# Patent

## Title
System and method for measuring a cable resistance in a power over Ethernet application

## Abstract

A system and method for measuring a cable resistance in a power over Ethernet (PoE) application. A short circuit module in a powered device is designed to produce a short circuit effect upon receipt of a cable resistance detection voltage. The cable resistance detection voltage can be designed to be greater than a voltage for detection or classification and less than a voltage for powering of the powered device. The measurement of the current at a time when a short circuit effect is produced at the powered device enables a calculation of the actual resistance of the cable on a given PoE port.

## Background
This application is a division of non-provisional application Ser. No. 11/849,336, filed Sep. 3, 2007, which is incorporated by reference herein, in its entirety, for all purposes.


BACKGROUND
1. Field of the Invention
The present invention relates generally to Power over Ethernet (PoE) and, more particularly, to a system and method for measuring a cable resistance in a PoE application.
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. One factor that can impact the power budget for the particular port is the physical characteristics (e.g., resistance) of the cable connecting the PSE and the PD. The more information that is available regarding the physical characteristics of the cable, the greater the accuracy in the power budget assigned to the individual ports. What is needed therefore is a mechanism for accurately identifying the cable characteristics in a PoE application.
SUMMARY
A system and/or method for measuring a cable resistance in a PoE application, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIG. 1 illustrates an embodiment of a PoE system.
FIGS. 2A and 2B illustrate a circuit model for the powering of a PD.
FIG. 3 illustrates an embodiment of a mechanism that enables measurement of a cable resistance.
FIG. 4 illustrates an embodiment of a cable resistance detection in a PoE process.
FIG. 5 illustrates an embodiment of a short circuit module.
FIG. 6 illustrates a transient analysis of a short circuit module.
FIG. 7 illustrates a flowchart of a PoE process that incorporates cable resistance detection.



