# Patent

## Title
Method and apparatus for detecting and supplying power by a first network device to a second network device

## Abstract

A first network device supplies power to a second network device in communication therewith. The first network device comprises a physical layer device which includes a pulse generator to generate a test signal comprising n sub-pulses to be transmitted to the second network device, wherein in n being greater than 2. A detector is responsive to the second network device, and a controller is in communication with the detector and the pulse generator. When the detector detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller, responsive to the detector, enables power to be transmitted to the second network device.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. 119(e) to U.S. provisional Application Ser. No. 60/280,735, entitled “Apparatus For DTE Power Via MDI and Method Thereof”, filed Apr. 3, 2001, the contents of which are incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to autonegotiation controllers within the physical layer of devices that are connected to an Ethernet network. More particularly, the present invention is directed to an autonegotiation controller in which the physical layer of one network device is able to supply power to another network device, if required, over the data cable connecting the devices.
2. Description of the Related Art
FIG. 1 illustrates a network device 10 in communication with another network device 12 over cable 18. These devices are well known. Network devices include, by way of example, network switches, computers, servers, network enabled appliances and the like. Heretofore, network devices have generally required external power from an AC power source. This methodology suffers from a number of drawbacks, including requiring an external power supply, which can be costly. Accordingly, it would be desirable to implement a system in which the power for one network device 12 can be supplied from the other network device 10 via the data cable 18. This approach, however, would require a physical layer of network device 10 to determine whether a DTE device is connected to cable 18 and whether DTE device 12 requires power. The capability of supplying power over cable 18 is referred to as power on Ethernet cable or POE. In this application, the term “cable-powered DTE device” shall refer to a network device that requires power being supplied from another network device via a data cable, and the term “self-powered DTE device” shall refer to a network device in which power not supplied by the data cable. Self-powered DTE devices may be supplied by external power supplies or internal power supplies, such as, batteries. Cable-powered DTE devices generally comprise a filter to provide a return path of a test signal used in detection of the cable-powered DTE device.
In addition to detecting power, the physical layer of network device 10 also negotiates the highest common operating speed with network device 12. Referring again to FIG. 1, first and second devices 10 and 12 include physical layers 14-1 and 14-2 that are connected by a compliant cable 18 that includes four pairs of twisted pair wires (A, B, C and D). One type of compliant cable is referred to as Category 5. The physical layers 14-1 and 14-2 usually include digital signal processors (DSPs) and autonegotiation controllers (both not shown). The DSP of the first device receives and decodes signals from the second device. The DSP of the first device codes and transmits signals to the second device. The four pairs of twisted pair wires are typically labeled A (1,2), B (3, 6), C (4,5), and D (7,8). In 10BASE-T and 100BASE-TX mode, only pairs A (1,2) and B (3,6) are required to autonegotiate, to establish a link, and to communicate. In 1000BASE-T mode, however, two pairs of twisted pair wires are required to autonegotiate and four pairs are required to establish a link and to communicate.
In 10BASE-T, 100BASE-TX, and 1000BASE-T modes, the physical layer performs autonegotiation before a link is established. During autonegotiation, the devices 10 and 12 negotiate the operating speed of the link as well as other functional capabilities of the devices. A device can advertise operating speeds that are less than or equal to the maximum operating speed of the device.
SUMMARY OF THE INVENTION
The present invention is intended to address the need for a system in which the DTE power is drawn directly from the transmission line, with an implemented technique for detecting whether a DTE is connected to the transmission line and whether the DTE requires power.
According to a first aspect of the present invention aspect of the present invention, a first network device supplies power to a second network device in communication therewith. The first network device comprises first and second transformers in communication with the second network device, and a power supply in communication with first and second transformers via a switch. A physical layer device is provided which comprises a transmitter in communication with the first transformer, a receiver in communication with the second transformer, a signal generator in communication with the transmitter, a detection circuit in communication with the receiver, and a controller in communication with the switch, the signal generator and the detection circuit. The signal generator generates a test signal comprising n sub-pulses to be transmitted by the transmitter, wherein in n being greater than 2; and when the detection circuit, in response to the receiver, detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller, responsive to the detection circuit, enables the switch to supply power from the power supply to the second network device.
In accordance with a second aspect of the present invention, when the detection circuit detects q pulses which are greater than a predetermined threshold, j<q≦n, the controller, responsive to the detection circuit, does not enable the switch and power is not supplied from the power supply to the second network device.
In accordance with a third aspect of the present invention, when the detection circuit detects 0 pulses which are greater than a predetermined threshold, the controller, responsive to the detection circuit, does not enable the switch and power is not supplied from the power supply to the second network device.
In accordance with a fourth aspect of the present invention, j=1.
In accordance with a fifth aspect of the present invention, n=3.
In accordance with a sixth aspect of the present invention, the transmitter transmits plural test signals, each successive test signal separated by a predetermined interval.
In accordance with a seventh aspect of the present invention, the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than the second pulse width.
In accordance with an eighth aspect of the present invention, q=3.
In accordance with a ninth aspect of the present invention, a physical layer device of a first network device supplies power to a second network device in communication therewith. A pulse generator generates a test signal comprising n sub-pulses to be transmitted to the second network device, wherein in n being greater than 2. A detector is responsive to the second network device, and a controller is in communication with the detector and the pulse generator. When the detector detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller, responsive to the detector, enables power to be transmitted to the second network device.
In accordance with a tenth aspect of the present invention, a network comprises first, second network devices, and a cable connecting them. The first network device comprises a first transformer, a second transformer, a power supply in communication with the first and second transformers via a switch, and a physical layer device. The physical layer device comprises a transmitter in communication with the first transformer, a receiver in communication with the second transformer, a signal generator in communication with the transmitter, a detection circuit in communication with the receiver, and a controller in communication with the switch, the receiver and the detection circuit. The signal generator generates a test signal comprising n sub-pulses to be transmitted by the transmitter, wherein in n being greater than 2. The detection circuit, in response to the receiver, detects j pulses, which are greater than a predetermined threshold, 1≦j<n, the controller, responsive to the detection circuit, enables the switch to supply power from the power supply to the second network device.
In accordance with an eleventh aspect of the present invention, a first network device is provided for supplying power to a second network device in communication therewith. The first network device comprises first transformer means for communicating with the second network device, second transformer means for communicating with the second network device, power supply means for supplying power in communication with first and second transformer means, switch means for enabling and disabling the power supply means, and physical layer means. The physical layer means comprises transmitter means for transmitting a signal to the first transformer means, receiver means for receiving a signal from the second transformer means, signal generator means for generating a signal to the transmitter means, detection means for detecting a signal from the receiver means; and controller means for controlling the switch means and the signal generator means, and responsive to the detection means. The signal generator means generates a test signal comprising n sub-pulses to be transmitted by the transmitter means, wherein in n being greater than 2. When the detection means, in response to the receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller means, responsive to the detection means, enables the switch means to supply power from the power supply means to the second network device.
In accordance with a twelfth aspect of the present invention, when the detection means detects q pulses which are greater than a predetermined threshold, j<q≦n, the controller means, responsive to the detection means, does not enable the switch means and power is not supplied from the power supply means to the second network device.
In accordance with a thirteenth aspect of the present invention, when the detection means detects 0 pulses which are greater than a predetermined threshold, the controller means, responsive to the detection means, does not enable the switch means and power is not supplied from the power supply means to the second network device.
In accordance with a fourteenth aspect of the present invention, the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than the second pulse width.
