IP Cameras Wasting Power Using Poe?

In class, we had a comment:

"I've had cameras that are not capable of speaking LLDP which, among other things, means that when the switch feeds the device PoE they will be over allocating/wasting power because the device cant tell the switch how much it actually needs. Seems like a logical concern from the IT teams, yet I don't hear much discussion on this topic from vendors."

Any experiences like this or thoughts here?


Most simple unmagaed PoE switches don't support LLDP either.

In a fully managed PoE switch the switch will track power budget and allocate power according to what the devices report. A PoE device will full implementation can say "I'm a Class III device, but I only need a max of 13 Watts, not 15.4" and the switch can then reserve 13 watts of the power budget for that device. If the device only reports it's class, then the switch would reserve a full 15.4 watts.

In simple PoE switches there is no power budgeting at all. Your switch/power supply has a max output wattage, and power is just allocated up to the limit based on total draw at a given moment.

Neither scenario will cause the power supply to push more power to the device causing wasted power in the sense of increasing your electrical consumption.

In most cases the cost delta between a managed switch that supports full LLDP and power budgeting and a cheap switch is large enough that it's not really worth buying the more expensive switch just to be able to put an extra device on it.

Agree with 1 that there is never any power wasting in the sense of more power being delivered than required.

Also agree that in unmanaged Poe switches the power is not regulated, when it's gone it's gone.

But would agree with OP that if the switch is pre-allocating ~15 W for a channel when it needs only 3, that this would be undesirable because it might deny another port power that was actually available.

If that is the case, the OP should be able go into the managed switch and set the power allocation manually. The switches I have seen allow power control on every port.

...if the switch is pre-allocating ~15 W for a channel when it needs only 3, that this would be undesirable because it might deny another port power that was actually available.

Agree, this is the only real concern, and only if the switch has a total power budget less than the product of ports-times-15.4, such as with a cheaper switch that does up to 7.5W/port for all ports, or 15.4W/port on half of them. If it's capable of doing a full 15.4W (or 25.5W if you're dealing with PoE+) on all ports, then per-port allocation doesn't really matter.

Other than that, basic physics come into play: a device will only draw as much current (and thus power) as it needs, regardless of how much extra the source CAN supply.

Interesting, you learn something knew everyday.

...you learn something knew everyday

I learn something I knew everyday, or at least I thought knew it.

The sneaky part about the POE cam power draws is the 'dynamic' one that is typically 'visible' as a camera is powering on. These are higher amounts than the steady state draw.

I have had POE sources fail to provide ample dynamic power to cams with the IR LEDs...causing all the cams to fail as the POE overall power was not maintained.

This is one argument why you'd want a camera and a switch that do full PoE power negotiation. The camera would report its *max* power requirement, and the switch would reserve that much power (or, it would round up to the top of the power class, if the camera reported it needed 4W some switches would reserve 6.49 watts, the Class II max).

However your overall problem is still that your switch is underpowered for the max load required and it's probably cheaper and easier to get a switch that supports full Class III power on all ports and not have to "do the math" to make sure the switch is adequate.

Switches with full-power support on all ports are pretty common these days, much more so than when the whole LLDP power-negotiation stuff was a common design and discussion topic. A basic 24 port managed switch that supports full PoE on all ports can readily be found under $500. At some point the cost savings of a switch with only a 180W power supply vs. a switch with a 375W power supply is not worth the hassle (IMO), especially when you factor in the larger project budget.

"At some point the cost savings of a switch with only a 180W power supply vs. a switch with a 375W power supply is not worth the hassle (IMO), especially when you factor in the larger project budget."

Couldn't agree more. Too many times (and I've been guilty of it too) a person can be mired in one model price versus another model price, but taken as a whole on the project makes a less than 1% difference, it's not worth wasting the time fretting the difference.

It’s important to note that POE power negotiation can happen at two layers. The first is at the physical layer when a device powers up. POE devices signal a Power class from 1-4 that sets a max at 4W,7W,15.4W,30W of supply power respectively. Non-compliant devices are categorized as class 0 and given the 15.4W default.

Once a device is powered up it can renegotiate power using LLDP. LLDP has the advantage of granularity (.1W increments) and dynamic allocation.

So even cameras that don't support LLDP still have an option to tell the PSE to allocate less power. However, LLDP is definitely superior to physical layer negotiation only.

