Ethernet power - new features as POE shifts up a gear
10 February 2009
Alison Steer and Clay Stanford guide us through the Power over Ethernet standard, its progress, and what the benefits are.
Power over Ethernet (PoE) was defined by the IEEE 802.3af task force in 2003 and has enjoyed wide industry acceptance. This standard defined nominal power delivery at 12.95W which was more than adequate for the early adopter applications such as standard VoIP phones, security cameras and wireless access points (WAPs). Today, network switches from all the major manufacturers can be purchased with
PoE-enabled ports, and with this infrastructure now in place the demand for additional features and higher power has increased.
In 2006, to combat the limited power budget and open PoE to new markets, a group from the IEEE started a new task force to explore raising the power limits of this international power standard. From this was born the IEEE 802.3at task force and the soon to be released PoE+ standard. The PoE+ standard will enable a whole new breed of power hungry devices from longer range WAPs, full-featured video phones, video teleconferencing stations and high performance pan-tilt-zoom security cameras, to name a few.
One of the areas that required careful engineering design was the new classification mechanisms that would be used to allow the Power Sourcing Equipment (PSE) and Powered Devices (PDs) to mutually
identify each other. With this mutual identification comes the ability for PSEs to properly power both .af (also known as Type 1 hardware) and .at (Type 2 hardware) PDs, as well as the ability for .af PDs to be powered by .at PSEs and for .at PDs to know if they have available the full power that their higher loads demand. Every combination needs to have a well defined and consistent behaviour so that the mainstay of the 802.3
standard, interoperability, is maintained. This mutual identification has been implemented in 802.3at with the use of a more elaborate hardware classification mechanism along with a new data-layer mechanism.
The 802.3at hardware classification is called 2-event classification (also known as ping-pong) and involves the PSE essentially repeating the 802.3af voltage probe twice. Each voltage probe of the PD results in a pulse of current being drawn (figure 1) which corresponds to a particular power level. To begin, the PSE asserts a voltage pulse in the range of 15.5V to 20.5V on the data or spare pairs. The PD responds with a current of up to 40mA which conveys one of four power classes to the PSE. The double pulse is a signal to the PD that the connected PSE is indeed a high-power PSE able to source the higher power levels associated with 802.3at power. The 802.3at PD responds with a class 4 current, thereby telling the PSE that it is a high-power PD requiring the full power available. The layer 1 classification method in 802.3af was an optional method for the PSE to query the PD to determine the PD power needs. In the 802.3at specification, the PSE is mandated to perform this classification.
In addition to the above mentioned hardware classification, the PoE+ task force defined a new data layer (layer 2) classification known as the Link Layer Discovery Protocol (LLDP) for communication between the PSE and PD. Once the link is powered, the PSE and PD can use LLDP to determine the power needs of the PD. The use of LLDP allows the PSE to query the PD repeatedly to know the status of the PD and its power needs. With this mechanism it is now possible to implement dynamic power allocation where the PSE can
continuously issue the PD new power levels and the PD can request and then subsequently relinquish power requests. The communication over layer 2 enables advanced features to poll for more information like peak power, average power and duty cycle. This new dynamic power allocation is certain to become an important feature as systems move toward a greener power environment. This new layer 2 classification mechanism is optional for the PSE, but is required to be implemented by the PD. Figure 1 shows the classification methods used in PoE+. The 802.3at group is still working on the details of this portion of the standard, so we can expect some changes as the group finishes defining the communication protocol. The final specification is likely to be released in spring '09.
For PoE systems, there are two distinct locations where power is defined; at the PSE output connector and at the PD input connector. One of the more important developments on the PoE+ specification is the capping of the current to 600mA. The PSE must now be able to continually source at least 600mA with a minimum output voltage of 50V. This translates to a PSE output power of 30W. The cable resistance is modeled no larger than 12.5Ohms, putting the available power at the PD connector at 26.5W. It is necessary to take into account the 48V conversion efficiency so that in the end, there is about 24.6W available at the PD load.
The need for higher power is of course driven by the market and today there is already strong demand for PD power solutions above the existing 12.95W limit. So while we wait for the 802.3at standard to be completed and become established in the marketplace, how do PD vendors solve their high-power needs today?
The answer is to design a PD front end that will be compatible with the emerging standard. The standard is far enough along that we can design PDs today with a high likelihood of being able to interoperate with 802.3at PSEs.
Linear Technology has pre-standard high power PD offerings that can form the basis of an 802.3at compatible PD solution. These parts typically have the high current power path needed for dealing with the higher power levels involved, but lack the 2-event classification mechanism. By adding a simple discrete circuit alongside the LTC4268-1, we can provide a complete 802.3at solution today. Figure 2 shows the required add-on circuitry needed to deal with the 802.3at 2- event classification using our existing PD products.
ALISON STEER is Product Marketing Manager, Mixed Signal Products, and CLAY STANFORD is Design Manager, Power over Ethernet Products, both for Linear Technology Corporation
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