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Key Technology of POE--Part II

Posted on 5th May 2016 @ 11:52 AM

 IEEE802.3af description:

1、 Outline
As early as 1999year, Power over Ethernet is not new concept. In fact, at that time there have been many companies offer dedicated solutions. Including 3Com,PowerDsine and Cisco and so on. But a relatively new idea is to developing an industry standard. Among  various devices to ensure comprehensive interoperability.

In 1999year, IEEE develop the standard, the earliest participating manufactures is including 3Com,Intel, PowerDsine, Nortel, Mitel and National Semiconductor. But, the standard has restricted the expansion of the market, until June 2003,IEEE ratify the standard of 802.3af, It clearly defines power detection and control matters in the remote system.
And routers, switches and hubs throught  Ethernet cable to supply power way for IP phones,security systems and wireless LAN access points and other provisions were adopted.

2、 POE Key Technology
1)POE System configuration and power parameters
A complete POE system includes two parts, including supply -side equipment (PSE, Power Sourcing Equipment) and Powered Device (PD, Power Device).

2、PSE device is for Ethernet Client device.also is the Managers of the entire POE process, The PD device is PSE load powered acceptance,for example, IP telephone,Network security cameras,AP and PDA or mobile phone adapter and so on. 

3、POE standard powering system’s detail parameter:
a、 a.Voltage is between 44~57v,Typical values is 48V.
b、 Maximum allowable current is 550mA,Maximum starting current is 500mA.
c、Typical operating current is 10~350Ma.
d、Under no-load conditions, maximum electric current is 5mA
e、Provide 3.84 ~ 12.95W five levels of electric power to the PD request , the maximum does not exceed 13W.

4、The detail of the five level power:
IEEE 802.3af PoE standard allows the designer through an Ethernet cable provides up to 12.95W power to the powered device (PD), also comply with safety extra low voltage (SELV) requirements.

According to the IEEE 802.3af standard , PD according to required power is divided into four different levels ( fifth grade Class 4 is reserved for the standard of the new higher power)

  ◆Maximum operating power of Class 0 equipment required is 0 ~ 12.95W

1) Class 1 equipment  required operating power between 0~3.84w
Class 2 equipment required operating power between 3.85~6.49w
Class 3 equipment required operating power between 6.5~12.95w

Designers can place their equipment designated for a specific level based on power requirements. Low-cost solution can be used to specify general default level (Class 0), PD represents the highest operating power required 12.95W

2) POE operation during supply POE Power 
In the classification stage , PSE will be applied to PD with a voltage of 15-20V , and is determined by measuring the specific level PD current size . At this stage , the power will be part of the PD undervoltage lockout (UVLO) circuit maintained in a passive state , in order to isolate the switch level until the signature and classification phases. Once the grading is completed , PSE will provide full operating voltage to the PD


When arranged PSE power supply terminal equipment in a network , POE Power over Ethernet working process is as follows

Detection: First, PSE will send a test voltage to the network device to detect powered device in a common mode resistance 24.9kΩ. Test signal begins to 2.5V, then raised to 10V, which will help compensate the loss of  Cat-5 cable own .  Because this cable up to 100m. If the PSE detects the proper impedance characteristic from the PD (24.9kΩ), it will continue to enhance the voltage. If you can not detect the characteristic impedance , PSE will not power up the cable .
Powered device circuit Zener diode rest of the system will ensure that the interference signal from the test

PD Equipment Category : When it is detected after the receiving end to the PD , PSE will be applied to PD 15 ~20V voltage, and is determined by measuring the specific level PD current size . If, in addition to the first stage of resistance detected outside,it did not find other classification circuit , the device is defined as zero level. At this stage , the power will be part of the PD undervoltage lockout (UVLO) circuit which maintained in a passive state , in order to isolate the switch level until the signature and classification.

  ◆Start Offering power: After completion of grading , in a configurable time ( generally less than 15μs)with start-up period , PSE equipment is froming low voltage to  PD device

a)Offering Power, until provide 48V DC power.
b)Offering power,to provide a  stable and reliable 48V DC for PD device, the device does not meet the PD across the 12.95W power.
c)Disconnect power : If the PD device is disconnected from the network , PSE will quickly ( generally within the 300 ~ 400ms ) stop power supply to the PD , and repeat the detection process to detect the cable terminal connecting PD

◆In the above process , mainly the following description of the process
Step1:Detection, PSE detect whether the PD exists. 
(1) This major operation is , PSE capacitance value detected by the resistance between the power output line to determine whether the PD exists.
(2) The output voltage of detection is 2.8v~10v,it is consistent with the -48V output voltage.
(3) The features of PD existing
     The DC resistance between 19K~26.5hm
     Capacitance value does not exceed 150nF

