Cable Tests
61
connected between a switch or a router without PoE / PoE+ and the PD. These
Midspans inject the current required to feed the PD.
To deliver power to the PD, the PoE / PoE+ standards define to different options. The
alternative A injects power between wires 1/2 and 3/6 while the Alternative B uses pairs
4/5 and 7/8 to deliver power. The PoE / PoE+ implements a simple protocol to avoid
injecting power in non-PoE devices. Before injecting a large amount of current in the
line, the PSE senses the PD to see how much current it consumes. Once the PD
identification has finished the PSE increases the voltage and classifies the PD
depending on the consumed current in five categories numbered from 0 to 4. Finally
Figure 3.3: PoE / PoE+ powering alternatives: (a) Alternative A, power is injected between
wires 1/2 and 3/6. (b) Alternative B, power is injected between wires 4/5 and 7/8.
(a)
(b)
1
2
4
5
7
8
3
6
1
2
4
5
7
8
3
6
PSE
PD
48 V
1
2
4
5
7
8
3
6
1
2
4
5
7
8
3
6
PSE
PD
48 V
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