Many networks such as local and wide area networks (LAN/WAN) structures are used to carry and distribute data communication signals between devices. Various network elements include hubs, switches, routers, and bridges, peripheral devices, such as, but not limited to, printers, data servers, desktop personal computers (PCs), portable PCs and personal data assistants (PDAs) equipped with network interface cards. Devices that connect to the network structure use power to enable operation. Power of the devices may be supplied by either an internal or an external power supply such as batteries or an AC power via a connection to an electrical outlet.
Some network solutions can distribute power over the network in combination with data communications. Power distribution over a network consolidates power and data communications over a single network connection to reduce installation costs, ensures power to network elements in the event of a traditional power failure, and enables reduction in the number of power cables, AC to DC adapters, and/or AC power supplies which may create fire and physical hazards. Additionally, power distributed over a network such as an Ethernet network may function as an uninterruptible power supply (UPS) to components or devices that normally would be powered using a dedicated UPS.
Additionally, network appliances, for example voice-over-Internet-Protocol (VOIP) telephones and other devices, are increasingly deployed and consume power. When compared to traditional counterparts, network appliances use an additional power feed. One drawback of VOIP telephony is that in the event of a power failure the ability to contact emergency services via an independently powered telephone is removed. The ability to distribute power to network appliances or circuits enable network appliances such as a VOIP telephone to operate in a fashion similar to ordinary analog telephone networks currently in use.
Distribution of power over Ethernet (PoE) network connections is in part governed by the Institute of Electrical and Electronics Engineers (IEEE) Standard 802.3 and other relevant standards, standards that are incorporated herein by reference. However, power distribution schemes within a network environment typically employ cumbersome, real estate intensive, magnetic transformers. Additionally, power-over-Ethernet (PoE) specifications under the IEEE 802.3 standard are stringent and often limit allowable power.
An IEEE 802.3af specification defines requirements for designing PoE equipment. The standard sets forth two types of devices including Power Sourcing Equipment (PSE) and Powered Devices (PD). According to the standard, the PSE supplies 48 volts with a current limit of 350 mA to the PD which may be one of a wide variety of devices such as Voice-over-Internet-Protocol (VoIP) telephones, wireless access points, and many others. The standard limits the PSE to a continuous maximum power delivery of 15.2 watts, which after line losses amounts to a power delivery of 12.95 watts at the PD interface.
Different devices can require significantly different power levels. For example, a VoIP telephone can typically consume four to six watts while a dual-radio wireless access point can have a requirement of about 14-18 watts. In a conventional system, several power classification levels can be specified using handshakes between the PSE and the PD. The handshake operations typically begin when a PD product is connected to a PoE cable. The PSE reacts by sending a test voltage to determine whether the PD has a valid IEEE 802.3af signature. The detection signature results from a small current-limited voltage that is applied to the network cable. The voltage responds to the presence of a 25 KΩ resistor in the PD.
Such probing by the PSE can create erroneous results for various reasons such as variations in cable length, presence of diode bridges at the PD interface, and other conditions or phenomena can potentially lead to errors in selection of supplied power, either failure to supply adequate power or, more typically, supplying of substantially more power than is necessary. For example, the output voltage for the PSE to drive the PD varies for different cable lengths. The diode bridge, which is required under the IEEE 802.3 standard to supply polarity protection in case a connector is attached backwards and to enable operation with a PSE that sources either −48V or +48V, also can result in a voltage drop that may be difficult to quantify using static techniques.
If the PD responds with to the PSE detection signal with a valid signature, a test is performed to determine the PD power consumption classification. During the classification test, the PD attempts to sink a known current according to the IEEE 802.3af classification table. If the PD does not supply a proper current sink, the PSE assumes a default type of Class 0 for the PD. The PSE supplies 48V to the PD only after results of the classification test are complete.
In the conventional operation, power allocation is a static process. The classification current identifies the amount of power to be supplied. A PD is designed to request a typically worst-case amount of power during the classification test, an amount that is typically estimated with a sufficient margin, often twenty to fifty percent, to prevent inoperability or failure due to lack of power due to coarse steps defined in the power classification table. Often the margin is extended to account for variability in actual power delivered by identical power supply models, a variability that may be significant even within the products of a particular manufacturer and within a particular model from the same manufacturer.
In the standard power management process, IEEE 802.3af handshake operations between the PSE and PD operate in the physical layer to set up a current, measure a current, then request a power supply level based on the measured current. The current can only be changed by terminating the link, then reinstating the link, and initializing using the handshake operation. Accordingly, the standard system is inherently static and cannot easily respond to dynamically changing demand without terminating the link.