In accordance with a fifteenth aspect of the present invention, a physical layer device of a first network device for supplies power to a second network device in communication therewith. The physical layer device comprises pulse generator means for generating a test signal comprising n sub-pulses to be transmitted to the second network device, wherein in n being greater than 2, detector means responsive to the second network device, and controller means for controlling the pulse generator means and responsive to the detector means. When the detector means detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller means, responsive to the detector means, enables power to be transmitted to the second network device.
In accordance with a sixteenth aspect of the present invention, a network comprises first networking means, second networking means and a cable means connecting them. The first networking means comprises first transformer means for transforming a signal, second transformer means for transforming a signal, power supply means for supplying power to the first and second transformer means, switch means for enabling/disabling the power supply means, and physical layer means. The physical layer means comprises transmitter means for transmitting a signal to the first transformer means, receiver means for receiving a signal from the second transformer means, signal generator means for generating a signal to the transmitter means, detection means for detecting a signal from the receiver means; and controller means for controlling the switch means, the signal generator means and responsive to the detection means. The signal generator means generates a test signal comprising n sub-pulses to be transmitted by the transmitter means, wherein in n being greater than 2. When the detection means, in response to the receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, the controller means, responsive to the detection means, enables the switch means to supply power from the power supply means to the second networking means.
In accordance with a seventeenth aspect of the present invention a method of supplying power from a first network device to a second network device in communication therewith, comprises the steps of (a) transmitting a test signal comprising n sub-pulses to the second network device, wherein in n being greater than 2, (b) detecting a signal from the second network device in response to step (a); and (c) enabling power to be transmitted to the second network device when in step (b) j pulses are detected which are greater than a predetermined threshold, 1≦j<n.
In accordance with an eighteenth aspect of the present invention, the method further comprises the step of not enabling power to be transmitted to the second network device when in step (b) q pulses are detected which are greater than a predetermined threshold, j<q≦n.
In accordance with a nineteenth aspect of the present invention, the method further comprises the step of not enabling power to be transmitted to the second network device when in step (b) 0 pulses are detected which are greater than a predetermined threshold.
In accordance with a twentieth aspect of the present invention, a computer program is provided for controlling a first network device to supply power to a second network device in communication therewith, comprises the steps of (a) transmitting a test signal comprising n sub-pulses to the second network device, wherein in n being greater than 2, (b) detecting a signal from the second network device in response to step (a), and (c) enabling power to be transmitted to the second network device when in step (b) j pulses are detected which are greater than a predetermined threshold, 1≦j<n.
In accordance with a twenty-first aspect of the present invention, the computer program further comprising the step of not enabling power to be transmitted to the second network device when in step (b) q pulses are detected which are greater than a predetermined threshold, j<q≦n.
In accordance with a twenty-second aspect of the present invention, the computer program further comprising the step of not enabling power to be transmitted to the second network device when in step (b) 0 pulses are detected which are greater than a predetermined threshold.
In accordance with a twenty-third aspect of the present invention, a first network device in communication with a second network device via a data cable is provided for supplying power thereto. The first network device comprises a first transformer in communication with the second network device, a second transformer in communication with the second network device, and a power supply in communication with first and second transformers via a switch. A physical layer device comprises a transmitter in communication with the first transformer, a receiver in communication with the second transformer, a signal generator in communication with the transmitter, a detector in communication with the receiver; and a controller in communication with the switch, the signal generator and the detector. The signal generator generates a test signal be transmitted by the transmitter; and the detector selects a threshold in accordance with a length of the data cable. The controller, in accordance with a comparison by the detector of the selected threshold and a received test signal from received by the receiver, controls the switch to enable or disable the power supply. In accordance with a twenty-third aspect of the present invention, when the detector measures a peak-to-peak voltage of the received test signal.
In accordance with a twenty-fifth aspect of the present invention, when the peak-to-peak voltage is less than the threshold and greater than zero, the controller controls the switch to enable the power supply.
In accordance with a twenty-sixth aspect of the present invention, the detector comprises a memory to store a plurality of threshold values and a corresponding plurality of cable lengths.
In accordance with a twenty-seventh aspect of the present invention, the detector determines the length of the cable.
In accordance with a twenty-eighth aspect of the present invention, the detector compares a phase of the test signal to a phase of the received signal to determine the length of the cable.
In accordance with a twenty-ninth aspect of the present invention, when the peak-to-peak voltage is zero or greater than the threshold, the controller controls the switch to not enable the power supply.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein like reference symbols refer to like parts:
FIG. 1 illustrates two network devices that are connected together by a data cable with four pairs of twisted pair wires according to the prior art;
FIG. 2 illustrates a first network device connected to a second network by a data cable, the second network device requiring power via the data cable, in accordance with the present invention;
FIG. 2A illustrates a first network device connected to a second network by a data cable, the second network device not requiring power via the data cable;
FIG. 3A illustrates power detection signal generated by the signal generator of the first network device shown in FIGS. 2 and 2A, in accordance with a first embodiment of the present invention;
FIG. 3B illustrates power detection signal generated by the signal generator of the first network device shown in FIGS. 2 and 2A, in accordance with a second embodiment of the present invention;
FIG. 4A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 0 meters, in accordance with the first embodiment of the present invention;
FIG. 4B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 0 meters, in accordance with the second embodiment of the present invention;
FIG. 5A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 0 meters in which the distal end of the cable is short circuited, in accordance with the first embodiment of the present invention;
FIG. 5B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 0 meters in which the distal end of the cable is short circuited, in accordance with the second embodiment of the present invention;
FIG. 6A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 100 meters, in accordance with the first embodiment of the present invention;
FIG. 6B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 100 meters, in accordance with the second embodiment of the present invention;
FIG. 7A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 100 meters in which the distal end of the cable is short circuited, in accordance with the first embodiment of the present invention;
FIG. 7B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 100 meters in which the distal end of the cable is short circuited, in accordance with the second embodiment of the present invention;
FIG. 8A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 150 meters, in accordance with the first embodiment of the present invention;
FIG. 8B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 150 meters, in accordance with the second embodiment of the present invention;
FIG. 9A is a graph of the signal of FIG. 3A transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 150 meters in which the distal end of the cable is short circuited, in accordance with the first embodiment of the present invention;
FIG. 9B is a graph of the signal of FIG. 3B transmitted by a first network device to a cable-powered DTE device over a cable received by a detector of the first network device, the cable having a length of 150 meters in which the distal end of the cable is short circuited, in accordance with the second embodiment of the present invention;
FIG. 10A illustrates the pulse signatures of the received detection signal, in accordance with the present invention, in accordance with the first embodiment of the present invention;
FIG. 10B illustrates the pulse signatures of the received detection signal, in accordance with the present invention, in accordance with the second embodiment of the present invention;
FIG. 11 is a state diagram of the detection algorithm, in accordance with the present invention;
FIG. 12 is a functional block diagram of a device with a physical layer that includes an autonegotiation controller according to the present invention;
FIG. 13 is a schematic diagram of a switch in accordance with the present invention;
FIG. 14 show illustrates the pulses received from a cable-powered DTE device having a faulty or leaky filter;
FIG. 15 is a block diagram of the autonegotiation controller in accordance with the present invention;
FIG. 16A show illustrates the pulses received within a window from a cable-powered DTE device having a leaky filter;
FIG. 16B show illustrates the pulses received outside a window from a second network device;
FIG. 17 is a flowchart of the autonegotiation controller of the first network device, in accordance with the present invention; and
FIG. 18 is a graph of the differences between the maximum pulse vs. the cable length in accordance with the first embodiment of the present invention.
FIG. 19 is a flowchart illustrating detection of a cable-powered device.