Does anyone have any experience with how this works on Cisco switches? i have a camera at class 3, using around 9.6 watts. The Cisco switch is showing 15.4w, but it wont show me the actual consumption, rather the budgeted allocation. other switches I have worked with will show me both allocated and budgeted. IT is concerned that they are going to may out older switches that are in place if each camera reserves 15.4w.

I found a cisco document advising tonse the port allocation to a lower number, such as 8 or 9w. As long as the switch isnt maxed out, it will flag the port as an issue, but allow it to exceed the 8 watts set and get its full 9.6, in this case, without taking the full 15.4w from the budget.

Any comments or ideas, or is this just an old cisco switch?

What switches are you using? We use mainly the SG300 series, and a few SG200 series, and even those show actual power use per port:

Sorry for thr delay and thanks for the comments. This is with a Cisco WS-C2960S-48LPS-L. IT is stating and the manual seems to read that the camera will be allocated thr full class worth of POE reserving it from being used on other ports.

Amy suggestions?

Any way to see actual consumption or lower reserved to actual usage?

We cant tell the customer to replace all of their cisco switches....

I'm not familiar with the Catalyst line. Looking at Cisco's website, it appears they're primarily managed through external software. Looking at the Quick Setup Guide though, it appears it should also have a web interface, though that only details how to connect to it... maybe try that, see if it looks anything like my screenshot above?

The switch has a rich CLI for POE management, have you tried it?

Yes i have tried. these settings dont really do anything. they arent feature rich. you can specify auto mode or a static reservation or disable poe as well as set a max power amount.

Pretty standard on all smart or web switches i have worked with.

I just got a similar model a 48 port 3560. it does the same thing - allocate the full class. thus the camera needs 9w and 15.4w is reserved.

seems like cisco is being heavy handed trying to encourage end to end solutions which would use their proprietary cdp protocol to accurately detect wattage. somehow all of the other switch manufacturers and even other switches can do this without needing cdp.

Yes i have tried. these settings dont really do anything. they arent feature rich.

To be clear, you are saying that when you use power inline max or static that it limits the power, but does not limit the allocation?

Try

power inline consumption 9500

on the port that draws max 9w.

If your switch is a 3560-E, I believe it's supported.

On our Cat3k switches, there is a command for this. Not sure if it applies to the 2960 though:

> show power inline police

Note that this shows the actual power in Watts, where just "show power inline" shows the power class.

I was in HVAC school years ago and there was a section on electrical, and we had this lab where we would take out bulbs, measure amperage, resistance etc. There was a question as to what would happen if we removed one bulb in series with other bulbs. (not open the circuit, just remove the resistance of the bulb) And I said the current will increase as there is less resistance.

I had a 20 minute debate with the instructor as he told me that circuit was pulling 20 amps regardless, because there was a 20A breaker in the panel. At first I thought we were not talking about the same thing. I had to start teaching HIM about Ohms law....

Doesnt matter if the budget is 500W, that device is only drawing what it needs. Ohms Law

Doesnt matter if the budget is 500W, that device is only drawing what it needs. Ohms Law.

What it draws is not the issue here, it's what the switch reserves that is the concern. When the power is held in reserve, it can't be distributed to other devices. This can be a good thing when the device can really draw the max at some point, because then it won't be starved.

On the other hand, if the device never will draw anywhere near that, its a waste.

they will be over allocating/wasting power because

IP Cameras Wasting Power Using PoE?

The way I read that was there was a thought that somehow the switch was pushing out XX wattage regardless of whether the camera needed it, and power was actually being wasted. Not the budget being wasted.

But I could be confused, wouldn't be the first time....

BTW I coped that from the subject and it is just large, not yelling... :)

It's true that the device is only drawing what it needs, but the if the switch is allocating it as a Class 3 (15.4W) and it's only using 6W, you have 9W that aren't available for other devices. The switch will simply not supply power to other devices once it allocates its budget, whether the actual consumption is hitting the budget limit or not.

So it's not wasting power technically, but it's effectively a waste.

...but it's effectively a waste.

Yes, assuming the device can never draw that much, if it can then some would argue its better to not power a new device than risk not powering an old one.

IMHO, it seemed like this issue was slowly going away as more and more switches provided 15.4w on every port.

But with POE+ now becoming the norm and POE++ on the horizon, I would imagine we will be typically working with switches that can not provide their full per port power on all ports at the same time, so power management will continue to be an issue.

I see a lot of cameras today that don't negotiate the proper PoE class. They just say 'class 0' and move on.

Not sure if it's the camera manufacturers, switch manufacturers (or both) getting lazy about the small details.