111111
`
`1111111111111111111111111111111111111111111111111111111111111111111111111111
`US 20060109728Al
`
`(19) United States
`c12) Patent Application Publication
`Dwelley et al.
`
`(10) Pub. No.: US 2006/0109728 Al
`May 25, 2006
`(43) Pub. Date:
`
`(54) COMMON-MODE DATA TRANSMISSION
`FOR POWER OVER ETHERNET SYSTEM
`
`(22) Filed:
`
`Nov. 19, 2004
`
`(75)
`
`Inventors: David McLean Dwelley, Santa
`Barbara, CA (US); John Arthur
`Stineman JR., Carpinteria, CA (US)
`
`Correspondence Address:
`MCDERMOTT WILL & EMERY LLP
`600 13TH STREET, N.W.
`WASHINGTON, DC 20005-3096 (US)
`
`(73) Assignee: LINEAR TECHNOLOGY CORPO(cid:173)
`RATION
`
`(21) Appl. No.:
`
`10/991,387
`
`Publication Classification
`
`(51)
`
`Int. Cl.
`(2006.01)
`GllC 7100
`(52) U.S. Cl. .............................................................. 365/222
`
`ABSTRACT
`(57)
`Circuitry and methodology for providing data transmission
`in a Power over Ethernet (PoE) system having a Power
`Sourcing Equipment (PSE) for providing power to a PoE
`link, and a Powered Device (PD) coupled to the PoE link for
`receiving the power from the PSE. The PSE and PD support
`data communication with each other in a common mode
`between two pairs of lines in an Ethernet twisted pair cable.
`
`Switch/Hub
`
`18
`
`l.S
`
`

`

`Patent Application Publication May 25, 2006 Sheet 1 of 4
`
`US 2006/0109728 A1
`
`Switch/Hub
`
`18
`
`Switch/Hub
`
`Power
`Sourcing
`Equipment
`(PSE)
`
`12
`
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`
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`
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`FIG. 1
`
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`
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`
`

`

`Patent Application Publication May 25, 2006 Sheet 2 of 4
`
`US 2006/0109728 Al
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`Patent Application Publication May 25, 2006 Sheet 3 of 4
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`Patent Application Publication May 25, 2006 Sheet 4 of 4
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`US 2006/0109728 Al
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`