DETAILED DESCRIPTION OF THE INVENTION
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
Power Detection
The first embodiment of the present invention is directed to a physical layer of a network device which can determine if the network device it is communicating with requires power to be supplied by via the data cable therebetween. Examples of network devices requiring power via the data cable include IP telephones, fax machines, other Internet appliances and the like.
Reference is now made to FIG. 2. As shown therein a first network device 10 is connected to a second network device 12 via a transmission line 18 or data cable. In this example, first network device 10 detects and provides power to second network device 12 via data cable 18. FIG. 2A illustrates an example in which second network device 12′ is supplied with power from an external source, and does not need to be supplied with power from first network device 10 via data cable 18. It is noted that only the A and B pairs of cable 18 are shown for purposes of simplicity. Additionally, the first and second network devices are shown as either a 10BASE-T or 100BASE-TX device. During the autonegotiation phase and power detection phase, a network device that is 1000BASE-TX compliant is operated as a 10BASE-T device, and can be schematically represented as shown in FIG. 2. In this application, the term “cable-powered DTE device” shall refer to a network device that requires power being supplied from another network device via a data cable, and the term “self-powered DTE device” shall refer to a network device in which power not supplied by the data cable. Self-powered DTE devices may be supplied by external power supplies or internal power supplies, such as, batteries.
The following discussion will focus on the detection of a cable-powered DTE by first network device 10. Referring the specifically to FIG. 2, first network device 10 comprises, inter alia, a transmitter 312, a first transformer 316, a receiver 314, a second transformer 318, a power supply 320, a signal generator 322, a detector 324 and controller 326. The output of transmitter 312 is coupled to the secondary side of transformer 316, and the primary side of transformer is connected to pair A of data cable 18. Pair B of data cable B is connected to the primary side of transformer 318, and the secondary side of transformer 318 is connected to receiver 314. The primary sides of transformer 316 and 318 comprise center taps, which are connected to power supply 320 to supply power over data cable 18 to cable-powered DTE device 12. The output of signal generator 322 is connected to the input of transmitter 312, and the output of receiver 314 is connected to the input of detector 324. Controller 326 controls the operation of signal generator 322 and power supply 320, and controller 326 is responsive to the output of detector 324. Transmitter 312 and receiver 314 each operates in a conventional manner, and no further discussion will be presented herein.
In response to controller 326, signal generator 322 generates test signals to be transmitted by transmitter 312 to the second network device 12 over pair A of data cable 18. Receiver 314 may receive a signal on pair B of data cable 18 and outputs it to detector 324, as described in detail hereinbelow. If detector 324 detects that the second network device is a cable-powered DTE device, controller energizes power supply 320, which provides power to the cable-powered DTE device via data cable 18. If, however, detector 324 does not detect a cable-powered DTE device, power supply remains disabled.
FIG. 2 shows and example of a cable-powered DTE device 12. As shown in that figure, pair A of cable 18 is connected to the primary of transformer 334. The secondary of transformer 334 is connected to selector 333, which selects either receiver 342 or filter 352. Pair B of cable 18 is connected to the primary of transformer 330, and the secondary of transformer 333 is connected to selector 333, which selects either transmitter 344 or filter 352. Load 350 and controller 352 are connected across the center taps of the primaries of transformers 334 and 330. Load 350 comprises for example the load of the receiver 342, transmitter 344 and other circuits constituting the cable-powered DTE device. Controller 352 controls selector 333. In the deenergized state or when power is not supplied over data cable 18, selector 333 connects the secondaries of transformer 334 and 333 to filter 352. Typically filter 352 is a low-pass filter. Controller 352 detects when network device supplies 10 power to cable 18. Since load 350 is in parallel to controller 352, power is also supplied to load 350 at the same time as power is supplied to controller 352. When power is supplied to controller 352, selector 333 is controlled to connect the secondary of transformer 334 to receiver 342 and the secondary of transformer 330 to transmitter 334. At substantially the same time, power is supplied to receiver 342, transmitter 344 and the other circuits of cable-powered DTE device 12. At this point cable-powered DTE device 12 can begin the autonegotiating with network device 10.
FIG. 2A is an example of network device 10 in communication with self-powered DTE device 12′. As shown in that figure, pair A of cable 18 is connected to the primary of transformer 334. The secondary of transformer 334 is connected to receiver 342. Pair B of cable 18 is connected to the primary of transformer 330, and the secondary of transformer 333 is connected to transmitter 344. Since self-powered DTE device 12′ is powered externally, self-powered DTE device 12′ can begin autonegotiation with network device 10.
A more detailed description of signal generator 322 and detector 324 is presented herein below.
Referring again to FIG. 2, the cutoff frequency of the low-pass filter 352 is set to filter out the 100-ns fast link pulses (FLPs). As described hereinbelow, the FLPs are utilized by network devices in the autonegotiation process. Thus, in this embodiment, first network device 10 transmits test signals having pulse widths greater than 100 ns, which will pass through low-pass filter 352.
Referring to FIG. 11, a state diagram of the detection algorithm. The RESET state 1101 is the starting point for the algorithm. The algorithm may be activated, for example, by reception of a Reset signal, as indicated by the input arrow to the RESET block. A variable labeled REQ_PWR is set to zero to indicate that the default state is that power is not required. A counter variable, labeled CNT, is initialized to zero. In the SENSE state 1102, a variable labeled ANEG_EN is set to one to indicate that network device 10 is enabled to autonegotiate the highest common transmission speed with network device 12 (link partner). If the autonegotiation function is disabled ANEG_EN is set to zero), it will stay in the SENSE state 1102 until link is established, otherwise, Timer 1 is set to time out after 1 second. Timer 1 can be reset by detecting an incoming link pulse or other signal activities. Signal generator 322 will either generate continuous streams of data or a link pulse nominally once per 16 ms (10BASE-T or Auto-Negotiation enabled). If network device 12 is a self-powered DTE device then network device 10 and network device 12 will attempt to autonegotiate. If network device 12 is a cable-powered DTE device, no activity will be seen during the 1 second interval, and Timer 1 will time out, and the algorithm will proceed to PULSE&RCV state 1103.
In PULSE&RCV state 1103, the test signal (as shown in FIGS. 3A and 3B) is generated by signal generator 322, and ANEG_EN is set to zero (i.e., Auto-Negotiation mode is disabled) to indicate that the circuit is in a test state rather than a normal operational state. Signal 312 generates the detection signal. The second timer, Timer 2, is set to 5 μs. The purpose of Timer 2 is to time out if no return signal is detected, which means the cable is open. Under this condition, theit will enter the FAIL state 1104. If the returned signal is detected in 5 μs, by comparing the difference of the transmitted signal and the returned signal, the cable length can be calculated and a peak-to-peak amplitude threshold can be determined as a function of the cable length. A lookup table can preferably be implemented to accomplish this function. By comparing this threshold with the peak-to-peak amplitude of the returned pulse, the present invention can determine if there is a filter at the far end or the far end is shorted.
If the far end is shorted, the FAIL state 1104 is entered. If a filter is detected the WAIT state 1105 is entered. In this state, the counter CNT is incremented; in the first instance, CNT is set equal to one. The third timer, Timer 3, is set to 156 ms. The purpose of Timer 3 is to wait until the next test signal is to be generated. Referring again to FIGS. 3A and 3B, the pulses are spaced by at least 156 ms. therefore another pulse can be expected before 130 ms. expires. The 130-ms interval prevents false 10BASE-T detection by second network device 12, because 125 ms of inactivity ensures that network device 12 will reset. If Timer 3 expires and the counter has not reached the Limit value, then PULSE&RCV state 1102 is reentered, and the process is repeated. This process is repeated several times to ensure that an anomalous determination that power is required does not occur. A typical Limit value may be 3.
Once the counter reaches the Limit value, the process proceeds to the WAIT FOR LINK state 1106. In this state, detector 324 has determined that second network device 12 is a cable-powered DTE device, so REQ_PWR is set to one. The counter CNT is reset to zero, ANEG_EN is set to one, and then the Sense state 1102 is entered. Then the present invention waits for the link partner to be powered up and to establish a link. If a successful link is established, then the process proceeds to the LINK GOOD state 1107. In this state, the counter CNT is again reset to zero, and the fifth timer, Timer 5, is set to 2 seconds. Once Timer 5 expires, the link is tested again. If the link is still good, Link Pass is indicated, and the algorithm stays in the LINK GOOD state and restarts Timer 5. Thus, the algorithm effectively waits until the link fails (e.g., the circuit has been disconnected for some reason). Once the link fails, the algorithm returns to the SENSE-PULSE cycle again.
