Performance And Capacity; Ge24Poe Card Operation; Hardware Limitations; Hardware Detection - Allied Telesis SwitchBlade x3100 Series Manual

Introduction

3.2 GE24POE Card Operation

3.2.1 Performance and Capacity

The maximum number of supported devices is determined by the number of installed PoE power supplies as
well as the class of PD installed at each PoE port. Refer to the following table.
TABLE 3-1
Configuration
2 PoE PSUs (2400W)
1 PoE PSU (1200W)
For example, with a single PSU supplying 1200 watts and Class 2 PDs drawing 7 watts each over 171 ports, 71%
of the total available ports can use power.

3.2.2 Hardware Limitations

Of the two solutions for providing power over Ethernet cable, only Mode A (using data pairs 1-2 and 3-6) is
supported on the GE24POE.
Only two ports per card at a time are able to detect PDs. The maximum detection period for a PD is 500ms.
This is followed by a 75ms classification period. Given 24 ports per card, this leads to an estimated recovery
time (for PoE) of 6.9 seconds, which is illustrated in the following calculation:
[24 (total ports) / 2 (ports at a time)] * [500ms (detection period) + 75ms (classification period)] = 6.9 seconds
Each GE24POE card has a minimum power allocation of 37W that it can draw from the shared pool. (By dis-
abling all POE ports on the card, you can reduce the allocation to 0W.)

3.2.3 Hardware Detection

Whenever any device is plugged into a port supporting PoE, the system must determine if it is a powered
device and, if so, how much power it needs. The PSE is responsible for managing this process.
The first step is to detect the presence of a PD on the port. When any device is attached to the port (whether
or not it supports PoE) the PSE applies a small amount of voltage to the line. It then looks for a 25 K-Ohm "sig-
nature" resistor on the device. This resistor is located on the front-end of the PD and is isolated from the rest
of the circuitry until detection is complete. If this resistor is detected, the device is accepted as a valid PD.
The next step is for the PSE to attempt to classify the device. To do this it applies a voltage of between 15.5 and
20.5 Vdc to the line for a short period of time (10-75 ms). The amount of current it draws indicates the power
class of the device. If the PD does not support classification, it is assigned the default class of 0 and a maximum
unclassified power consumption of 15.4W is assumed (Classes using greater than 15.4W MUST support classi-
fication). Otherwise the device is assigned a class indicating the maximum amount of power it will draw.
3-2
Software Reference for SwitchBlade x3100 Series Switches (Power over Ethernet (PoE))
Performance and Capacity for the PSUs (for PoE)
Class 2 PDs (7W)
240/240 ports (100%)
171/240 ports (71.3%)
Class 3 PDs (15.4W)
155/240 ports (64.6%)
77/240 ports (32.1%)
Performance and Capacity
Class 4 PDs (30W)
80/240 (33.3%)
40/240 ports (16.7%)