`

`US 2006/0109728 AI
`
`May 25,2006
`
`1
`
`COMMON-MODE DATA TRANSMISSION FOR
`POWER OVER ETHERNET SYSTEM
`
`TECHNICAL FIELD
`
`[0001] This disclosure relates to power supply systems,
`and more particularly, to a circuitry and methodology for
`providing data transmission in a Power over Ethernet (PoE)
`system in a common mode.
`
`BACKGROUND ART
`
`[0002] Over the years, Ethernet has become the most
`commonly used method for local area networking. The IEEE
`802.3 group, the originator of the Ethernet standard, has
`developed an extension to the standard, known as IEEE
`802.3af, that defines supplying power over Ethernet cabling.
`The IEEE 802.3af standard defines a Power over Ethernet
`(PoE)
`system
`that
`involves delivering power over
`unshielded twisted-pair wiring from Power Sourcing Equip(cid:173)
`ment (PSE) to a Powered Device (PD) located at opposite
`sides of a link. Traditionally, network devices such as IP
`phones, wireless LAN access points, personal computers
`and Web cameras, have required two connections: one to a
`LAN and another to a power supply system. The PoE system
`eliminates the need for additional outlets and wiring to
`supply power to network devices. Instead, power is supplied
`over Ethernet cabling used for data transmission.
`
`[0003] As defined in the IEEE 802.3af standard, PSE and
`PD are non-data entities allowing network devices to supply
`and draw power using the same generic cabling as is used for
`data transmission. A PSE is the equipment electrically
`specified at the point of the physical connection to the
`cabling, that provides the power to a link. APSE is typically
`associated with an Ethernet switch, router, hub or other
`network switching equipment or midspan device. A PD is a
`device that is either drawing power or requesting power. PDs
`may be associated with such devices as digital IP telephones,
`wireless network access points, PDA or notebook computer
`docking stations, cell phone chargers and HVAC thermo(cid:173)
`stats.
`
`[0004] PSE's main functions are to search the link for a
`PD requesting power, optionally classify the PD, supply
`power to the link if a PD is detected, monitor the power on
`the link, and disconnect power when it is no longer
`requested or required. A PD participates in the PD detection
`procedure by presenting a valid or non-valid detection
`signature to request power and indicate that power has been
`received. The PD detection signature has electrical charac(cid:173)
`teristics measured by the PSE.
`
`[0005] Electrical signals can be carried over cables in a
`normal, differential or common mode. A normal-mode sig(cid:173)
`nal appears between a pair of wires or on a single wire
`referenced to (or returned via) the earth, chassis or shield.
`Normal-mode signals are read between two wires in a
`balanced or unbalanced transmission path. A differential(cid:173)
`mode signal appears differentially on a pair of wires in an
`ungrounded cable configuration. A common-mode signal
`appears equally with respect to local common or ground on
`both lines of a 2-wire transmission system.
`
`[0006] Traditional data transmission over Ethernet is car(cid:173)
`ried out between the Ethernet twisted pairs in a differential
`mode, where one pair is used for transmitting data from a
`
`first node to a second node, and the other pair is used for
`receiving data sent from the second node to the first node. In
`a PoE system, power between a PSE and a PD is transmitted
`in a common mode as a voltage between two of the Ethernet
`twisted pairs, typically by powering the center-taps of the
`isolation transformers used to couple the Ethernet data
`signals to the wire. Since Ethernet data are sent differen(cid:173)
`tially, the power transmitted in a common mode between the
`Ethernet transmit pairs and receive pairs does not affect the
`Ethernet data transmitted in a differential mode. A PD
`detection and powering protocol in a PoE system involves
`an analog scheme to sense impedance or current on the
`common-mode connection of the Ethernet twisted pairs.
`
`[0007] Although PSE and PD are non-data entities, which
`are not involved in transmission of Ethernet data, it would
`be desirable in some cases to provide data transmission
`between the PSE and the PD, for example, for identification
`purposes.
`
`SUMMARY OF THE DISCLOSURE
`
`[0008] The present disclosure offers novel circuitry and
`methodology for providing data transmission in a Power
`over Ethernet (PoE) system comprising a Power Sourcing
`Equipment (PSE) for providing power to a PoE link, and a
`Powered Device (PD) coupled to the PoE link for receiving
`the power from the PSE. The PSE and PD are configured for
`supporting data communication with each other in a com(cid:173)
`mon mode between two pairs of lines in an Ethernet twisted
`pair cable.
`
`[0009]
`In accordance with an embodiment of the disclo(cid:173)
`sure, the PD may comprise a variable load for drawing from
`the PSE a first value ofload current in a first operating mode
`and a second value of load current in a second operating
`mode. The first and second values ofload current correspond
`to respective logic levels of a data signal to be transmitted
`to the PSE.
`
`[0010] The PD may further comprise a voltage detector for
`detecting first and second voltage levels corresponding to
`respective logic levels of a data signal transmitted from the
`PSE. The voltage detector may include a comparator for
`comparing a voltage level of a signal received from the PSE
`with a reference voltage to detect a logic level of the data
`signal transmitted from the PSE.
`
`[0011] The PSE may comprise a voltage generator for
`producing first and second voltage levels corresponding to
`respective logic levels of a data signal to be transmitted to
`the PD, and a current detector for detecting a value of the
`load current drawn by the PD.
`
`[0012] The current detector may include a current to
`voltage converter for producing a voltage representing the
`value of current detected by the current detector, and a
`comparator for comparing the produced voltage with a
`reference voltage to determine a logic level of a data signal
`transmitted from the PD.
`
`[0013]
`In accordance with one aspect of the disclosure, a
`protocol for data communication between a PD and a PSE
`in a PoE system involves:
`
`[0014] after receiving power from the PSE, initiation of
`data transmission by the PD, and
`
`

