Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 588)

Power over Ethernet Introduction
Software Reference for x310 Series Switches
22.4 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
PoE+ (IEEE 802.3at)
The IEEE 802.3at-2009 standard specifies how power is distributed together with data on
twisted pair Ethernet LAN cables. PoE+ supplies the higher power required by a new
generation of network attached devices. These devices, such as, multiple radio IEEE
802.11n wireless access points, powered pan tilt and zoom IP security cameras, thin
clients, door locks, touch screen displays, and video phones frequently require more than
the 12.9 W (at a nominal 54 VDC) available under the IEEE 802.3af standard. The IEEE
802.3at specification provides for up to 30 W of power at the PSE, of which 25.5 W is
available to the PD.
The standard also requires that PDs support a flexible Layer 2 power classification method
using Link Layer Discovery Protocol Media Endpoint Devices (LLDP-MED). The use of LLDP-
MED for power classification provides PoE power allocation in steps of 1 watt, along with
an ability to reallocate power, for improved power allocation and management between
the PSE and PD. For more information see
“LLDP-MED (TIA-1057) with PoE+ (IEEE
802.3at)” on page 22.4 . The IEEE 802.3at specification is backwards compatible with the
IEEE 802.3af specification. Devices that support the IEEE 802.3at specification are
optimized to operate with IEEE 802.3at PSEs to support dynamic power management.
PSEs that support the IEEE 802.3af specification can still interoperate with IEEE 802.3at
compliant PDs, providing that the PD can operate using 12.95 W of power. However, these
PDs will operate without the dynamic power allocation and management feature.
Differences between PoE and PoE+
The major differences between the IEEE 802.3af (PoE) and the IEEE 802.3at (PoE+)
standards relate to the power that the PDs are allowed to consume, and the ability to
dynamically manage the power supplied to each PD. The following table summarizes the
major differences in terms of their applied voltages and power ratings.
LLDP-MED (TIA-1057) with PoE+ (IEEE 802.3at)
The IEEE 802.1AB standard, Link Layer Discovery Protocol (LLDP) was designed to provide
a multi-vendor solution for the discovery of network devices and accurate physical
topology of how these devices are connected to one another. LLDP allows network
devices to advertise their basic configuration and device capabilities to other network
devices on the same LAN.
The IEEE 802.1AB standard was extended by the Telecommunications Industry Association
(TIA) to fill the need for multi-vendor VoIP deployments. The TIA created the TIA-1057
standard, Link Layer Discovery Protocol Media Endpoint Devices (LLDP-MED), which
allows for Media Endpoint Devices, such as VoIP phones, to exchange configuration
information, including Power over Ethernet management. The TIA-1057 standard and the
IEEE 802.3at standard provide for the following advanced PoE management capabilities:
■ Fine grain PoE power allocation (1 watt granularity instead of wider power class
bands)
Standard
Voltage DC
at PSE
Cabling
Power
Supplied
by PSE Power
Available
to PD
Nominal
Current
IEEE 802.3af 44 V 2 pairs (CAT 3
or better) 15.4 W 12.95 W 350 mA
IEEE 802.3at (44 to 57) V 2 pairs (CAT 5
or better) 30 W 25.5 W 600 mA