PoE Midspan With Switch Tested

Published Mar 23, 2016 04:00 AM

Both PoE switches and midspans generally provide power to network devices.

One question that is regularly raised in IPVM classes is whether combining the two will cause problems, e.g., the camera to be fried.

Using a PoE switch with a midspan injector may occur when devices depending on especially high power (long range IR, PTZs, heaters / blowers) are used, needing more than the 15 or 30 watts typical switches supply.

We know from experience that this combination is not a risk but to show students, we tested this. In this report, we share our findings including videos. 

PoE For Surveillance Background

PoE is commonly used to power IP cameras, eliminating a separate cable and supply for power. In most cases, a PoE switch combines both switching and power supply into a single unit. However, for older non-PoE switches or where PoE demand outstrips supply, a separate unit called a midspan is often used. More: PoE: Switch vs. Midspan and PoE Guide for IP Video Surveillance.

Test Equipment

In this test, we used a Cisco SF302 SwitchHikvision MidspanPelco IXP51 IP Camera, & a Byte Brothers Power Panel [link no longer available]. Related, we have tested more than 20 cameras power consumption, results in this report.

Measuring Power

We own a Byte Brother power panel and use that. Here is a video of how it works with IP cameras:

Midspans Inline with Switch

In the video below, we show a camera connected to a midspan injector and PoE switch. Despite the switch port also being configured to send power, the switch interface shows no power being sent to the midspan. Only the camera is receiving PoE power.

In the video below, we show the meter inline between the switch and midspan. The midspan is still the only device providing power.  Notice that power is not passed from the switch to the midspan, with the meter showing 0. However, all equipment is still powered/working as expected.

Camera Powered By Switch

Just as an example to show the same camera powered directly by the PoE switch. Power measurements are similar to those seen between midspan and camera.

Explaining Results: No Doubled Power

So to recap the results:

  • The device directly connected to the camera is the only device providing power to camera.
  • With the midspan inline, the switch is NOT providing power to the camera nor to the injector despite being connected to them.
  • Only the camera is receiving PoE power in this scenario.

The reason this is the case is due to the PoE negotiation process between camera and source.  In our setup, the camera identifies itself as requesting PoE from the midspan injector, which sends it power.  However, the midspan injector does not request PoE from the switch, so no power is sent.  In this way, safety is preserved and no power is delivered to any devices unless specifically requested.  

PoE Specs Prevent This

The underlying power-over-ethernet specification, IEEE 802.3af, prevents power from being dangerously doubled or added to in a network chain of devices. When using a PoE switch and a midspan injector together, the switch simply will only send data and not power.

All 802.3af 'PDs', or powered devices like cameras, will identify themselves as requiring PoE even when not powered. This negotiation process happens where the power source, or 'PSE' like a switch or midspan sends low voltage levels on the port, looking for a characteristic impedance in response. If no response, or an incorrect response is returned, no power is sent.

In the case of our midspan and switch, no request is made nor returned by the midspan, so the switch sends no potentially damaging power.

Comments are shown for subscribers only. Login or Join