Check out the datasheet on the product, i have copied the POE section of that datasheet below.
I would have preferred to understand the Class of POE device or the maximum power consumed by the camera.
The Video camera manufacturers datasheet, should be able to tell you exactly how much power should be consumed and the power class of the camera, to better allow you to plan for the appropriate switch..
Anyway, there is a power budget within the switch that is spread across the switch ports, or directed at a few switch ports.. It depends on how the Powered Devices negotaite POE.
I will use the SF300-24P and the statistics from the table above in my example below.
The switch as you can see from the table above can support 180W over 24switch ports. or ;
180Watts / 24 ports = around 7.5 watts of power can be supplied to all ports at the same time.
In your case the calculation would be ;
180Watts / 21 ports = around 8.5 watts of power can be supplied to 21 ports at the same time.
Or, if a Powered Devices (PD) needs more power on a switch port, it can be supplied, depending on how the Powered Device negotiates POE..
But my switch is very smart in how it manages and allows you to manage Powered Devices (PD), such as your video cameras.
PoE implements on the switch in the following 3 stages:
Detection—Sends special pulses on the copper cable. When a PoE device is located at the other end, that device responds to these pulses.
Classification—Negotiation between the Power Sourcing Equipment (PSE) and the Powered Device (PD) commences after the Detection stage. During negotiation, the PD specifies its class, which is the amount of maximum power that the PD consumes.
Power Consumption—After the classification stage completes, the PSE provides power to the PD. If the PD supports PoE, but without classification, the switch assumes it to be class 0 powered device (the maximum). If a PD tries to consume more power than permitted by the standard, the PSE stops supplying power to the port. Just some added protection for the port
PoE supports two modes:
Port Limit—The maximum power the switch agrees to supply is limited to the value the system administrator configures, regardless of the Classification result. This is something you can set if you want.
Class Power Limit—The maximum power the switch agrees to supply is determined by the results of the Classification stage. This means that it is set as per the Client's (video camera) request.
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