Power Over Ethernet
From Internetworkpro
Power Over Ethernet or PoE refers to either IEEE 802.3af or Cisco's inline power standard. Moving forward, it may also refer to IEEE 802.3at, a future standard.
Contents |
[edit] IEEE 802.3af
The IEEE 802.3af inline power standard uses a signiture resistance of 25K Ohm to determine if a device is PoE capable. If this is detected on the return path, power is provided. This process is shown below:
The general process is upon connection, the switch will send a voltage between -2.8V and -10V to test if we have a 25K ohm resistance on the connected device. If this is detected, PoE will be provided.
There are two modes in IEEE 802.3af of providing PoE. Mode A, where power is provided inline using pins 1-2 and 3-6 (inline, using the same pins as 10Base-T and 100Base-TX) and Mode B, where power is provided using the 'unused' pins 4-5 and 7-8 are used. These pins are only unused in 10Base-T and 100Base-TX. 1000Base-TX uses these for data delivery and thus should use Mode A.
While the standard allows a switch to select either mode A or B, a PoE device must accept both. Cisco use Mode A.
[edit] Cisco Inline Power
The Cisco inline power standard uses fast link pulses to determine if the end device is PoE capable. The process is shown below:
The general process is upon connection, the Cisco switch will send an FLP (fast link pulse) to the connected device in order to negotiate speed and duplex settings. In the case of an inline power capable device, these FLP's will be looped back. Once we have determined this, we power the device using PoE.
Cisco's PoE standard delivers 48V DC at a maximum of 7.6W. This power is always delivered "inline" using pins 1-2 for delivery and pins 3-6 return. These pairs are also used for transmission in 10Base-T and 100Base-TX Ethernet.
[edit] IEEE 802.3at
IEEE 802.3at is a standard in progress intended to succeed 802.3af (above). Basing itself on a similar design, 802.3at will provide up to 56W of power. This will be achieved by using four pairs of Category 5 cable to provide power (Category 3 is supported with 802.3af). A major goal of the standard is to be backwards compatible with existing 802.3af devices.
The increased power of this standard could power next generation access points, thin clients, video phones and similar devices. Delivering such power in high densities will be an engineering challenge, especially regarding efficient heat and power consumption on the switch.
[edit] Technical Information
[edit] Diagnosing Power over Ethernet issues
[edit] Calculating anticipated Power over Ethernet usage
The cisco power calculator can be used as a 'best-guess' effort toward power over ethernet capacity based on most known Cisco models, phones, and switchports. CCO Login required [Cisco Power Calculator]
[edit] Displaying actual Power over Ethernet usage
Switch# show power detail
Power Fan Inline
Supply Model No Type Status Sensor Status
------ ---------------- --------- ----------- ------- -------
PS1 PWR-C45-1300ACV AC 1300W good good good
PS2 none -- -- -- --
Power supplies needed by system : 1
Power supplies currently available : 1
Power Summary Maximum
(in Watts) Used Available
---------------------- ---- ---------
System Power (12V) 480 1000
Inline Power (-50V) 138 800
Backplane Power (3.3V) 0 0
---------------------- ---- ---------
Total 618 (not to exceed Total Maximum Available = 1300)
Module Inline Power Summary (Watts)
(12V -> -48V on board conversion)
---------------------------------
Maximum
Mod Used Available
--- ---- ---------
1 128 158
--- ---- ---------
Watts Used of System Power (12V)
Mod Model currently out of reset in reset
---- ----------------- --------- ------------ --------
1 WS-X4013+TS 330 330 330
2 WS-X4548-GB-RJ45V 60 60 20
3 WS-X4548-GB-RJ45V 60 60 20
-- Fan Tray 30 -- --
----------------------- --------- ------------ -------
Total 480 450 370
Watts used of Chassis Inline Power (-50V)
Inline Power Admin Inline Power Oper
Mod Model PS Device PS Device Efficiency
---- ----------------- ---------------- ---------------- ----------
2 WS-X4548-GB-RJ45V 138 123 73 65 89
3 WS-X4548-GB-RJ45V 0 0 22 20 89
----------------------- ---------------- ---------------- ----------
Total 138 123 95 85
Watts used of Module Inline Power (12V -> -50V)
Inline Power Admin Inline Power Oper
Mod Model PS Device PS Device Efficiency
---- ----------------- ---------------- ---------------- ----------
1 WS-X4013+TS 128 128 64 64 100
----------------------- ---------------- ---------------- ----------
Switch#
Switch# show power inline g1/1
Module 1 Inline Power Supply: Available:158(w) Used:128(w) Remaining:30(w)
Interface Admin Oper Power(Watts) Device Class
From PS To Device
--------- ------ ---------- ---------- ---------- ------------------- -----
Gi1/1 auto on 10.3 10.3 CNU Platform 3
Interface AdminPowerMax AdminConsumption
(Watts) (Watts)
---------- --------------- --------------------
Gi1/1 15.4 15.4
Thanks to http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/25sg/configuration/guide/PoE.html
[edit] Overriding default values
In some networks, use of PoE power cubes may be used in front of DC-powered devices that are not ethernet-aware. These devices tend to be hungry, and may draw the maximum 802.3af standard (15.4 watts) - far more than the devices often use. PoE can be overridden with power inline max consumption on a port.