DETAILED DESCRIPTION
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.
FIG. 1 illustrates an embodiment of a power over Ethernet (PoE) system. As illustrated, the PoE system includes power sourcing equipment (PSE) 120 that transmits power to powered device (PD) 140. Power delivered by the PSE to the PD is provided through the application of a voltage across the center taps of transformers that are coupled to a transmit (TX) pair and a receive (RX) pair of wires carried within an Ethernet cable. In general, the TX/RX pair can be found in, but not limited to structured cabling. The two TX and RX pairs enable data communication between Ethernet PHYs 110 and 130 in accordance with 10BASE-T, 100BASE-TX, 1000BASE-T, 10 GBASE-T and/or any other layer 2 PHY technology.
As is further illustrated in FIG. 1, PD 140 includes PoE module 142. PoE module 142 includes the electronics that would enable PD 140 to communicate with PSE 120 in accordance with a PoE standard such as IEEE 802.3af, 802.3at, legacy PoE transmission, or any other type of PoE transmission. PD 140 also includes pulse width modulation (PWM) DC:DC controller 144 that controls power FET 146, which in turn provides constant power to load 150.
In the example of the IEEE 802.3af standard, PSE 120 can deliver up to 15.4 W of power to a plurality of PDs (only one PD is shown in FIG. 1 for simplicity). In the IEEE 802.at specification, on the other hand, a PSE can deliver up to 30 W of power to a PD over 2-pairs or 60 W of power to a PD over 4-pairs. Other proprietary solutions can potentially deliver even higher levels of power to a PD. In general, high power solutions are often limited by the limitations of the cabling.
The delivery of power from PSE 120 to load 150 can be modeled by the circuit model illustrated in FIG. 2A. As illustrated, a power source provides a voltage VPSE to a circuit that includes a first parallel pair of resistors (R1, R2), a load resistance RLOAD, and a second parallel pair of resistors (R3, R4). Here, the first parallel pair of resistors R1, R2 represents the resistances of the TX pair of wires, while the second parallel pair of resistors R3, R4 represents the resistances of the RX pair of wires.
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 FIG. 2B.
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 FIG. 2B, where the PD includes a DC:DC converter, the load resistance RL, would receive constant power, PL, and see a voltage VL on its input. Since PL is fixed at the load, PL=I*VL, where I is the current going through the whole circuit. The power loss of the cable would then be Ploss=I2*Rcable.
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.
In accordance with the present invention, the resistance of the cable is determined through a direct measurement. This is in contrast to conventional techniques that are based on cable resistance estimation or worst-case assumptions.
FIG. 3 illustrates an embodiment of a mechanism by which the resistance of the cable can be measured. As illustrated, the Ethernet cable includes two data wire pairs and two spare wire pairs. The two data wire pairs are used for data transmission (TX) and reception (RX). The PD can receive power that is provided by the data wire pairs and/or the spare wire pairs. Under 802.3af, the two data wire pairs would be used in Alternative A powering, while the two spare wire pairs would be used in Alternative B powering. Power extracted from the two data wire pairs would be routed through diode bridge 312. Power extracted from the two spare wire pairs would be routed through diode bridge 314. DC/DC converter 320 ultimately provides the power to the load in the PD.
In the present invention, the 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 FIG. 2B, a short circuit in the PD would remove the load from the overall circuit. The result is a circuit that includes simply the PSE voltage and the cable resistance. With knowledge of both the voltage and the current through the circuit, the PSE can then calculate the resistance of the cable.
FIG. 4 illustrates an embodiment of a sequence during which SCM 330 produces a short circuit in the PD. In the illustration of FIG. 4, a two-point detection occurs during the 500 ms Detection time. This 500 ms Detection time is followed by a 400 ms Turn On time, the expiration of which would commence powering of the PD. As noted above, an optional classification can also occur after PD detection. This optional classification can occur within the 75 ms Classification time as shown.
In one embodiment, the cable resistance detection is designed to occur in the time slot between the Classification time and the Turn On time. This embodiment is illustrated in the sequence of FIG. 4. In an alternative embodiment, 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.
FIG. 5 illustrates an embodiment of SCM 330. As noted, the circuit of FIG. 5 is coupled on one end to diode bridges 312, 314, and on the other end to the ground of the PD. SCM 330 is placed behind diode bridges 312, 314 because the polarity of the voltage received on the two data wire pairs and/or the two spare wire pairs may not be known. In an alternative embodiment, SCM 330 can be placed in front of diode bridges 312, 314. In this arrangement, SCM 330 can be coupled directly to the two spare wire pairs and/or the taps of the transformers of the two data wire pairs.
To illustrate the operation of the SCM of FIG. 5, reference is made to the PoE process of FIG. 4. As illustrated, the cable resistance detection is based on a two-point detection scheme. In this two-point detection scheme, the PSE generates two voltages V1 and V2 (V2>V1) between 21V to 30V. Here the voltage V1 is chosen to be greater than the voltage used for classification (e.g., 15V-20V), while the voltage V2 is chosen to be less than the voltage used during powering of the PD (e.g., 36V). SCM 330 can therefore be designed to work between 21V to 30V. In operation, SCM 330 is designed to keep the input voltage at a certain stable level (i.e., V1 and V2) for a period of time. During these times, the PSE can then calculate the resistance of the cable precisely by measuring the DC current during the established voltage windows. When the incoming voltage is outside of the window of 21V to 30V, SCM 330 would be an open circuit for the DC voltage.
In the embodiment of FIG. 5, zener diode D1=Vz+_V, where _V is the voltage tolerance of the zener diode. Here, V2>V1>Vz+_V. During the Detection time or Classification time, although the input voltage (max 20V) may let the FET Q1 to turn on, D1 will not work. There would then be no impact on the remaining circuit. When the input voltage=V1>22V, both Q1 and D1 would work. The PSE can then measure the current I1 passing through D1, Q1 and Rcable. When the input voltage=V2>V1, both Q1 and D1 keep working. The PSE can then measure the current I2 passing through D1, Q1 and Rcable. According to the measured values of I1 and I2 and known voltages of V1 and V2, the PSE can calculate the cable resistance of Rcable={[V2−(Vz+_V)]−[V1−(Vz+_V)]}/(I2−I1)=(V2−V1)/(I2−I1). As this calculation illustrates, the two-point detection removes the impact of the zener diode. After finishing the measurement of cable resistance, the PSE can turn on power for the PD (e.g., apply 48V DC to power the PD). When this happens, then D2 turns on, Q2 turns on, and Q1 turns off, thereby isolating D1. More specifically, when the PSE starts to apply 48V DC to the PD, the voltage will ramp up from 0V to 48V. During this period, Q2 will be turned on due to the body capacitor of D2 to hold the voltage of Vgs of Q1=0, which ensures that no current passes through D1 until the voltage passes 30V (i.e., D2 turns on). FIG. 6 illustrates the transient analysis of the SCM for the period of voltage ramping up on the PD side. As illustrated the voltage from the PSE is shown along with Vgs of Q1.
To further illustrate the principles of the present invention, reference is now made to the flowchart of FIG. 7, which illustrates a PoE process that includes cable resistance detection. As illustrated, the process begins at step 702 where the PSE detect the PD. This PD detection can occur during the Detection time illustrated in FIG. 4. Next, at step 704, the PSE generates a cable resistance detection voltage. Here, it should be noted that step 704 can be designed to follow the Detection time or the optional Classification time. In one embodiment, the cable resistance detection voltage has a value greater than a voltage used for detection and/or classification, but less than a voltage used for powering.
At step 706, the generation of the cable resistance detection voltage activates a short circuit module at the PD. At step 708, the PSE can then measure the current that flows through the circuit. The measurement of the current then enables the PSE to determine the cable resistance at step 710. As noted above, two different cable resistance detection voltages can be used to produce two different current measurements. These two current measurements can be used to cancel out the impact of additional elements in the circuit being measured. Where the uncertainty produced by those additional elements is not considered significant, a single current measurement can be used.
In one embodiment, the current measurement is performed by the PD. This current measurement can then be communicated by the PD to the PSE for use in calculating the cable resistance.
After the cable resistance is determined, the PSE can then determine an impact on the PoE system. For example, the cable resistance can change the power loss attributed to the cable, and hence the power budget needed for a given port. Regardless of the determined impact, the PSE can implement any needed adjustments to the PoE system operation or configuration at step 712. Significantly, these adjustments are based on an accurate measurement of the cable resistance instead of a resistance value produced through estimation or worst-case assumptions.
Once the adjustments have been implemented, the PSE can then continue in the ramp of the Turn On voltage at step 714. Once the Turn On voltage exceeds a certain threshold, the short circuit module would then deactivate at steps 716.
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.

