It's time to run power over Ethernet

David Berlind: Save hours of work, and make your network more reliable, by combining power and data. With the IEEE 802.3af standard, the time is finally at hand!
Written by David Berlind, Inactive

If you haven't done so already, perhaps now is a good time to start looking into migrating your existing network infrastructure to one that supports the IEEE 802.3af draft standard, which makes possible the distribution of power over Ethernet cabling (PoE). If you're at a green field where a new network is going in -- especially a wireless one -- then you should be looking at making PoE an integral part of your design.

The chief benefit is rather obvious. With PoE, a single wire delivers both communications and power, so you don't need an electrical outlet near the device at the end of an Ethernet run (referred to by the draft standard as the Data Terminal Equipment or DTE). This can be particularly handy for devices that need to be located where a power source isn't convenient: wireless access points (for WLANs), net-enabled security cameras, and Voice over IP-based telephones are good examples. Of course, you'll need devices at both ends of the wire that support 802.3af. Most new and legacy equipment does not support PoE. In fact, manufacturers are just getting around to doing interoperability testing of PoE equipment at the University of New Hampshire.

You should know some other things about the conveniences of running PoE. One of them has to do with servicing the DTE. In the two years I've been operating my two-access point WLAN, I've only had to perform a hard reboot a few times. However, in a corporate scenario with hundreds or thousands of access points, power cycling could become a daily activity. Depending on how your technicians bill for time, getting to and from the access point could become expensive. Imagine all that wasted time just to get to the equipment, pull the plug out and put it back in.

Now imagine that you're sitting at an Simple Network Management Protocol (SNMP) management console like HP OpenView and, instead of sending a technician out to the misbehaving access points, you use a few mouse clicks to turn off the port on an 802.3af-compliant Ethernet switch that corresponds to the access point and then, with another mouse click, you turn it back on.

It gets better. How many times has your power gone out but the phones kept working? It's a lot easier to keep your WLAN (and your VoIP telephones, Net cams, and other DTE) running during a power outage when you deliver the power to them over Ethernet. For example, if you want to keep a non-PoE access point running during an outage, it would need a separate uninterruptible power supply (UPS). With PoE, you only need to back up the switch in your wiring closet with a UPS. Granted, the UPS might have to be a big honkin' UPS (depending on how many devices are connected to the switch), but centralising UPS activity in situations where the DTE can't go could yield dramatic cost savings. One of the big reasons that companies have shied away from VoIP-based telephony is because they are so accustomed to not losing their phone service -- even during a power outage.

But now, thanks to 802.3af, things are changing for the better. By the end of this year, there should be a flurry of devices that support 802.3af. Even if you're not prepared to do a wholesale swap of your networking infrastructure, at least one company -- PowerDsine -- is coming forward with technologies to help you manage the transition. In fact, PowerDsine's circuitry and chipsets can be found in the majority of PoE solutions today.

Of particular note is PowerDsine's 6000 series of midspan devices. As opposed to an endspan device, which is a networking and power switch in one, a midspan device can take the feed from the port of an existing Ethernet switch, and enrich that feed with power. So, for each DTE, a midspan device has two ports: one for the inbound Ethernet signal from the switch, and the other for the outgoing data and power feed to the DTE.

In terms of port density, however, each set of two ports is the equivalent of one functional power-enriched Ethernet port. PowerDsine has four midspan offerings ranging in port density (number of DTEs supported) from one to 24 and in price from $80 per port for the one-port device to approximately $50 per port for the 24-port devices.

Perhaps more interesting is a splitter that PowerDsine is making available in order to support legacy equipment. PowerDsine's $30 splitter splits off the 48-volt current coming down the Ethernet wire and converts it into an external supply that can provide local power to a 5-, 12- or 24-volt device looking for an electrical outlet.

According to PowerDsine marketing vice president Amir Lehr, the company is introducing a proprietary Management Information Base (MIB) to expose users of SNMP-based management consoles to a richer level of detailed information about what is happening in both endspan and midspan devices.

In an interview with me, Lehr discussed how network managers can set up network power consumption profiles. There could be one profile for full utility power and another profile for when the infrastructure is living off of a UPS. Because of the limited amount of time that a UPS can deliver power to the device it's backing up, the latter profile might deprioritise certain DTE in order to conserve power. For example, it could give security cameras first priority, phones second priority, and access points third priority. In a situation like that, if the UPS is suddenly engaged, it could notify (via RS-232 port or SNMP interface) the management console that power has been lost. The management console could then send a message to network users that there has been an interruption in power and that they have a few minutes to log out of the network. Then, the console could shut power off to the lowest priority DTE (in this case, the access points).

It all sounds very cool. We've had this sort of non-stop power for our telephones for ages. It's about time the same functionality worked its way into our networking gear. I just hope it works as advertised.

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