The PoE provides the solution to transmit the power and network communicating over single Ethernet cable. It saves the installation cost. However the PoE has limited of 100 meters which has relative to both network communicating and voltage drop of the power. In the IEEE802, the maximum network distance is 100 meters between two IP devices. The PoE system require the remain voltage need to be higher than DC38V at the front end device. Otherwise the power system will stop working. Usually the PoE switch inject DC 48V power to the Ethernet cable. After 100 meters transmitting, the remain voltage will drop down to DC 39V which can still satisfied the requirement. however if the power is sending over 100 meters without proper design, the correlated power loss will become issue. Here we need to deal with both network and power constraints to get longer distance PoE.
There are three way to extend the Power over Ethernet. The first one is the PoE extender, which can repeat the PoE up to 400 meters with daisy chain setup. Each PoE extender is capable of to boost up the network and the power. However, the energy loss inline the cable still need to be considered. As only two twisted pair is used to transmit the power, similar as 22AWG copper cable. Only 7.91W remain power left at the end of 400 meters. the missing power is converted to the heat in the twisted pairs. The power loss not only depends on transmitting distance but also the quality of Ethernet cable. Ethernet cable with pure copper is the best choice for repeating the PoE. Basing on our test, the maximum PoE distance can shorten down to 60 meters when we use low cost copper clad aluminum cable, and it can achieve up to 120 meters after we replaced with cat6 pure copper cable. The cat5e and cat6 grade only specific the data rate, it doesn’t specific the material of the Ethernet cable. When choosing Ethernet cable, always make sure the twisted pairs are made of pure copper.
The second solution is the PoE switch with CCTV mode or VLAN support. Once the CCTV mode is turned on in the switch, it can support up to 250 meters transmitting. There is downside with CCTV mode. The Network speed will be down grade to 10 Mbps network. The network speed (data rate) is trade off for the distance. As most of the IP camera request less than 8mbps bandwidth, this kind of solution will work for IP surveillance’s camera. However if you want to extend the network for access point or small cell, the CCTV mode PoE switch is not the right solution. After all 10Mbps is too small bandwidth for network which could have multiple users accessing at the same time. The CCTV mode also will activate the VLAN automatically. The VLAN make sure each ports has independent 10 Mbps bandwidth. Port-1 can’t take up the bandwidth from port-2. What is VLAN? the VLAN isolates the PoE ports. Each PoE ports only can communicate to uplink ports, but not other each. Basically the PoE ports don’t see each other in the LAN. The next question is how CCTV mode switch deal with the voltage drop? The CCTV mode PoE switch usually inject the higher voltage power than DC48V, such as DC55V, so it can have more margin to adapt to voltage drop after 250 meters transmitting.
The last solution is the fiber power system. The fiber optical has barely limited for network communicating. 10 kilometers distance is the typical specification for most of the fiber network. The challenge left is the voltage drop. Thanks to the composite fiber cable which had copper cable integrated. Now both fiber optical and the power can be transmitting in the single composite cable.
The fiber power system work similar as conventional PoE system. In the center the power is injected to the composite fiber cable. When the power arrive at the front end together with fiber optical, there is PoE fiber extender which has integrated both power management and media conversions will convert to standard PoE output, so the PoE extender can supply conventional PoE device, such as PoE IP camera. The voltage drop depends on the thickness of the copper cable. The thickest size of composite copper is 12AWG in the market. This can minimize the voltage drop and supply the sufficient voltage after long distance transmitting. With properly design, the fiber power system can send the power over 2500 – 3000 meters far from the center with standard PoE output.
Other than long distance, people are also looking for high power output for new IoT thing. In the IEEE802.3 af/at standard, the maximum power output is 25.5W. The new PoE standard IEEE802.3bt has come up which employs all 4 twist pairs to send the power. The IEEE802.3bt devices are capable of output 71W maximum power. This will benefits the power hungry device such as LED lighting, PTZ camera, Point of Sale terminals, Thin Client, etc. The transmit the high power in long distance will become the new challenge in the next few year. As the higher current inline, the more correlated power loss will be. We are looking forward to see how the technology evolves.