The present invention is preferably implemented in a network switch. Referring to FIG. 13, a network switch 400 comprising a plurality of ports is shown therein. Each port is capable of communicating with self-powered DTE devices, cable-powered DTE devices and other network switches. Each port comprises a physical layer device configured to can determine if the network device it is communicating with is a self-powered DTE device or a cable-powered DTE device. Ports 420-1–420-8 are connected to an internal data bus. A CPU controls the communication among Ports A–H by controlling which ports have access to the data bus. Each port has a detector described above that can be connected to another network device via respective cable 18-n. In the example of FIG. 13, ports A and B are not connected to any device. Ports C and E are connected to IP telephones A 430-3 and B 430-5, respectively. Ports D, F, and G are connected to computers A 430-4, B 430-6, and C430-7, respectively. Port H is connected to a facsimile machine 430-8.
In the default mode, each ports sends test signals to its respective device, and determine if the device connected thereto is a self-powered DTE device or cable-powered DTE device. In the example shown in FIG. 13 only IP telephone 430-3 is a cable-powered DTE device, and only port C 420-3 supplies power over data cable 18-3.
Each port a physical layer device arranged and constructed similarly to that shown in FIG. 2.
Although the detector is described in the context of a network switch, those skilled in the art will appreciate that the detector is likewise suitable for various other applications. Accordingly, the described exemplary application of the detector is by way of example only and not by way of limitation.
First Embodiment
The following is a detailed description for detecting whether the connected network device 12 (12′) is a cable-powered DTE device or self-powered DTE device. In network device 10, signal generator 322 generates test signals for transmission by transmitter 312 over pair A of data cable 18, the test signal returns through filter 18 and pair B of data cable 18, receiver 314 to detector 324. If the network device 12′ is a self-powered DTE device, as shown in FIG. 2B, or if there is an open circuit, detector 324 does not detect a return signal. As a result power is not supplied by network device 10 to network device 12′ over data cable 18.
Referring to FIG. 3A, an exemplary test signal generated by signal generator 322 comprising plural pulses is illustrated. An initial pulse having a magnitude of −1 volt is applied for 752 ns followed by, positive pulses (1 volt) having a width of about 152 ns, negative (−1 volt) pulses having a width of approximately 72 ns and ending with a negative (−1 volt) pulse having a width of 304 ns. Successive test signals are spaced by at least 156 ms. It will be appreciated that the test signal in FIG. 3A is shown for illustrative purposes only and other appropriate test signals may be utilized.
FIG. 4A illustrates the test signal received by detector 324 when the length of the data cable is approximately zero meters and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises a peak-to-peak voltage of about 1.25 volts. FIG. 5A illustrates the test signal received by detector 324 when the length of the data cable is approximately zero meters and the cable is short-circuited at the distal end thereof, with respect to network device 10. As can be seen in FIG. 5A, the difference between the maximum pulse and the minimum pulse received is approximately 2.25 volts.
FIG. 6A illustrates the test signal received by detector 324 when the length of the data cable is approximately 100 meters, and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises a peak-to-peak voltage of about 0.4 volts. FIG. 7A illustrates the test signal received by detector 324 when the length of the data cable is approximately 100 meters and the cable is short circuited at the distal end thereof with respect to network device 10, the difference between the maximum pulse and the minimum pulse received is approximately 0.9 volts.
FIG. 8A illustrates the test signal received by detector 324 when the length of the data cable is approximately 150 meters of a cable-powered DTE device, and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises a peak-to-peak voltage of about 0.35 volts. FIG. 9A illustrates the test signal received by detector 324 when the length of the data cable is approximately 150 meters and the cable is short circuited at the distal end thereof with respect to network device 10, the difference between the maximum pulse and the minimum pulse received is approximately 0.7 volts.
FIG. 18 is a graph of the differences between the maximum value of the three pulses and the minimum value of the three negative pulses (referred to hereinbelow as “peak-to-peak voltage”) vs. the cable length for various conditions. The two plots with the largest peak-to-peak voltages are short circuit conditions for CAT5 and CAT3 cables, and the smallest are the two plots with the smallest voltage are the CAT5 and CAT3 cables connected to a cable-powered DTE device. As illustrated therein, for any specified length, a short-circuited cable will always have a higher peak-to-peak voltage than cable that is not short-circuited. A threshold value is preferably defined as the average peak-to-peak voltage of the short-circuited CAT3 cable and the CAT5 cable connected to a cable-powered DTE device for a specified length. It will be appreciated by one of ordinary skill in the art that other appropriately selected threshold values may be used so long as it is between the largest peak-to-peak voltages and the smallest peak-to-peak voltages.
FIG. 19 is a flow chart utilized implemented by detector 324 to detect whether network device 12 is a cable-powered device. In step 180, the test signal illustrated in FIG. 3A is generated by signal generator 322 and then transmitted by transmitter 312 over cable 18. A returned test signal is then received by receiver 314 and processed by detector 324. The length of cable 18 is determined by the phase difference between the transmitted and received signal (step 181). Once the distance is determined, the threshold voltage can be determined as a function cable length, which is empirically determined as discussed above. In the preferred embodiment, detector 324 comprises a memory or look up table for storing the threshold values (step 182). Alternatively, the threshold value may be calculated directly based the functional relationship between the voltage and cable length.
Still referring FIG. 19, detector 324 measures the peaks values of the three positive pulses of the test signal and determines which one has the largest value. Detector 324 further measures the relative minimum of the 2 negative pulses of the test signal and determines which one has the smallest value. Detector 324 determines the difference between the largest peak value and the smallest relative minimum to calculate the peak-to-peak voltage. It will be appreciated by one of ordinary skill in the art that other algorithms may be utilized to calculate the peak-to-peak voltage, such as, for example only, by calculating the difference between an average of the peak values and an average of the relative minima or by determining a difference between an arbitrary peak voltage and an arbitrary relative minimum voltage.
If detector 324 does not detect the test signal on cable 18, either network device 10 is connected to a self-power DTE device 12′, there is no connection to network device 12, the distal end of cable 18 is not connected to any device or there is an open circuit (step 184). In any case, network device 12 does not supply power on cable 18 (step 187).
On the other hand if detector 324 detects a return signal, processing continues to step 185. In step 185, detector 324 determines if peak-to-peak voltage measured in step 183 is greater than the threshold determined in step 182. If so, cable 18 is either short-circuited or connected to another port in network device 10 or a device similar to network device 10. In either case, network device 12 does not supply power on cable 18 (step 187). Alternatively if the peak-to-peak voltage measured in step 183 is less than the threshold determined in step 182, network device 12 is a cable-powered DTE device and power controller 326 enables power supply 320. As such power is supplied on cable 18.
Second Embodiment
Referring to FIG. 3B, an exemplary test signal generated by signal generator 322 comprising plural pulses in accordance with the second embodiment is illustrated. The width of each positive pulse is about 150 ns, and the width of each negative pulse is approximately 70 ns. Successive test signals are spaced by at least 156 ms. When a cable powered DTE requiring power is not already being supplied with power, signal generator 322 generates test signals for transmission by transmitter 312 over pair A of data cable 18. The test signal returns through filter 18 and pair B of data cable 18, receiver 314 to detector 324. If the network device 12′ is a self-powered DTE device or there is an open circuit, as shown in FIG. 2B, detector 324 does not detect a return signal. As a result power is not supplied by network device 10 to network device 12′ over data cable 18.
