributed across the various connections. He can choose to set caps for certain ports or reserve a certain amount of power for them.

The standard specifies what individual data fields have to look like for the management of power source equipment. In December, the Internet Engineering Task Force (IETF) published a standardised MIB file for SNMP software.

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