Power Delivery Options; Poe Negotiation; Additional Poe Power-External Supplies - HP 5400zl Series Technical Overview

Power delivery options

The 802.3af standard provides two options for providing power over CAT 5 Ethernet cable. They are:
• Unused pairs—This option takes advantage of the fact that 10Base-T and 100Base-TX signals use
only two of the four twisted pairs in the cable. In this option, the pins on pairs 1/2 and 3/6 are used
for data, and the pins on pairs 4/5 and 7/8 are used for power. The 4/5 pair is twisted together to
form the positive supply, and the 7/8 pair is twisted together to form the negative supply.
• Data pairs—This option provides power over pairs that also provide data and is necessary because
1000Base-T signals use all four pairs. In this option, either the 1/2 pair or the 3/6 pair can form the
positive supply.
The 802.3af standard requires power sourcing equipment (PSE) to choose between the options. It is not
possible to mix and match or to use all pairs for power transmission. HP ProCurve switches provide
power over the data pairs. However, the standard requires PDs to be able to accept power over either
data pairs or unused pairs.

PoE negotiation

In most networks, PSE will be connected to some devices that support PoE and some that do not.
Consequently, in order to prevent damage to non-PoE devices, the 802.3af specification includes a
negotiation mechanism between PSEs and the stations connected to them. Under the specification,
PSEs apply a slight voltage on the power-delivery pairs. If there is a valid PD on the cable, it will
present a resistance and a capacitance.
Typically, this discovery process requires approximately 500 milliseconds. The PSE will apply the full
wattage if it discovers a PD, but the flow is current-limited to prevent damage to cable and equipment
under fault conditions.
After discovery, a PD must draw a minimum current for the power flow to remain active. If, for
example, the PD is unplugged, the PSE will discontinue power supply over the cable. The discovery
process will be repeated when the PD is returned to service.
The HP ProCurve switches described in this document that are PoE enabled are:
• HP ProCurve Switch 5406zl—supports up to 144 PoE ports
• HP ProCurve Switch 5412zl—supports up to 288 PoE ports
• HP ProCurve Switch 3500yl-24G-PWR—supports up to 24 PoE ports
• HP ProCurve Switch 3500yl-48G-PWR—supports up to 48 PoE ports
Additional PoE power—external supplies
The HP ProCurve 620 Redundant/External Power Supply can be used with up to two 3500yl Switches
as an external supply providing an extra 398 W of PoE power for each switch. This extra power can be
used to provide full PoE power redundancy at 15.4 W for all ports for the 3500yl-24G-PWR. It can
also be used to provide full 15.4 W to all 48 ports on the 3500yl-48G-PWR, or full PoE power
redundancy at 15.4 W for 24 ports on the 48-port switch.
The HP ProCurve Switch zl Power Supply Shelf provides a 3U-high enclosure to hold up to two extra
zl switch power supplies to provide additional PoE power to zl switches beyond the PoE power
available through the switch's internal power supplies alone. These extra supplies can also be used to
provide PoE power redundancy beyond what is possible with switch internal supplies. Large Switch zl
VoIP installations are ideal environments for the HP ProCurve Switch zl Power Supply Shelf.
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