Friday, 26 February 2016

802.3af Is Dead (For Surveys)

I’ve recently been involved with a project that has brought me into contact with a number of wireless engineers who are performing WLAN survey work using the traditional “AP on a stick” (APOAS) survey method. Yes, there are plenty of people out there who still prefer this method, or have customers who demand it.


One thing that has become apparent is that a number of people are still using power sources (generally a battery pack) for their survey AP that still only supports the 802.3af POE (power over Ethernet) standard.


The time has come for those using these legacy power supplies to make an investment and  upgrade their AP power packs to support the higher power provided by the 802.3at standard. The next generation of wireless access points simply won’t allow the continued use of 802.3af power packs, due to the enhanced power requirements of modern APs.


Background


Until relatively recently, many Enterprise grade wireless access points have been able to work with the power budget imposed by the 802.3af POE standard. This means that they are able to work with a power draw of 15.4W or less.


This was sufficient for many APs until the advent of the next generation 802.11ac (“Gigabit Wireless”), back in late 2013. Even after the emergence of this first "wave" of 802.11ac APs, some models could just about fit within the 15.4W power budget of 802.3af POE, or at least run with a reduced feature set that would allow it to be used for a basic RF passive survey.


Example of the types of power pack previously used included the Terrawave battery pack and the Pointsource POE Injector. Whilst these were a reasonable choice as an 802.3af power source, they simply can’t provide the power requirements of newer wireless APs.


802.11ac Wave 2


With the emergence of the “latest and greatest” 802.11ac “Wave 2” access points, the lowly 802.3af POE supply can no longer be used for APOAS surveying.


The additional power requirements of “Wave 2” APs means that a 802.3at power pack must now be available for survey work. This new generation of APs have additional features such as MU-MIMO (multi-user MIMO) and additional radio chains that mean that the legacy 802.3af (15W) power budget simply isn’t enough.

(Note: You may also see 802.3at referred to as "POE+" in some documents)


So, the time has come to put our old, trusty 802.3af power packs out to pasture and ensure we have a power supply that can support 802.3at, and is able to meet the higher power budgets of this next generation of APs.


Power packs that support the 802.3at standard will be able to supply a power draw in excess of 25W, providing a significant increase from the previous 15.4W that we could achieve with 802.3af.


Suggested Power Packs
A quick “Google” around the Internet doesn’t yield any useful results when trying to find a 802.3at POE battery pack - well none that I have found so far with my great sausage fingers…. (update: see end of this article for some suggestions)


A great solution is a home-brew solution that is the subject of a great blog article by Scott Stapleton. It details how to create a 802.3at power pack using a lithium battery pack and POE DC-DC converter. I suggest you get along to his article to find out how to build your own 802.3at power supply. It’s very straightforward to build and turns out to be smaller, lighter and has a higher capacity than the previous lead-acid battery based solutions.


If anyone has any other suggestions for any “off the shelf” 802.3at POE power pack solutions suitable for surveying, please include them in the comments section of this article.


My Power Pack
Here are some pictures of my current AP power pack, based on Scott’s great article, so you can get an idea of what a build-your-own solution might look like. Admittedly, mine looks a little “home made”, but it does the job and lasts many hours on-site - you can get a full day out of a pack like this, depending on the battery you buy, AP power settings etc.


Battery_Pack1.jpg
Fig 1 - Yes, I have to label everything


Fig 2 - A side-on view so that you can appreciate the full beauty of this work of art

(Note: the cut-off RJ45 lug is to stop me plugging the AP into the non-POE port, which causes me to swear under my breath)


I’m sure there are plenty of you out there that could come up with something far more aesthetically pleasing (and that looks less like a bomb…)


References


Get along to Scott’s blog article for full details of how to build your own 802.3at survey power pack.


My components:
  • Battery: Aukey 28000mAH External Battery Pack (UK Amazon link)
  • Converter: Tycon Power 24v-48V DC To DC Gigabit POE (UK Supplier Link)
  • Lots of gaffer tape


References


Here are some off the shelf survey batteries you might also like to take a look at: