With a large 802.af installation, your power budget is a big concern. Existing buildings offer few wiring choices. You can run new electrical lines to your wiring closets, but this is expensive and often unfeasible. Another method is to collapse your wiring closets back to the data center, but Ethernet's distance limitations may prevent this.
Alternatively, you can try to manage your power budget by using devices that support 802.3af's optional "power classification" feature. Many 802.3af phones, for example, support this feature. The last remaining option is to deploy PoE in conjunction with standard wall power at the PD, powering only those devices where AC power isn't conveniently available.
If you're constructing a facility with PoE, make sure multiple 20-amp circuits and/or 220-volt service are available in every wiring closet. In addition, consider deploying high-capacity DC power, which has several advantages over standard AC wall power. It can be converted more efficiently into usable power for your infrastructure, which means better capacity and less heat. DC power also can be set up for centralized fault tolerance, using large banks of DC batteries to store electricity. This eliminates the need for UPSs in data wiring closets, which cuts down on heat and saves on per-closet infrastructure costs.
Power classification is an 802.3af option that gives standards-compliant PDs the opportunity to tell the network how much power they need. This optional part of the standard defines four classes of power that the device can request. Class Zero, the default class, requests full power from the PSE. Class 1 requests no more than 4 watts from the PSE. Class 2 requests no more than 7 watts. Lastly, Class 3 requests full power and at least 7 watts. A fifth class is reserved for future use.
Here's an idea of how much power different devices require: A CDMA2000 cell phone uses 3.6 watts, an 802.11b AP needs 6 watts, a trimode AP needs 12 watts and an IP telephone needs 7 to 10 watts.
Discover the benefits of scalable backbone and network services for application and cloud providers.
Network automation is gaining momentum. Here's how you can drive this powerful technology to its full potential.
Subsea cable alternatives can provide a level of comfort for those concerned about disruptions. Realistically, they will continue to play a small, but critical role in the short term.