FIG. 4B illustrates the test signal received by detector 324 when the length of the data cable is approximately zero meters and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises two relative minima between the positive pulses to occur at in the range of approximately 0.7 to 0.8 volts which is significantly higher voltage levels than the level at which they were originally transmitted. FIG. 5B illustrates the test signal received by detector 324 when the length of the data cable is approximately zero meters and the cable is short-circuited at the distal end thereof, with respect to network device 10. As can be seen in FIG. 5B, the minima between the positive pulses, is approximately 0 volts.
FIG. 6B illustrates the test signal received by detector 324 when the length of the data cable is approximately 100 meters, and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises two relative minima between the positive pulses to occur at in the range of approximately 0.3 to 0.38 volts. FIG. 7B illustrates the test signal received by detector 324 when the length of the data cable is approximately 100 meters and the cable is short circuited at the distal end thereof with respect to network device 10, the minima between the positive pulses, is in the range of approximately 0.0 to 0.1 volts.
FIG. 8B illustrates the test signal received by detector 324 when the length of the data cable is approximately 150 meters of a cable-powered DTE device, and network device 10 is connected to cable-powered DTE device 12. The received test signal comprises two relative minima between the positive pulses to occur at in the range of approximately 0.3 to 0.38 volts. FIG. 9B illustrates the test signal received by detector 324 when the length of the data cable is approximately 150 meters and the cable is short circuited at the distal end thereof with respect to network device 10, the minima between the positive pulses, is in the range of approximately 0.1 to 0.14 volts.
Detector 324 comprises a slicer and compares the received test signal with a threshold level that is above the original relative minima but lower than the relative minima of the returned pulse. Referring to FIG. 10B, detector 324 processes the received signal to have the resultant one of three signatures as follows. The short circuit signature comprises three positive pulses. The signature of open circuit or of a self-powered DTE device is a 0 volt signal. The signature of network device 10 (which detects a cable-powered DTE device) connected to a device similar to network device 10 has substantially same signature as a short circuit. The signature of the signal of a cable-powered DTE device is a single pulse.
As such, detector 324 is able to distinguish between an open circuit (either a self-powered DTE device, a disconnected cable or an open conductor in the cable), a cable-powered DTE device (when the relative minima of the received signal are above the threshold level) and a short circuit (when the relative minima of the received signal are less than the threshold level).
Autonegotiation
In 10BASE-T, 100 BASE-TX, and 1000BASE-T networks, the physical layer executes autonegotiation protocols that initiate the data link between the network devices. Once the data link is lost, the physical layer notifies the network device. The cable usually provides the physical connection between the physical layers of network devices.
During autonegotiation, bursts of pulses called fast link pulse bursts (FLP) (each pulse in the burst is referred to as an NLP) are transmitted and received periodically by the physical layer. The purpose of the FLP bursts is to detect the presence of another network device and to initiate the exchange of data between the network devices. The initialization information typically includes configuration information such as the communication speed(s) that are available and other information that will be necessary for subsequent communications between the network devices.
When a physical layer of a network device is not connected to another network device, the physical layer still periodically transmits FLP bursts in an attempt to initiate connections to other network devices. FLP bursts usually include 17 to 33 link pulses that are generated every 16 ms. The physical layer remains powered up while attempting to connect to another network device. The autonegotiation function is defined more fully in IEEE 802.3, which is hereby incorporated by reference. In particular, Sections 22.2.4, 28, 32.5 and 40.5 of IEEE 802.3 address the autonegotiation capability. Referring now to FIG. 3, a physical layer 50 of a device includes an autonegotiation controller 52, a digital signal processor (DSP) 54 and other conventional physical layer circuits 58.
The inventors have observed that sometimes when performing the autonegotiation process, network device 10 may incorrectly attempt to complete autonegotiation with a cable-powered DTE device, which is not yet powered. In other words, in this situation in the SENSE state, shown in FIG. 11, network device 10 would send the autonegotiation FLP's and receive very similar FLP's. As a result network device 10 incorrectly believes that it has successfully autonegotiated. The inventors have determined that in cable-powered DTE devices 12, filter 352 may not be manufactured to specification (referred to hereinbelow as a “faulty filter” or “leaky filter”). These filters result in the FLP's not being completely filtered and network device 10 falsely autonegotiating. This problem is particularly exacerbated when the cable length is short. FIG. 14, illustrates the transmitted and received FLPs transmitted to a DTE device having a leaky filter. Additionally, the autonegotiation circuit may incorrectly autonegotiate when data cable 18 is short circuited or when the other end of the data cable is connected to another port of the same switch.
The inventors propose a modification to the autonegotiation controller 52 to prevent false autonegotiation. Autonegotiation controller 52 further comprises a counter circuit 522, windowing circuit 526 and a blinding circuit 524, and randomizer 528, the operation of which will be explained herein below.
FIG. 17 is a flow chart of the process implemented by autonegotiation controller 52 during the SENSE state shown in FIG. 11. Autonegotiation controller 52 initiates the transmission of the FLP's as described above (step 1204). Autonegotiation controller 52 then analyzes the received signal, if any. The received signal is compared to the transmitted NLP's, if the received signal does not contains the same number NLP's in the FLP transmitted within a window, and then the autonegotiation process continues. On the other hand if the received signal contains the same number of NLPs in the FLP, the network device still must determine whether it is autonegotiating with a self-powered network device or a cable-powered DTE device, which has a leaky filter. This is preferably implemented by counter circuit 522 counting both the number of NLPs transmitted (step 1206) and the number of pulses received within a window established by windowing circuit 526 (step 1208). FIG. 16A is an example of the NLP's being received within the window, and FIG. 16B is an example of some other signal being received. This other signal may be autonegotiation pulses generated by network device 12.
Referring back to FIG. 17, if the number of NLP's transmitted does not equal the number of received pulses (step 1210) then blinding circuit 524 is disabled (step 1218) and autonegotiation continues.
Alternatively in step 1210 if the number of NLP's transmitted equals the number of received pulses within the window, the autonegotiation process still is not certain whether it is autonegotiating with a self-powered network device or a cable-powered DTE device. The blinding circuit 524 is then enabled. When enabled, the blinding circuit 524 prevents the autonegotiation controller 52 from autonegotiating. (As noted above the blinding circuit 524 is enabled until the number of received pulses does not equal the number of transmitted NLPs within a window.) The timing between the next FLP bursts is randomized (step 1214) by randomizer 528. As noted above the normal timing between FLP bursts is 16 ms. Randomizer 528 randomly changes the timing between FLP bursts from 14 ms and 16 ms. The randomization will tend to eliminate network device 1—from counting pulses within the window from network device 12, which is attempting to autonegotiate with network device 10. In this situation the NLP's generated from network device 12 are coincidentally being received within the window. After the randomization the autonegotiation process is repeated. If the blinding circuit remains enabled sufficient enough time to cause timer 1 (130 ms) to time out (that is if the number of transmitted NLPs remains equal the number of received pulses), then the process exits the SENSE state and enters the PULSE state. In other words, the autonegotiation controller has detected that network device 12 contains a leaky filter, and start the detection states.
The blinding mode also facilitates detecting a cable-powered network device while in the sleep mode. An example of the sleep mode is discussed in commonly-assigned and copending patent application entitled “Apparatus for Automatic Energy Savings Mode For Ethernet Transceivers and Method Thereof” filed on Nov. 21, 2001 and assigned Ser. No. 09/990,137, the contents of which are incorporated by reference.
It is hereby noted that the best mode of the present invention entails the use of an Ethernet data transmission system, including Ethernet transmitters and receivers. However, while the present invention has been described with respect to what is presently considered to be the preferred embodiment, i.e., an implementation in an Ethernet system, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, it is to be understood that the invention is applicable to other types of data communication circuitry. The invention also may be implemented via an appropriately programmed general purpose computer. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