## Claims

1. A powered device that enables a measurement of cable resistance, the powered device receiving a voltage from at least two of four wire pairs that coupled the powered device to a power sourcing equipment, comprising:
a short circuit module that produces a short circuit to enable the measurement of cable resistance when a voltage received at a powered device is greater than a detection voltage transmitted by said power sourcing equipment on said at least two of said four wire pairs to determine whether the powered device is a valid powered device, is greater than a classification voltage transmitted by said power sourcing equipment on said at least two of said four wire pairs to determine a power classification of the powered device, and is less than an operating voltage transmitted by said power sourcing equipment on said at least two of said four wire pairs when the powered device receives an amount of power that has been allocated by said power sourcing equipment to the powered device.

2. The device of claim 1, wherein said at least two of said four wire pairs are data wire pairs.

3. The device of claim 1, wherein said at least two of said four wire pairs are unused wire pairs.

4. The device of claim 1, wherein said short circuit module is coupled directly to taps of transformers on data wire pairs.

5. The device of claim 1, wherein said short circuit module is coupled to diode bridges that are connected to wire pairs.

6. The device of claim 1, wherein said short circuit module includes a zener diode that turns on at a voltage greater than a voltage used during detection or classification.

7. A powered device that enables a measurement of cable resistance, comprising:
an input port that receives four wire pairs, said input port receiving a voltage from at least two of said four wire pairs; and a short circuit module that produces a short circuit when a received voltage is greater than a voltage used during detection or classification and less than a voltage when the powered device is being powered, wherein said short circuit module deactivates when a received voltage approaches a voltage when the powered device is being powered.

8. A powered device that enables a measurement of cable resistance, comprising:
an input port that receives at least two wire pairs for a delivery of power from a power sourcing equipment; and
a short circuit module that produces a short circuit when a voltage across said short circuit module is greater than a first threshold voltage and less than a second threshold voltage, said first threshold voltage being greater than a voltage used during detection or classification of the powered device, and said second threshold voltage being less than a voltage used to deliver an allocation of power for fully functional use of the powered device, wherein said short circuit module is coupled directly to taps of transformers.

9. The device of claim 8, wherein said short circuit module includes a zener diode that turns on substantially at said first threshold voltage.

10. The device of claim 9, wherein said short circuit module includes a second zener diode that turns on substantially at said second threshold voltage.

11. The device of claim 8, wherein one end of said short circuit module is coupled to a diode bridge.