My Quest for Power

I learned very quickly when I got started in wireless that Wi-Fi junkies love their IEEE standards documentation. As such, I decided to take a look at some IEEE standards and amendments. To get started, I downloaded and spent a bit of time reading IEEE Std 802.3-2008: CSMA/CD and IEEE Std 802.3at-2009.

I know what you're thinking: "What the heck? Those are wired standards, not wireless!". You are correct. However, the specifications for Power-over-Ethernet (PoE) and PoE+ can be found within these documents (Clause 33). A major concern for any wireless project is always going to be that of powering the access points. 

Here is a brief snippet of the information I managed to pull from the specs:

Classification of a Powered Device (PD)

Not all devices require the same amount of power. The Power Source Equipment (PSE) can try and classify the PD to help manage the available power and ensure that all devices get only as much power as they need. There are currently, 5 classes of PDs.

Class	Min. power level at PSE for 802.3af	Min. Power level at PSE for 802.3at
0	15.4 W	15.4 W
1	4.0 W	4.0 W
2	7.0 W	7.0 W
3	15.4 W	15.4 W
4	Treat as class 0	25.5 W

 Note: 802.3af is also referred to as poE and 802.3at is also referred to as PoE+.

As of IEEE Std 802.3at-2009, there are several different methods for determining a PDs classification: 

1. 1-Event Physical Layer classification (Can advertise classes 0,1,2, or 3.)
2. 2-Event Physical Layer classification (Can advertise class 4)
3. Data Link Layer classification (Can advertise class 4)

If a device only implements 1-Event classification, then it is referred to as a Type 1 device and can only draw upto 15.4 W at the PSE (PoE). If a device implements 2-Event classification and Data Link Layer classification, then it can draw upto 25.5 W (PoE+).

Types of PSEs

There are two types of PSEs: Endpoint PSEs (ie. a PoE-capable switch), and Midspan PSEs (ie. an injector). A few interesting details about the capabilities and allowances of both types: 

Medium	Endpoint 802.3af	Midspan 802.3af	Endpoint 802.3at	Midspan 802.3at
10BASE-T	Supported	Supported	Supported	Supported
100BASE-TX	Supported	Supported	Supported	Supported
1000BASE-T	Supported	Not Supported	Supported	Supported

So what am I getting at?

For many of you, this is nothing new. And quite honestly, aside from the different classes, it wasn't really new for me either. However, the exercise of reading throught the standard was really my goal here. There are some take-aways and questions though: 

• If you are using a  1000BASE-T PoE (802.3af) midspan injector, it technically is not supported under the standard. Results may very from injector to injector.
• There are some PSEs on the market that claim to provide upto 51 Watts of power. They do this by providing power over all four twisted pairs instead of only two. This is specifically noted as an unsupported configuration in IEEE 802.3at-2009 Clause 33. Therefore, these devices are also proprietary and may not play well with your gear. 
• What will power requirements for 802.11ac and 802.11ad look like? Currently, 3x3:3 APs are pushing the limits of 802.3af so it seems safe to assume that these newer APs will make the leap into 802.3at territory. Can your current switches serve up 802.3at power? If not, wired switch refreshes might need to be timed to coincide with 802.11ac/ad rollouts. What are you thoughts? 

As we deploy more and more access points, the need to properly manage power consumption is going to become more important. Reading through these standards and amendments was a good way to keep myself informed of the different methods and technologies availabe for doing so. At the very least, just learning what all the different acronyms and terms meant will make reading vendor specifications easier for me in the future.

Daniel

Got anything to add to my limited summary of the more interesting power-over-ethernet specs? Feel free to leave a comment and I'll try and get it added in.