Patent Application
20090222678
1. Field of the Invention
The present invention relates generally to Power over Ethernet (PoE) and, more particularly, to an active powered device (PD) for the application of PoE.
2. Introduction
The IEEE 802.3af and 802.3at PoE specifications provide a framework for delivery of power from power sourcing equipment (PSE) to a powered device (PD) over Ethernet cabling. In this framework, various PDs can be deployed such as voice over IP (VoIP) phones, wireless LAN access points, network cameras, computing devices, etc.
In the PoE process, a valid device detection is first performed. This detection process identifies whether or not it is connected to a valid device to ensure that power is not applied to non-PoE capable devices. After a valid PD is discovered, the PSE can optionally perform a Layer 1 power classification. For example, in the IEEE 802.3af standard, the classification step identifies a power classification of the PD from the various power classes of 15.4 W, 7.0 W, and 4.0 W. In various PoE implementations, a Layer 2 power classification process can be initiated to reclassify the power class or implement some form of dynamic classification.
After the classification process is complete, the PSE would allocate power to the port. In a typical usage scenario, the PSE has a fixed power budget that can easily be oversubscribed by the connected PDs. Management of such a fixed power budget can therefore dictate that lower priority PDs would not receive a power allocation from the PSE.
In this fixed power budget environment, it is important for the PSE to accurately determine a power budget for the various powered ports. Various factors can impact the power budget for the particular port. For example, physical characteristics such as the resistance of the cable connecting the PSE and the PD can impact the power budget for a port. Additionally, the actual power consumption by the PD can impact the power budget for a port. What is needed therefore is a mechanism that enables a PSE to monitor in an effective manner the provision of power on a port.
An active powered device (PD) for the application of PoE, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.
In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Various embodiments of the invention are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the invention.
As is further illustrated in
In the example of IEEE 802.3af, PSE 120 can deliver up to 15.4 W of power to a plurality of PDs (only one PD is shown in
The delivery of power from PSE 120 to load 150 can be modeled by the circuit model illustrated in
The values of resistors R1, R2, R3, and R4 depend on the type and length of Ethernet cable. Specifically, the resistors R1, R2, R3, and R4 have a certain resistance/length that is dependent on a type of Ethernet cable (e.g., Category 3, 5, 6, etc.). For example, for Category 3 Ethernet cable, resistors R1, R2, R3, and R4 would have a resistance of approximately 0.2 Ω/meter. Thus, for a 100-meter Category 3 Ethernet cable, each of resistors R1, R2, R3, and R4 would have a value of 20Ω. In this example, parallel resistors R1 and R2 would have an equivalent resistance of 10Ω, while parallel resistors R3 and R4 would also have an equivalent resistance of 10Ω. In combination, the total value of the Ethernet cable resistance (Rcable) can then be determined as the sum of 10Ω+10Ω=20Ω. A simplified PoE circuit model that includes the single cable resistance value Rcable is illustrated in
In a typical PoE application, the resistance of the cable is either estimated or assumed to have a worst-case value. For example, an 802.3af PoE application can assume a worst-case resistance of 20Ω, which is the resistance of 100 m of category 3 cable.
In the circuit model of
In specifying the minimum output power of 15.4 W for the PSE, the IEEE 802.3af standard assumes that the PD is connected to the PSE using 100 m of Category 3 cable. At a current limit of 350 mA, the worst-case power loss attributed to the cable is Ploss=(350 mA)2*20Ω=2.45 W. This worst-case power loss of 2.45 W is the difference between the PSE's minimum output power and the max power drawn by the PD (i.e., 15.4 W−12.95 W=2.45 W). As the amount of power loss attributable to the cable is directly proportional to the resistance of the cable, the accuracy of the cable resistance estimate plays a significant role in the accuracy of the power budget.
The direct measurement of the cable resistance is enabled through the operation of short circuit module (SCM) 330. In general, SCM 330 is designed to produce a short circuit in the PD at a time when the PSE intends to measure the current and voltage of the circuit. As illustrated in
In the illustration of
In the illustrated example, the cable resistance detection occurs in the time slot between the Classification time and the Turn On time. In another example, the cable resistance detection can be designed to occur in the time slot between the Detection time and the Classification time. As would be appreciated, the particular point in the PoE sequence during which the cable resistance detection would occur would be implementation dependent.
Regardless of the particular point in the PoE sequence during which the cable resistance detection occurs, the direct determination of the cable resistance enables the PSE to accurately establish an initial power budget for a particular PD. This follows since the power loss attributable to the cable would vary significantly depending, for example, on the length of the cable. As the resistance is proportional to the length of the cable, a 100 meter cable would have four times more resistance than a 25 meter cable.
