Many of you will understand where this is headed. The PBX worked perfectly fine, of course. It just wouldn't work with my Mikrotik devices. So I spent the best part of the day today reading up on what POE actually is, what standards exist, and how Mikrotik takes care of all that.
The PBX in question has the following POE specifications: 11W, 802.3at POE class 3. After reading up on the various POE standards, it's clear that this is a type 1 device, also called 802.3af. And, indeed, the supplier sent me a picture of the PBX being powered by a regular 802.3af POE switch.
So, why doesn't it work with Mikrotik? First and foremost, to my very big surprise, most Mikrotik routers and switches do not follow the 802.3af/at standard, also called active POE. Instead, they provide so called passive POE. There seem to be two major differences.
- First, the voltage provided to the powered device (PD): 802.3af requires a voltage of at least 37V at POE IN of the device which translates in a voltage of at least 44V needed at the power sourcing equipment (PSE). Mikrotik passive POE is available in two versions, low voltage up to 30V, and high voltage above 30V, depending on the device and power adapter use.
- Secondly, between compliant devices, a negotiation takes place and only the power needed by the PD is sent through by the PSE. Since my devices are not compliant, no such negotiation takes place.
Both my RB2011uias and the injector are using passive POE out with 30V so it is clear now why they won't power the PBX. After researching this topic a lot, I do have a better understanding of this but I also have some questions left.
- I'm really interested to understand, why did Mikrotik choose not to follow the standard, what was there to gain? Even some new routers such as RB4011 only support passive POE.
- It seems that some Mikrotik PSEs can be powered by different power adapters. For example, the specification for RB4011 says that it accepts an input voltage of 12V - 57V, yet it is being sold together with a 24V adapter. According to the Mikrotik Wiki on POE, the voltage provided by the PSE depends on the power adapter so by using an adapter with 48V output voltage, I could fulfill the 802.3af requirement of providing 37V to the PD. For some reason, the hardware product pages don't point to the higher powered adapters, though. Am I correct that I would need the 48POW adapter for the RB4011?
- According to the Mikrotik Wiki on POE, by using a higher powered adapter, 802.3af/at devices can be powered that do not require negotiation. Some questions here:
- At least from reading the respective Wikipedia article it seems that compliant devices always perform some sort of negotiation. Type 1 devices, i.e. the typical 802.3af devices, do a negotiation by way of power signature, i.e. hardware-based negotiation, type 2 (POE+) devices may but don't have to implement a separate protocol called LLDP to negotiate power, i.e. software based negotiation. Are both types of negotiation out of bonds for Mikrotik passive POE or just LLDP?
- If Mikrotik doesn't support any type of negotiation, how can we make sure that the correct power is transmitted?
- How can I find out if a compliant device requires POE negotiation or not? This information does not seem readily available for most devices. Is there some rule of thumb which devices will work and which won't?
- What are the risks, if any, of providing power to a compliant, non-Mikrotik PD through a non-compliant switch such as the RB4011, especially when the higher power adapters are used?
Some of the sources used for my research:
Mikrotik Manual on POE Out: https://wiki.mikrotik.com/wiki/Manual:PoE-Out
Netgear Blog on passive POE: https://blog.netgear.com/blog/active-or ... -question/
Wikipedia on POE: https://en.wikipedia.org/wiki/Power_over_Ethernet
Versatek on POE negotiation: https://www.versatek.com/blog/poe-negot ... -protocol/
Extreme Networks on POE device classification: https://gtacknowledge.extremenetworks.c ... ation-work
Product page RB2011: https://mikrotik.com/product/RB2011UiAS-2HnD-IN (max input and output voltage 30V)
Product page RB4011: https://mikrotik.com/product/rb4011igs_rm (can be powered by higher voltage power supply)
Product page RBGPOE: https://mikrotik.com/product/RBGPOE (can be powered by higher voltage power supply)
Product page 48POW: https://mikrotik.com/product/48POW