What is PoE
Power over Ethernet or PoE is a cost effective solution that allow both data and power to be provided over an (or same) ethernet cable to an endpoint.
Why use PoE
There are a number of reasons why you would use PoE, but in most cases the overriding reason is cost and easy of install.
For an example:
It can cost between $100 and $200 to run an ethernet cable to a wireless access point. But it can costs a further $400 to run an electrical conduit to power that access point.
The other alternative is to install Power Sourcing Equipment (PSE) called Endspan Equipment into the ethernet switch, or by adding Midspan Power Insertion Equipment after the ethernet switch.
This will combine the data and power at the source to be sent through the ethernet cable. The power from the cable can then be extracted at the endpoint with a Powered Device (PD) to operate the access point.
This solution removes the need to run expensive electrical conduit and will cost considerably less than $400.
What is the PoE Standard
Institute of Electrical Electronics Engineers (IEEE) 802.3af Task Force has ratified the technology into the Ethernet standard. The IEEE 802.3af standard covers the interoperability of using PoE products and covers the PSE - Endspan or Midspan, the Powered Device (PD) and the cabling requirements. It is endorsed by all network players that power IP-connected devices, such as voice-over-IP (VoIP) phones, wireless LAN (WLAN) access points (AP) and advanced security devices, over the existing Ethernet cabling infrastructure.
How the power is put onto the Ethernet cable
The power can be added to the ethernet cable if the switch has a built-in power source in the form of PSE is called Endspan Equipment, as shown below:
The POE Endspan method uses the Ethernet switch ports on power sourcing equipment, which have embedded POE technology, to deliver power over the network.
With the Endspan solution, four wires deliver data and power over the network. Specifically, power is carried over the live wire pairs that deliver data, as illustrated above - When using the same live wire pairs that deliver data, different frequencies enable power and data signals to traverse the same pairs of wires. Since data and power travel over different frequencies, the integrity of the data signals is not effected.
Alternatively the power can be added to the cable after the switch by using a PSE called Midspan Equipment, as show below (most used/needed):
The POE Midspan solution uses an intermediate device, usually a powered device, to inject power into the network. The intermediate device is positioned between the switch and the powered device and delivers power over the network using the spare pairs of wires. The intermediate device has multiple channels (typically 6 to 24), and each of the channels has data input and a data plus power RJ-45 output connector
PoE Injector below
Under normal operation the PSE supplies a nominal d.c. voltage of 48V, this is limited to a maximum of 15.4 Watts (at present time).
Endspan VS MidspanWhy choose PoE enabled Endspan Switches?:- There are no existing Ethernet Switches in place, or older Ethernet switches in place, buying new Ethernet Switches anyway.
- PoE required for End devices is all one type (generally IEEE 802.3af or legacy Cisco CDP style) and the installation has a large Port count.
- Limited space in 19" racks to mount Midspan units (more of an issue in large scale implementations) where IP nodes are in the high hundreds or thousands.
- Budget available to spend more on powered Switch PoE implementations.
- Does not like or want the extra patch cords involved with connecting an Ethernet Switch to a Midspan PoE Switch.
- Midspan and Switch (viewed) together as two separate potential points of failure.
- Have been subjected to pressure from Ethernet Switch Manufacturer to not induce any other Network products into the network.
- Corporate mandate for all Network devices to have an IP address.
- Consultant or Systems Integrator has specified Endspan technology.
- Do not need full power (15.4 watts) per Port, application OK with limited power per Port. Most Endspan PoE units develop only 200 watts total per 24 Ports or an average 8.3 watts per Port. This will work for powering most IP Phones but is not enough power for many other Network devices.
- End User views this as "easy way" to implement PoE, somehow requiring less work than an installation using Midspan Hubs.
Why choose Midspan PoE Switchs or Injectors?: - Have new or newer existing Ethernet switches in place, buying new Ethernet Switches just to get the PoE function would be a waste of resources.
- PoE required for the End devices is of mixed types because of different requirements for different brands and models of powered End devices.
- No limitation on rack space to mount the actual Midspan PoE hubs.
- Lower Budget, overall dollars spent are far less with Midspan PoE implementation compared to purchase of powered Switches.
- Is fine with patch cord connectivity between Switch and Midspan Hub.
- Consultant or Systems Integrator has specified Midspan technology.
- View PoE enabled Switch as a single point of failure and want to separate the two functions so that repair-ability is maintained should one function or the other experience failure.
- Not subject to pressure from Switch Manufacturer against introduction of other products into the Network.
- Viewed as "easy way" to implement PoE, requiring less work than implementing new Switches with all of their programming and set-up.
How the power is extracted from the cable
To extract power from the cable the connected device must be PoE enabled.
How does the PSE know that the connected device must be PoE enabled
Before the PSE supplies power, it has to verify that the device connected is a Powered Device (PD). It does this by checking that the PD has a conforming signature. All Intellinet PD modules have a built-in signature chip that does that automatically.
Only upon successful completion of a valid signature will the PSE apply power to the cable. This ensures that non Powered Devices are not damaged by the PSE, thus making the system backwards compatible.
What is Power Class
This is optional within the IEEE 802.3af specification, that can be used by the PSE to manage the overall power requirements of the system. Below is a table that shows the various power classes detailed within IEEE 802.3af.
| Class |
Usage |
Maximum Power |
| 0 |
Default |
15.4W |
| 1 |
Optional |
4.0W |
| 2 |
Optional |
7.0W |
| 3 |
Optional |
15.4W |
| 4 |
Reserved for future use |
Treat as CLASS 0 |
What is Power over LAN
Power over LAN or PoL is (bassically) another term used for Power over Ethernet or PoE.
Example below of another mid-span
More yet to consider
Businesses invest huge amounts of funds in their existing IP infrastructure, which includes high speed switching, security capabilities and guaranteed QoS features that are critical to VoIP implementation.
As deployment of IP telephony systems become more widespread, businesses also need consider methods of delivering power to their IP phones.
The approval of the IEEE 802.3af Power over Ethernet (PoE) standard effectively removed a significant barrier to the widespread adoption of IP telephony, in that it greatly reduced the challenge of providing power to VoIP devices. 803.2af enables electric power to be delivered along the same cables that transport Ethernet data, allowing PoE-enabled switches and Midspans to deliver power over standard Ethernet cabling to IP phones, Access Points, Routers, Switches and more.
PoE's advantage over the other protocols is the ubiquity of Ethernet, which encourages the development of products to support the standard—not only in the Enterprise market where it was born, but also in a broadening range of applications in industrial environments.
Increasingly, in new industrial system implementations and in system upgrades, the benefits of an all-Ethernet network are winning over specialized local networks and winning bids/projects.
Ethernet data transmission requires two of the four twisted pairs (most often) that are available in a standard CAT5 Ethernet cable. The CAT5 cable is the most widely used Ethernet connectivity medium on the market.
PoE also utilizes two twisted pairs, and the IEEE 802.3af standard allows either the unused pair or the data pair for power transmission.
A midspan power source injects power into the cables in current enterprise Ethernet deployments. The MPS (mid-span) sits between the central switch and the target PDs. In the PDs, a picker or splitter extracts the power current from the wires in the cable, while allowing the data signal to proceed as normal.
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