Verify the power available PER NETWORK CONNECTION is adequate for your cameras' needs! Often, PoE switches only supply ~7W (e.g., for VoIP phones) per drop. Or, may provide 15W but for only certain combinations of outputs, etc. (note that what the switch *delivers* is not the same as what is AVAILABLE at the PD! E.g., 15W translates to about 13W at the camera). PoE+ ups the power available...
PoE is delivered in two different "modes": A and B. In A, power is carried by the same conductors that carry Rx and Tx data to/from the switch. I.e., only two pairs are required in the cable (at
The camera will require a *normal* cable (not "crossover").
The switch is "just a switch". And, the camera is "just a network device". Nothing special about interconnecting them (though you will need all four pairs to be "intact" if it uses mode B for power delivery -- pull your drops with care! :> )
Why would one need unused pairs for power feeds ? The normal practice is to use separate DC center tap voltage for Tx and Rx pairs, just like phantom feed in audio circuits to power the electrostatic microphone.
The standard allows for power to be delivered over the data pairs (as you described) *or* on the two unused pairs (for 10/100).
Cisco (and others?) have an "unusual" implementation that uses the unused pairs to deliver power -- WITHOUT any of the signalling conventions specified by the standard. (i.e., they just attach a 48V wall wart to the unused pairs and use the cable as an "extension cord").
The same is true of my Motorola broadband radio (though I think they use 28V)
Right. For 10-BaseT and 100-BaseTX, "Mode-A" uses the center taps of the transformers on the data pairs to inject power. "Mode-B" uses the unused pairs for power. Polarity is undefined in either case. I believe a powered device is supposed to use either source to be compliant.
Again, it depends on how power is delivered by the PSE and expected by the PD. I would expect a switch and camera to both be designed as endspan devices expecting power over the same pairs as data (PoE mode A). If your camera (or switch) requires a "midspan injector", then it is using mode B.
Assuming "fully compliant" devices, you have no problems. The issue is when corners are cut and one mode of power delivery is expected/implemented over the other.
[Likewise, the non-standard implementations I mentioned]
The standard assumes the maximum losses in standard cable at maximum length. That's why, for example, the ~15W that you might expect to get from a PoE switch only ends up as ~12.5 USABLE watts (the balance potentially dissipated in the cable)
A compliant PD (i.e., your camera) should *signal* the PSE (your switch) what it's power needs are (this happens automatically when the camera is initially "plugged in" to the network). The switch should either provide the requested power -- or not (if the switch's capabilities aren't up to it).
There are different power levels available -- I think 3, 7 and 15 (nominal) Watts. Regardless of the nameplate on your cameras, they may be sloppy and "request" more power than they need (e.g., the 15W class is the default even if your camera only USES 3!). This could impact the switch's ability to supply as many loads as you THINK it should be able to supply (10 cameras at 3W = 30W! Ah, but the cameras are telling the switch that they NEED 15W so the switch thinks it has to accommodate a 150W load... even though it really is 30W!)
-------------- Thnx for replies. I was on the place today and this is the situation; One part of the cable (where the camera should be connected ) is NOT crimped but the other part is patched to the rack. I will use patch cable to connect between rack and POE switch.
So, the camera can not give more then 10/100 base so i should use MODE-A to connect this. OK. Thank you !
I think this maybe a language issue. I.e., "twisted" as in "not straight thru" (and not "no twisted pairs").
To be more precise, you want CAT5/5e cable (which gives you twisted pairs with *specific* twisting patterns in those pairs) but wired "straight thru" -- i.e., as a "patch cord" not a "crossover cable".
The camera will have an MDI pinout and the switch will be MDIX.