## Claims

1. A first network device for supplying power to a second network device in communication therewith comprising:
a first transformer in communication with the second network device;
a second transformer in communication with the second network device;
a switch;
a power supply in communication with first and second transformers via said switch; a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;
a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said signal generator and said detector;
wherein said signal generator generates a test signal comprising n sub-pulses to be transmitted by said transmitter, wherein n being greater than 2; and
wherein when said detector, in response to said receiver, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, enables said switch to supply power from said power supply to the second network device.

2. The first network device according to claim 1, wherein when said detector detects q pulses which are greater than a predetermined threshold, j<q≦n, said controller, responsive to said detector, does not enable said switch and power is not supplied from said power supply to the second network device.

3. The first network device according to claim 1, wherein when said detector detects 0 pulses which are greater than a predetermined threshold, said controller, responsive to said detector, does not enable said switch and power is not supplied from said power supply to the second network device.

4. The first network device according to claim 2, wherein q=3.

5. The first network device according to claim 1, wherein j=1.

6. The first network device according to claim 1, wherein n=3.

7. The first network device according to claim 1, wherein said transmitter transmits plural test signals, each successive test signal separated by a predetermined interval.

8. The first network device according to claim 1, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

9. The first network device according to claim 1, wherein when said detector, in response to said receiver, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, performs one of providing an indication, providing an interrupt and setting a bit.

10. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
a pulse generator to generate a test signal comprising n sub-pulses to be transmitted to the second network device, wherein n being greater than 2;
a detector responsive to the second network device;
a controller in communication with said detector and said pulse generator; and wherein when said detector detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, enables power to be transmitted to the second network device.

11. A physical layer device according to 10, wherein when said detects q pulses which are greater than a predetermined threshold, j<q≦n said controller, responsive to said detector, does not enable power to be transmitted to the second network device.

12. A physical layer device according to claim 10, wherein when said detector detects 0 pulses which are greater than a predetermined threshold, said controller, responsive to said detector, does not enable power to be transmitted to the second network device.

13. A physical layer device according to claim 10, wherein j=1.

14. The first network device according to claim 10, wherein n=3.

15. A physical layer device according to claim 10, wherein said pulse generator transmits plural test signals, each successive test signal separated by a predetermined interval.

16. A physical layer device according to claim 10, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

17. A physical layer device according to 10, wherein when said detector detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, performs one of providing an indication, providing an interrupt and setting a bit.

18. A network comprising: a first network device comprising:
a first transformer;
a second transformer; a switch;
a power supply in communication with said first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;
a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said receiver and said detector;

a cable in communication with said first network device; and
a second network device in communication with said cable,
wherein said signal generator generates a test signal comprising n sub-pulses to be transmitted by said transmitter, wherein in n being greater than 2; and
wherein when said detector, in response to said receiver, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, enables said switch to supply power from said power supply to the second network device.

19. The network according to claim 18, wherein when said detector detects q pulses which are greater than a predetermined threshold, j<q≦n, said controller, responsive to said detector, does not enable said switch and power is not supplied from said power supply to the second network device.

20. The network according to claim 18, wherein when said detector detects 0 pulses which are greater than a predetermined threshold, said controller, responsive to said detector, does not enable said switch and power is not supplied from said power supply to the second network device.

21. The network according to claim 18, wherein j=1.

22. The network according to claim 18, wherein n=3.

23. The network according to claim 18, wherein said transmitter transmits plural test signals, each successive test signal separated by a predetermined interval.

24. The network according to claim 18, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

25. The network according to claim 18, wherein when said detector, in response to said receiver, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller, responsive to said detector, performs one of providing an indication, providing an interrupt and setting a bit.

26. A first network device for supplying power to a second network device in communication therewith comprising:
first transformer means for communicating with the second network device
second transformer means for communicating with the second network device power supply means for supplying power in communication with first and second transformer means;
switch means for enabling and disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means; and
controller means for controlling said switch means and said signal generator means, and responsive to said detection means;

wherein said signal generator means generates a test signal comprising n sub-pulses to be transmitted by said transmitter means, wherein in being greater than 2;
and wherein when said detection means, in response to said receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detection means, enables said switch means to supply power from said power supply means to the second network device.

27. The first network device according to claim 26, wherein when said detection means detects q pulses which are greater than a predetermined threshold, j<q≦n, said controller means, responsive to said detection means, does not enable said switch means and power is not supplied from said power supply means to the second network device.

28. The first network device according to claim 26, wherein when said detection means detects 0 pulses which are greater than a predetermined threshold, said controller means, responsive to said detection means, does not enable said switch means and power is not supplied from said power supply means to the second network device.

29. The first network device according to claim 26, wherein j=1.

30. The first network device according to claim 26, wherein n=3.

31. The first network device according to claim 26, wherein said transmitter means transmits plural test signals, each successive test signal separated by a predetermined interval.

32. The first network device according to claim 26, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

33. The first network device according to claim 26, wherein when said detection means, in response to said receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detection means, performs one of providing an indication, providing an interrupt and setting a bit.

34. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising: pulse generator means for generating a test signal comprising n sub-pulses to be transmitted to the second network device, wherein in being greater than 2;
detector means responsive to the second network device;
controller means for controlling said pulse generator means and responsive to said detector means; and
wherein when said detector means detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detector means, enables power to be transmitted to the second network device.

35. A physical layer device according to 34, wherein when said detector means detects q pulses which are greater than a predetermined threshold, j<q≦n said controller means, responsive to said detector means, does not enable power to be transmitted to the second network device.

36. A physical layer device according to claim 34, wherein when said detector means detects 0 pulses which are greater than a predetermined threshold, said controller means, responsive to said detector means, does not enable power to be transmitted to the second network device.

37. A physical layer device according to claim 34, wherein j=1.

38. The first network device according to claim 34, wherein n=3.

39. A physical layer device according to claim 34, wherein said pulse generator means transmits plural test signals, each successive test signal separated by a predetermined interval.

40. A physical layer device according to claim 34, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

41. A physical layer device according to 34, wherein when said detector means detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detector means, performs one of providing an indication, providing an interrupt and setting a bit.

42. A network comprising:
first networking means comprising:
first transformer means for transforming a signal;
second transformer means for transforming a signal;
power supply means for supplying power to said first and second transformer means;
switch means for enabling/disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;

receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means; and
controller means for controlling said switch means, said signal generator means and responsive to said detection means;

second networking means; and
cable means for electrically connecting said first and second networking means;
wherein said signal generator means generates a test signal comprising n subpulses to be transmitted by said transmitter means, wherein in n being greater than 2; and
wherein when said detection means, in response to said receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detection means, enables said switch means to supply power from said power supply means to said second networking means.

43. The network according to claim 42, wherein when said detection means detects q pulses which are greater than a predetermined threshold, j<q≦n, said controller means, responsive to said detection means, does not enable said switch means and power is not supplied from said power supply means to the second networking means.

44. The network according to claim 42, wherein when said detection means detects 0 pulses which are greater than a predetermined threshold, said controller means, responsive to said detection means, does not enable said switch means and power is not supplied from said power supply means to said second networking means.

45. The network according to claim 42, wherein j=1.

46. The network according to claim 42, wherein n=3.

47. The network according to claim 42, wherein said transmitter means transmits plural test signals, each successive test signal separated by a predetermined interval.

48. The network according to claim 42, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

49. The network according to claim 42, wherein when said detection means, in response to said receiver means, detects j pulses which are greater than a predetermined threshold, 1≦j<n, said controller means, responsive to said detection means, performs one of providing an indication, providing an interrupt and setting a bit.

