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Power priority mechanism
The priority mechanism is implemented in order to handle cases where the power requested by
the PDs exceeds the switch PoE capacity. This priority mechanism determines the order in
which ports will be powered on after boot, and powered off if the power resources of the module
are exhausted. Three user-configurable port power priority levels are available: low, high &
critical. Within each priority level the lower the port number, the higher the priority (by default all
the ports have low priority).
Disconnected power will be automatically reconnected to the PDs based on their priority,
whenever there is an available power budget. Immediately after the PoE has booted up, it starts
to supply power to the ports where a load is detected. Ports are powered up one after another,
based on the port priority, until the limit is reached. Power calculation is based on the actual
power consumption of the PD. After this, no more ports are powered up until the total power
consumption drops lower than the limit. The limit is 18 Watts below the maximum PoE capacity.
The remaining 18W are reserve power for a change in the power draw of PDs.
Midspan Power Unit
Description
The official name for this device is the 1152A1 Power Unit, but the Midspan Power Unit can also
be called a powered data unit (PDU) or a power over Ethernet (POE) device. The Midspan
Power Unit is 1U in height (1.75 inches or 4.44 cm) and has 24 RJ45 data input jacks on the
bottom row, and 24 data and power output RJ45 jacks. Data flow is unaffected if power is
disrupted and if the endpoint does not require power. An example is a laptop computer that is
connected to the 1152A1. The computer does not receive power from the 1152A1. If the
120-volt power is disrupted to the 1152A1, the computer data stream would not be affected. The
1152A1 unit provides a maximum of 200 watts or a peak of 16.8 watts per port. This unit powers
any device that conforms to the 25-K Ohm resistive signature defined in the IEEE 802.3-2003 af
standard. This unit also powers devices that use the nonstandard capacitive signature, such as
Cisco IP telephones. The 1152A1 provides positive voltage on pins 4/5 and negative voltage on
pins 7/8, which is one of the three methods as described by the IEEE 802.3af standard.
Designed usage
The Midspan Power Unit is designed to mount in a 19-inch data rack, or can be stacked up to
four units high using the optional rubber feet. Its niche is to provide power to only those IP
endpoints that need power. The alternative is to have a switch that incorporates power.
However, any nonpowered device that uses that switch is not using the power capabilities of the
switch, and does not justify the higher price per port of that switch. The Midspan Power Unit
solves this problem by providing power without altering the network topology.
Midspan Power Unit
Issue 3.4.1 June 2005
119
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