Step2:Classification,PSE confirm PD power.
(1)The major operation of this step is PSE confirming PD power level determined by detecting the output current.
(2)Classification Phase output voltage Port size 15.5V ~ 20.5V. -48V Output voltage polarity and consistent
Step3: Power-up,PSE offering power for PD.
A major operation of this step , when it is detected at the port on a device belonging to legitimate the PD and PSE to complete this PD classification ( optional) 

Step4: RTP & Power management,real-time monitoring,power management

Step5:Disconnection, PSE detects whether PD disconnected
◆Detection:The major operation of this step is, PSE through specific testing   methods to determine whether the PD has been disconnected ,  if the PD is disconnected ,PSE will close the port  output voltage ,port state returns to detection.

1.When connecting any network device to a PSE, first PSE must test that equipment are not PD, to  ensure that does not give POE Ethernet Device providing current standards, as this may cause damage.
As an extension of the discovery process , PD can also claim the PSE power supplying classify ,help making PSE provides power in an efficient manner . Once PSE started to provide power , it will continuously monitor PD current input , when PD current consumption drops to a minimum value, such as when you unplug the device or experiencing PD device power consumption overload , short circuit, over load, etc. PSE power supply , PSE will disconnect the power and start testing process.

2.Power supply device may also be ability to provide system management, such as the application SNMP (SNMP). This feature can provide services such as night shutdown, restart the distal end of the class.。
Seen from studying POE power supply mode, there are two key issues to consider in the supply process,A PD device identification, and the other is the capacity of the UPS system.

3.The Principle of offering power through cable
Standard UTP cable has four twisted pairs , but l0M BASE-T and 100M BASE-T used in only one of the two pairs.
IEEE80 2.3af allow two uses in Figures 1 and 2 as below.

Powered through the date pin
When the application is idle foot-powered , 4 and 5 -pin connector for the positive electrode , a negative electrode 7-8 pin connector

When the foot-powered application data , the DC power applied to the midpoint of the transmission transformer,it does not affect the data transmission. In this way offline 1,2 and 3,6 pairs can be of any polarity.

Standard does not allow simultaneous application of these in two cases. PSE power supply devices offer only one usage , but the power PD applications must be able to simultaneously accommodate two cases, The standard power supply is usually 48V, 13W. PD device provides 48V to low voltage conversion,it is relatively easy, but at the same time there should be safety insulation voltage of 1500V .
POE standards also regulate that the transmission of electric power should be used unshielded twisted pair cable , it  is including 3,5,5e or Category 6 cable . Clear the existing cable facilities , together with the job does not require any changes , including 3,5,5e or Category 6 cable , various short- wiring board and wiring , electrical outlets and wiring hardware connections. POE standard is compatible with the IEEE 802.3 standard series .

POE Ethernet standard for the use of direct current transmission cables conveyed to POE compatible device ,it  defines two methods : one called " mid-span Law " (Mid -Span), idle-line, using an Ethernet cable is not used for to transmit direct current, corresponding Endpoint PSE function POE support Ethernet switch , router, hub or other network switching .Another method is the End-span.

The following figure is an example of POE power supply system , the power supply equipment PSE, powered device PD and related equipment and Ethernet transmission cable.
Examples of the power supply system with the IEEE 802.3af standard for Ethernet .
When POE PD devices are compatible with the standard, it  can be directly through the RJ-45 Ethernet cable from the power socket for the device,it is not compatible with POE -DC converter used the method of tap dividing means , which is converted into a voltage compatible with POE Voltage. These devices are sometimes referred to active Ethernet splitter (Sputters), it can be a DC voltage Ethernet cable and taken out by a conventional DC jack for PD devices

4、IEEE802.3at description
Broaden PoE applications, PoE (Power over Ethernet, Power over Ethernet ) is a LAN technology , through the twisted pair to provide DC powered device,It has been widely used in undustrial applications in the enterprise. 

IEEE in 2005 ,develop new POE standard 802.3at(POE Plus) to improve POE power transferred.

Dynamic PowerChallenges of the IEEE 802.3  task force is to determine how to extend the classification system. Generally use two classifications. As on the basic of PoE , and now all of the PSE chip can send a first classification pulse , but PoE Plus PSE will send a second pulse . Standard PoE PD will respond to the usual way as a first pulse , and PoE PlusPD can send that it is the fourth kind of PD

The second pulse PSE issued tell that PD is now connected to a high -power PSE. PD response to each pulse show that PSE is connected to a high power PD.