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.
IPVMU Certified | 10/20/15 06:14pm
Interesting, you learn something knew everyday.
Seneca | IPVMU Certified | 10/20/15 07:14pm
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.
IPVMU Certified | 10/20/15 07:43pm
"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.
IPVMU Certified | 10/21/15 02:31am
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?
IPVMU Certified | 12/23/16 03:17pm
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