50. A method of supplying power from a first network device to a second network device in communication therewith, comprising the steps of:
(a) transmitting a test signal comprising n sub-pulses to the second network device, wherein n being greater than 2;
(b) detecting a signal from the second network device in response to step (a); and
(c) enabling power to be transmitted to the second network device when in step (b) j pulses are detected which are greater than a predetermined threshold, 1≦j<n.

51. A method according to 50, further comprising the step of not enabling power to be transmitted to the second network device when in step (b) q pulses are detected which are greater than a predetermined threshold, j<q≦n.

52. A method according to 50, further comprising the step of not enabling power to be transmitted to the second network device when in step (b) 0 pulses are detected which are greater than a predetermined threshold.

53. A method according to 50, wherein j=1.

54. A method according to 50, wherein n=3.

55. A method according to 50, wherein step (a) is performed plural times such that each successive test signal separated by a predetermined interval.

56. A method according to 50, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

57. A method according to claim 50, further comprising the step of performing one of providing an indication, providing an interrupt and setting a bit.

58. A computer program stored on a tangible storage medium for controlling a first network device to supply power to a second network device in communication therewith, comprising the steps of:
(a) transmitting a test signal comprising n sub-pulses to the second network device, wherein n being greater than 2;
(b) detecting a signal from the second network device in response to step (a);
(c) enabling power to be transmitted to the second network device when in step (b) j pulses are detected which are greater than a predetermined threshold, 1≦j<n.

59. A computer program according to 58, further comprising the step of not enabling power to be transmitted to the second network device when in step (b) q pulses are detected which are greater than a predetermined threshold, j<q≦n.

60. A computer program according to 58, further comprising the step of not enabling power to be transmitted to the second network device when in step (b) 0 pulses are detected which are greater than a predetermined threshold.

61. A computer program according to 58, wherein j=1.

62. A computer program according to 58, wherein n=3.

63. A computer program according to 58, wherein step (a) is performed plural times such that each successive test signal is separated by a predetermined interval.

64. A computer program according to 58, wherein the n subpulses comprise a subpulse of a first polarity having a first pulse width and a subpulse of a second polarity having a second pulse width, wherein the first pulse width is greater than said second pulse width.

65. A computer program according to claim 58, further comprising the step of performing one of providing an indication, providing an interrupt and setting a bit.

66. A first network device in communication with a second network device via a data cable for supplying power thereto, said first network device comprising:
a first transformer in communication with the second network device;
a second transformer in communication with the second network device;
a switch;
a power supply in communication with first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;

a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said signal generator and said detector;
wherein said signal generator generates a test signal be transmitted by said transmitter;
wherein said detector selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply, and
wherein said detector comprises a memory to store a plurality of threshold values and a corresponding plurality of cable lengths.

67. The first network device according to claim 66, wherein when said detector measures a peak-to-peak voltage of the received test signal.

68. The first network device according to claim 67, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller controls said switch to enable said power supply.

69. The first network device according to claim 67, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller controls said switch to not enable said power supply.

70. A first network device in communication with a second network device via a data cable for supplying power thereto, said first network device comprising:
a first transformer in communication with the second network device;
a second transformer in communication with the second network device;
a switch;
a power supply in communication with first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;

a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said signal generator and said detector;
wherein said signal generator generates a test signal be transmitted by said transmitter;
wherein said detector selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply, and wherein said detector determines the length of the cable.

71. The first network device according to claim 70, wherein said detector compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

72. A first network device for supplying power to a second network device in communication therewith comprising:
first transformer means for communicating with the second network device;
second transformer means for communicating with the second network device;
power supply means for supplying power in communication with first and second transformer means;
switch means for enabling and disabling said power supply means;
Physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means; and
controller means for controlling said switch means and said signal generator means, and responsive to said detection means;
wherein said signal generator means generates a test signal be transmitted by said transmitter means;
wherein said detection means selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal received by the receiver means, controls said switch means to enable or disable said power supply means, and wherein said detection means comprises a memory means to store a plurality of threshold values and a corresponding plurality of cable lengths.

73. The first network device according to claim 72, wherein when said detection means measures a peak-to-peak voltage of the received test signal.

74. The first network device according to claim 73, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller means controls said switch to enable said power supply.

75. The first network device according to claim 73, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller means controls said switch to not enable said power supply.

76. A first network device for supplying power to a second network device in communication therewith comprising:
first transformer means for communicating with the second network device;
second transformer means for communicating with the second network device;
power supply means for supplying power in communication with first and second transformer means;
switch means for enabling and disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means; and
controller means for controlling said switch means and said signal generator means, and responsive to said detection means;
wherein said signal generator means generates a test signal be transmitted by said transmitter means;
wherein said detection means selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal received by the receiver means, controls said switch means to enable or disable said power supply means, and wherein said detection means determines the length of the cable.

77. The first network device according to claim 76, wherein said detection means compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

78. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
a pulse generator to generate a test signal comprising;
a detector, responsive to the second network device, to receive a received test signal;
a controller in communication with said detector and said pulse generator;
wherein said detector circuit selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and the received test signal, enables or disables supplying power to the second network device, and
wherein said detector comprises a memory to store a plurality of threshold values and a corresponding plurality of cable lengths.

79. The physical layer device according to claim 78, wherein when said detector measures a peak-to-peak voltage of the received test signal.

80. The physical layer device according to claim 79, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller enables supplying power to the second network device.

81. The physical layer device according to claim 79, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller does not enable supplying power to the second network device.

82. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
a pulse generator to generate a test signal comprising;
a detector, responsive to the second network device, to receive a received test signal;
a controller in communication with said detector and said pulse generator;
wherein said detector circuit selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and the received test signal, enables or disables supplying power to the second network device, and
wherein said detector determines the length of the cable.

83. The physical layer device according to claim 82, wherein said detector compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

84. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
pulse generator means for generating a test signal comprising;
detector means, responsive to the second network device, for receiving a received test signal;
controller means in communication with said detector means for controlling said pulse generator means; and
wherein said detector means circuit selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detector means enables or disables supplying power to the second network device, and
wherein said detector means comprises memory means for storing a plurality of threshold values and a corresponding plurality of cable lengths.

85. The physical layer device according to claim 84, wherein when said detector means measures a peak-to-peak voltage of the received test signal.

86. The physical layer device according to claim 85, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller means enables supplying power to the second network device.

87. The physical layer device according to claim 85, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller means does not enable supplying power to the second network device.

88. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
pulse generator means for generating a test signal comprising;
detector means, responsive to the second network device, for receiving a received test signal;
controller means in communication with said detector means for controlling said pulse generator means; and
wherein said detector means circuit selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detector means enables or disables supplying power to the second network device, and
wherein said detector means determines the length of the cable.

89. The physical layer device according to claim 88, wherein said detector means compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

90. A network comprising:
a first network device comprising:
a first transformer:
a second transformer:
a switch;
a power supply in communication with said first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;
a signal generator in communication with said transmitter;
a detector in communication with said receiver; and

a controller in communication with said switch, said receiver and said detector;
a cable in communication with said first network device; and

a second network device in communication with said cable,
wherein said signal generator generates a test signal be transmitted by said transmitter;
wherein said detector selects a threshold in accordance with a length of said cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply, and
wherein said detector comprises a memory to store a plurality of threshold values and a corresponding plurality of cable lengths.

91. The network according to claim 90, wherein when said detector measures a peak-to-peak voltage of the received test signal.

92. The network according to claim 91, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller controls said switch to enable said power supply.

93. The network according to claim 91, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller controls said switch to not enable said power supply.

94. A network comprising:
a first network device comprising:
a first transformer:
a second transformer:
a switch;
a power supply in communication with said first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;
a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said receiver and said detector;
a cable in communication with said first network device; and


a second network device in communication with said cable,
wherein said signal generator generates a test signal be transmitted by said transmitter;
wherein said detector selects a threshold in accordance with a length of said cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply, and
wherein said detector determines the length of the cable.

