poe - Jiri Brejcha

Unboxing Cisco 802.3bt 60W PoE power injector CW-INJ-8

To enable full radio capability of some of the latest access points, 802.3bt power source is required. If you don’t or can’t have a 802.3bt capable switch, power injector might be the right choice for you.

For Cisco’s CW9179F, CW9178I, CW9172H, and other APs, the correct injector is CW-INJ-8. It takes AC power and injects 802.3bt Type 3 Class 6 (up to 60 Watts) into your twisted pair copper cable, and it is designed for up to 10 Gigabit Ethernet.

Apart from documentation, there is just the power injector in the box. If you don’t have one in your “box of cables” already, order an IEC C13 power cable separately. It is not included.

When is injector not the best choice?

Here is my rule of thumb. If you can, power your AP using a switch. Why? Power injectors in general introduce extra points of failure. Unlike switches, they can’t be remotely monitored or controlled.

Now, if there is no switch available, use a power injector. Site survey, assuming that AC power or battery pack with inverter is available, is another good use case. Or lab use – for when you don’t have a 802.3bt switch.

Hardware overview

There are no surprises on the top side. Note the LED on the top right.

Here is a quick look at the label.

The injector has RJ-45 input, RJ-45 output, and AC power connector which accepts standard IEC C13 cable.

Status LED

Solid green LED indicates that a valid IEEE 802.3bt or 802.3at AP is detected, and the AP consuming PoE power.

Very occasional flashing green indicates that AC power is provided to the injector itself. Don’t expect it to be solid green with no AP connected.

Let’s test it

Microchip makes this pocket-sized PoE tester. Let’s see what we get at the output of this power injector. As we expected, 802.3bt Type 3 Class 6 (up to 60 Watts) it is.

Official documentation

Always refer to the official documentation, please.

How to power Cisco Meraki access point by USB-C battery pack

Battery-powered access points are typically used for site surveys. The engineer uses a professional battery pack with 802.3at/802.3bt PoE output which delivers power to the AP via Ethernet. I do love professional battery packs. Accelerator and VenVolt are your friends. I haven’t had a chance to test either one, but I’ve seen them in very capable hands of others.

Now, when you don’t have a PoE battery pack by hand and want to achieve a similar outcome, this tip might help you.

Use case and requirements

I am building a compact pocket-sized package consisting of an AP, small battery pack and I want to achieve at least 2-3 hours of battery life.

What am I going to use it for? I need to temporary install (and hide) an AP which we will then locate during my presentation using WLAN Pi Go – a mobile Wi-Fi tool which snaps magnetically to iPhone.

In the coming weeks, I am going to use the same setup for point-to-point Wi-Fi testing. So, this will help us kill two birds with one … battery pack I guess 😊

Solution

Cisco Meraki APs do have 12V DC input via barrel jack 5.5 mm x 2.5 mm connector. I then researched USB-C battery packs with USB Power Delivery 12V output. This part is important! Not all USB-C packs support this output voltage.

Finally, I found a USB-C PD cable which negotiates 12V with the battery pack, and outputs 12V DC to the AP’s barrel jack input.

I run the APs in Site Survey mode. To enable this mode connect to this AP’s SSID in normal cloud-managed mode, and browse to the Local Status Page (LSP). It is powered by a web server running locally on the AP. Access LSP via http://10.128.128.126/ while associated to the AP. I do recommend MR32 or newer software release which has some great survey mode enhancements.

Now, it is time to test and see if we can break things. How long does the battery last for? We get 6+ hours of battery life.

Let’s appreciate that the battery pack is nearly 7x smaller in volume than the already compact access point.

Kit list

Cisco Meraki MR20, MR78, or other AP which accepts 12V barrel jack input. Newer Cisco Wireless APs do have a barrel jack connector but they require 54V input. This approach won’t work.

USB-C PD cable with 5.5 mm x 2.5 mm barrel jack connector

Anker 533 Power Bank PowerCore 30W or any other battery pack – just make sure it supports USB PD 12V output, not all do

It doesn’t stop there

Many other devices like routers, modems, home automation hubs use the same barrel jack connector and take 12V input. The main fact to keep in mind is that this Anker battery pack can output up to 12 Volts at 1.5 Amps on its USB-C port. We don’t want to overshoot these 18 Watts.

Incompatible power injector with Cisco Catalyst Wireless CW9166 Wi-Fi access point

Just a very quick reminder that if you power your CW AP using an incompatible 802.3at power injector, you will likely see the AP successfully boot up, but it disables its radios few seconds later. The result is no SSID put on the air.

What to look for in the logs?

If you console or SSH into the AP, you will see this error message. Followed by radio interfaces going down.

set_sys_cond_state: condition critical state 4

condition critical state 4 error message

That’s it. Use officially supported injectors, and save yourself from the trouble I ran into 😊

Portable Catalyst 9136 Wi-Fi 6E demo powered by Zyxel 802.3bt power injector

I am building a portable Wi-fi 6E demo in a box solution. What do I use for that?

PoE powered FriendlyElec’s NanoPi R5S runs iperf3 server. Here a quick iperf3 performance review of this little, 2.5 GbE, and mighty Linux box.

My Catalyst 9800-CL controller is hosted on a cloud, so I don’t need any hardware for that. Finally, my Catalyst 9136 Wi-Fi 6E AP is powered by a Catalyst 3560CX 10 Gigabit Ethernet multigigabit switch.

Catalyst 9136 is Cisco’s premium AP with all the bells and whistles including hexa-radio architecture and built-in environmental sensors for smart building use cases. It requires an 802.3bt/UPOE power source to enable 6 GHz radio in full performance 4×4 MIMO mode. The switch I use supports 802.3at/PoE+, which is great, but 6 GHz radio downshifts to 2×2. And that’s where an 802.3bt power injector comes to the rescue.

Zyxel 5G PoE++ Injector

Cisco’s 5 GbE 802.11bt power injector (AIR-PWRINJ7=) is now available, and that’s my go to option for production use.

Since the Cisco injector isn’t widely available yet, I decided to test this Zyxel one. It provides 802.3bt power and allows the AP to run in full power and full 4×4 6 GHz radio mode with no compromise.

Do I like power injectors in production?

Absolutely not! Ideally you should design for 802.3bt/UPOE switches to power all your new APs via PoE.

It allows you to:

easily, centrally and remotely monitor how much power the APs use
enable/disable power on a port to bounce an AP
leverage redundant Platinum-rated power supplies for the AC to DC power conversion
manage the solution with ease – just think how difficult it is to manage more than 1 power injector, the number of AC power sockets, and what happens when someone disconnects the injector?

I still use C3650 UPOE mGig switch in my lab. Catalysts 9300 and 9400 the best choice these days.

UPOE and mGig capable C3650 providing full power to the AP

Final look

Carrying a full-size switch is not really an option for me, because small form factor is my main goal. So a power injector works best for me. But if I could I would love to use a compact 802.3bt switch.

Are you wondering if the PoE splitter connected to my iperf3 server (the little black box with 3 Ethernet interfaces) actually negotiated 2.5 Gbps Full duplex with the switch? Yes, it did. But keep in mind that the PoE splitter is technically only rated for 1 GbE. So use as short patch cable as possible and ideally CAT6.