The new IEEE 802.3at definition, also known as the PoE+ standard, simultaneously expands the power budget of the existing 802.3af specification and improves the power classification mechanism used by Power Sourcing Equipment (PSE) and PDs to identify each other. In the new PoE+ standard, the PD power level is now 25.5W. There are several notable differences between the legacy PoE specification and the new PoE+ standard that are highlighted here. Firstly, the minimum PSE output voltage was raised from 44V to 50V, while the current was raised from 350mA to 600mA. PoE+ also mandates the use of CAT-5 cable (or better) which provides lower cable resistance. This lowers the overall ‘channel’ resistance including the cable and connectors etc. to less than 12.5 Ω. In addition, PoE+ compliant systems, also known as Type 2 hardware will provide two methods for PD power classification. A Type 2 PSE may obtain a PD’s power classification by hardware-based 2-event classification and/or by software via the data connection. 2-event classification is supported by Layer 1 hardware and involves essentially repeating the 802.3af voltage probe twice. Although Layer 1 power classification was optional in 802.3af, a single event classification is now required for Type 2 PSEs under the 802.3at specification. The PoE+ task force also defined a new data layer classification known as the Link Layer Discovery Protocol (LLDP). This Layer 2 classification occurs over the data layer and is transparent to the PD controller, as this communication occurs directly between the PSE and CPU of the powered device. This type of classification has the advantage of being able to dynamically allocate power in 0.1W increments for more power efficient operation. The specification calls out for 2-pair only. Proprietary systems may be configured for 4-pair to achieve higher power than the PoE+ standard permits.