95. The network according to claim 94, wherein said detector compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

96. A network comprising:
first networking means comprising:
first transformer means for transforming a signal:
second transformer means for transforming a signal:
power supply means for supplying power to said first and second transformer means;
switch means for enabling/disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means;
controller means for controlling said switch means, said signal generator means and responsive to said detection means;


second networking means; and
cable means for electrically connecting said first and second networking means;

wherein said signal generator means generates a test signal be transmitted by said transmitter means; and
wherein said detection means selects a threshold in accordance with a length of said cable means,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal from received by the receiver means, controls said switch means to enable or disable said power supply means, wherein said detection means comprises a memory means to store a plurality of threshold values and a corresponding plurality of cable lengths.

97. The first network device according to claim 96, wherein when said detection means measures a peak-to-peak voltage of the received test signal.

98. The first network device according to claim 97, wherein when the peak-to-peak voltage is less than the threshold and greater than zero, said controller means controls said switch to enable said power supply.

99. The first network device according to claim 97, wherein when the peak-to-peak voltage is zero or greater than the threshold, said controller means controls said switch to not enable said power supply.

100. A network comprising:
first networking means comprising:
first transformer means for transforming a signal:
second transformer means for transforming a signal:
power supply means for supplying power to said first and second transformer means;
switch means for enabling/disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means;
controller means for controlling said switch means, said signal generator means and responsive to said detection means;


second networking means; and
cable means for electrically connecting said first and second networking means;

wherein said signal generator means generates a test signal be transmitted by said transmitter means; and
wherein said detection means selects a threshold in accordance with a length of said cable means,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal from received by the receiver means, controls said switch means to enable or disable said power supply means, wherein said detection means determines the length of the cable.

101. The first network device according to claim 100, wherein said detection means compares a phase of the test signal to a phase of the received signal to determine the length of the cable.

102. A method of supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
storing a plurality of threshold values and a corresponding plurality of cable lengths.

103. A method according to 102, wherein step (b) further comprises the step of
(b1) measuring a peak-to-peak voltage of the signal.

104. A method according to 103, step (e) enables power to be transmitted to the second network device when the peak-to-peak voltage measured in step (b1) is less than the threshold selected in step (c) and greater than zero.

105. A method according to 103, step (e) does not enable power to be transmitted to the second network device when the peak-to-peak voltage measured in step (b1) is zero or greater than the threshold selected in step (c).

106. A method of supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
measuring the length of the cable.

107. A method of supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d),
wherein the measuring step comprises the step of comparing a phase of the test signal to a phase of the signal detected in step (b).

108. A computer program stored on a tangible storage medium for supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
storing a plurality of threshold values and a corresponding plurality of cable lengths.

109. A computer program according to 108, wherein step (b) further comprises the step of
(b1) measuring a peak-to-peak voltage of the signal.

110. A computer program according to 109, wherein step (e) enables power to be transmitted to the second network device when the peak-to-peak voltage measured in step (b1) is less than the threshold selected in step (c) and greater than zero.

111. A computer program according to 109, wherein step (e) does not enable power to be transmitted to the second network device when the peak-to-peak voltage measured in step (b1l) is zero or greater than the threshold selected in step (c).

112. A computer program stored on a tangible storage medium for supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
measuring the length of the cable.

113. A computer program according to 112, wherein the measuring step comprises the step of comparing a phase of the test signal to a phase of the signal detected in step (b).

114. A first network device in communication with a second network device via a data cable for supplying power thereto, said first network device comprising:
a first transformer in communication with the second network device;
a second transformer in communication with the second network device;
a switch;
a power supply in communication with first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;

a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said signal generator and said detector;
wherein said signal generator generates a test signal be transmitted by said transmitter; and
wherein said detector selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal received by the receiver, performs one of providing an indication, providing an interrupt and setting a bit.

115. A first network device for supplying power to a second network device in communication therewith comprising:
first transformer means for communicating with the second network device;
second transformer means for communicating with the second network device;
power supply means for supplying power in communication with first and second transformer means;
switch means for enabling and disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means; and
controller means for controlling said switch means and said signal generator means, and responsive to said detection means;
wherein said signal generator means generates a test signal be transmitted by said transmitter means; and
wherein said detection means selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal received by the receiver means, controls said switch means to enable or disable said power supply means, wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal received by the receiver means, performs one of providing an indication, providing an interrupt and setting a bit.

116. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
a pulse generator to generate a test signal comprising;
a detector, responsive to the second network device, to receive a received test signal;
a controller in communication with said detector and said pulse generator; and
wherein said detector circuit selects a threshold in accordance with a length of the data cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and the received test signal, enables or disables supplying power to the second network device, wherein said controller, in accordance with a comparison by said detector of the selected threshold and the received test signal, performs one of providing an indication, providing an interrupt and setting a bit.

117. A physical layer device of a first network device for supplying power to a second network device in communication therewith, comprising:
pulse generator means for generating a test signal comprising;
detector means, responsive to the second network device, for receiving a received test signal;
controller means in communication with said detector means for controlling said pulse generator means; and
wherein said detector means circuit selects a threshold in accordance with a length of the data cable,
wherein said controller means, in accordance with a comparison by said detector means enables or disables supplying power to the second network device, wherein said controller means, in accordance with a comparison by said detector means, performs one of providing an indication, providing an interrupt and setting a bit.

118. A network comprising:
a first network device comprising:
a first transformer:
a second transformer:
a switch;
a power supply in communication with said first and second transformers via said switch;
a physical layer device comprising:
a transmitter in communication with said first transformer;
a receiver in communication with said second transformer;
a signal generator in communication with said transmitter;
a detector in communication with said receiver; and
a controller in communication with said switch, said receiver and said detector;
a cable in communication with said first network device; and


a second network device in communication with said cable,
wherein said signal generator generates a test signal be transmitted by said transmitter; and
wherein said detector selects a threshold in accordance with a length of said cable,
wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal from received by the receiver, controls said switch to enable or disable said power supply, wherein said controller, in accordance with a comparison by said detector of the selected threshold and a received test signal received by the receiver, performs one of providing an indication, providing an interrupt and setting a bit.

119. A network comprising:
first networking means comprising:
first transformer means for transforming a signal:
second transformer means for transforming a signal:
power supply means for supplying power to said first and second transformer means;
switch means for enabling/disabling said power supply means;
physical layer means comprising:
transmitter means for transmitting a signal to said first transformer means;
receiver means for receiving a signal from said second transformer means;
signal generator means for generating a signal to said transmitter means;
detection means for detecting a signal from said receiver means;
controller means for controlling said switch means, said signal generator means and responsive to said detection means;


second networking means; and
cable means for electrically connecting said first and second networking means;

wherein said signal generator means generates a test signal be transmitted by said transmitter means; and
wherein said detection means selects a threshold in accordance with a length of said cable means,
wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal from received by the receiver means, controls said switch means to enable or disable said power supply means, wherein said controller means, in accordance with a comparison by said detection means of the selected threshold and a received test signal received by the receiver means, performs one of providing an indication, providing an interrupt and setting a bit.

120. A method of supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
performing one of providing an indication, providing an interrupt and setting a bit.

121. A computer program stored on a tangible storage medium for supplying power via a cable from a first network device to a second network device in communication therewith, comprising:
(a) transmitting a test signal the second network device;
(b) detecting a signal from the second network device in response to step (a);
(c) selecting a threshold in accordance with a length of the cable;
(d) comparing the signal detected in step (b) with a threshold selected in step (c);
(e) enabling power to be transmitted to the second network device when in accordance with a comparison of step (d); and
performing one of providing an indication, providing an interrupt and setting a bit.