Even with an initial direct determination of the cable resistance, an accurate power budget for a port may still not be possible. This results since the characteristics of the PSE-PD link during operation have not been considered. For example, the cable resistance can change with temperature as the cable heats up during its own operation, or due to the impact of neighboring cables. In another example, the actual power consumption information (e.g., input voltage and current) of the PD may not be known. Typically, a manufacturer would specify a worst-case assumption of power consumption to ensure that the actual operation of the device would not exceed the designed specification. This may lead to an oversubscription of power as requested by the PD.
Conventional PD designs are commonly based on passive designs. This limits the flow of information to the PD. In accordance with the present invention, a PD design is provided that enables the provision of dynamically-gathered information from the PD to the PSE. This information can then be used by the PSE in configuring various elements of PoE system operation. For example, the PD-provided information can be used to alter a power budget attributable to that port.
In this configuration, the active PD device is designed to gather information that is useful by the PSE in monitoring the performance on a particular port. In the current example, the PSE can use the current and voltage information to manage the power dynamically and efficiently. More specifically, the PSE can use the current and voltage to determine the power consumed by the PD load, as well as determine the resistance of the cable.
Across the cable, the voltage drop can be defined as VPSE−VL=I*Rcable. This equation can be solved for the cable resistance Rcable=(VPSE−VL)/I. Since VPSE is known by the PSE, the PSE can then determine the resistance of the cable dynamically using the information provided by the PD. With the determined cable resistance, the PSE can then determine the power loss in the cable as Pcable=I2*Rcable. This determined result of the power loss in the cable can then be used in combination with the fixed power at the PD (i.e., PL=I*VL) to determine the total power budget attributed to the PSE port.
In general, the principles of the present invention enable a dynamic monitoring process using information that is dynamically gathered by the PD. As would be appreciated, the principles of the present invention are not dependent on a particular type of information that is gathered by the PD. Other types of information such as the status of the PD (e.g., working, idle or sleeping), future power needs, diagnostic information, etc. can be gathered by the PD to aid the PSE in managing power or other operational budgets.
To further illustrate the principles of the present invention, reference is now made to the flowchart of
In one example, the initial circuit measurements enable the PSE to determine a cable resistance of the link between the PSE and PD. This cable resistance determination would then enable the PSE to establish initial operation parameters at step 508. For example, the determined cable resistance can be used to establish an initial power budget for the PSE port.
After the turn on voltage has been applied and the PD receives power, the PD can then gather additional information (e.g., current and input voltage) during operation at step 510. As noted, this additional information gathering would be useful to determine whether conditions on the port have changed over time. In one example, the cable resistance may have changed due to a rise in temperature. In another example, the power consumption of the PD may have changed due to a malfunction or other change in operation in the PD. Regardless, the additional information is needed to confirm whether or not the initial operational parameters established at the outset remain true. The active nature of the PD in the present invention is able to facilitate such additional information gathering.
At step 512, the additional information gathered by the PD is transmitted to the PSE at step 512. In one embodiment, this transmission of additional data is facilitated by a microcontroller that leverages Layer 2 communication through a PHY. The receipt of this additional information by the PSE enables the PSE to determine an impact on the PoE system, At step 514, the PSE can determine whether an adjustment of any operation parameters on the port. For example, the current and input voltage can be used to determine if the cable resistance has changed. Any determined change could then impact the power budget for the port.
As has been described, the principles of the present invention enable the PD to provide information that can be useful in establishing and monitoring the operation of a particular port. This increased granularity is key to enabling an increased efficiency in the management of PoE power budgets.
In this environment, the power budgets attributable to a single PSE-PD link can have further consequences upstream as the power budgets are dependent upon each other. An accurate understanding of the power budget actually needed for a link is therefore key to ensuring that an overly conservative estimate does not cascade through the PoE network in a manner that produces great inefficiencies. With the principles of the present invention, each PD can be actively involved in the acquisition of information that is useful by a PSE in effecting proper resource management.
These and other aspects of the present invention will become apparent to those skilled in the art by a review of the preceding detailed description. Although a number of salient features of the present invention have been described above, the invention is capable of other embodiments and of being practiced and carried out in various ways that would be apparent to one of ordinary skill in the art after reading the disclosed invention, therefore the above description should not be considered to be exclusive of these other embodiments. Also, it is to be understood that the phraseology and terminology employed herein are for the purposes of description and should not be regarded as limiting.