`

`US 2006/0109728 AI
`
`May 25,2006
`
`2
`
`[0015] acknowledgement by the PSE receipt of a data
`signal from the PD.
`[0016] For example, the data transmission may be initiated
`during a low-current period defined by the IEEE 802.3af
`Standard. Duration of the data signal transmitted by the PD
`may be limited by a 250 ms window defined by the IEEE
`802.3af Standard.
`[0017] The PD may initiate data communication with the
`PSE to identify itself. For example, the PD may transmit to
`the PSE data to determine whether the PSE is capable of
`providing power higher than an amount of power required
`by the IEEE 802.3af Standard. The PSE may respond by
`transmitting data confirming ability to provide a higher
`amount of power
`[0018]
`In accordance with a further aspect of the disclo(cid:173)
`sure, a PD interface device is provided for enabling a PD to
`support data communication with a PSE in a PoE system.
`The PD interface device may comprise a variable load for
`transmitting data having logic levels defined by values of PD
`operating current drawn from the PSE, and a voltage detec(cid:173)
`tor for detecting line voltage levels corresponding to logic
`levels of data transmitted by the PSE.
`[0019]
`In accordance with another aspect, aPSE device is
`provided with ability to support data communication with a
`PD in a PoE system. The PSE device may comprise a current
`detector for detecting first and second values of current
`drawn by the PD, and a voltage generator for producing first
`and second line voltage levels corresponding to respective
`logic levels of a data signal to be transmitted to the PD.
`[0020] Additional advantages and aspects of the disclo(cid:173)
`sure will become readily apparent to those skilled in the art
`from the following detailed description, wherein embodi(cid:173)
`ments of the present disclosure are shown and described,
`simply by way of illustration of the best mode contemplated
`for practicing the present disclosure. As will be described,
`the disclosure is capable of other and different embodiments,
`and its several details are susceptible of modification in
`various obvious respects, all without departing from the
`spirit of the disclosure. Accordingly, the drawings and
`description are to be regarded as illustrative in nature, and
`not as limitative.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`[0021] The following detailed description of the embodi(cid:173)
`ments of the present disclosure can best be understood when
`read in conjunction with the following drawings, in which
`the features are not necessarily drawn to scale but rather are
`drawn as to best illustrate the pertinent features, wherein:
`[0022] FIGS. 1-3 illustrate PoE arrangements, in which a
`data transmission system of the present invention may be
`implemented.
`[0023] FIG. 4 illustrates the data communication system
`of the present invention.
`[0024] FIG. 5 illustrates data transmission from a PD to a
`PSE.
`[0025] FIG. 6 illustrates data transmission from the PSE
`to the PD.
`
`DETAILED DISCLOSURE OF THE
`EMBODIMENTS
`[0026] The present disclosure will be made with the
`example of data transmission between PSE and PD in an
`
`IEEE 802.3af-compliant PoE system. It will become appar(cid:173)
`ent, however, that the concepts described herein are appli(cid:173)
`cable to data communications in any system for supplying
`power over a network.
`
`[0027] FIG. 1 illustrates a PoE system, in which the data
`transmission scheme of the present disclosure may be imple(cid:173)
`mented. The PoE system includes a PSE 12 and a PD 14.
`The PSE 12 may be associated with a switch or hub 16,
`whereas the PD 14 is associated with a powered end station
`18 coupled via Ethernet cabling 20 to the switch/hub 16. The
`Ethernet cabling 20 includes four twisted pairs of wire.
`However, 1 OBase-T and 1 OOBase-TX Ethernet systems use
`only two of the twisted pairs for transmission of Ethernet
`data in a differential mode. These twisted pairs are referred
`to as data pairs 22. Two twisted pairs, which are referred to
`as spare pairs 24, remain unused. One of the data pairs 22
`provides transmission of Ethernet data from the switch/hub
`16 to the end station 18, whereas the other data pair 22 is
`used for receiving Ethernet data sent from the end station 18
`to the switch/hub 16. A 1 OOOBase-T Ethernet system uses all
`4 twisted pairs for Ethernet data communications in a
`differential mode.
`
`[0028] The PSE 12 provides a nominal 48V DC between
`the data pairs 22. The power provided from the PSE 12 to the
`PD 14 is applied in a common mode between two data pairs
`22 by powering the center-taps of isolation transformers 26
`used to couple the differential Ethernet data signals to the
`data pairs 22 at the switch/hub side. The PD 14 receives the
`power from the center-taps of isolation transformers 28 used
`to couple the differential Ethernet data signals to the data
`pairs 22 at the end station side.
`
`[0029] FIG. 2 illustrates another example of a PoE system
`in which the data transmission scheme of the present dis(cid:173)
`closure may be implemented. In this example, the power
`provided from the PSE 12 to the PD 14 is applied in a
`common mode as a voltage between two spare pairs 24. The
`data pairs 22 are used for transmitting and receiving Ether(cid:173)
`net data in a differential mode between the switch/hub 16
`and the powered end station 18.
`
`[0030] The examples shown in FIGS. 1 and 2 illustrate
`endpoint PSE arrangements, where a PSE is coincident with
`a Data Terminal Equipment (DTE) or a repeater. Endpoint
`PSEs in the arrangements shown in FIGS. 1 and 2 may be
`compatible with 1 OBase-T, 1 OOBase-TX and 1 OOOBase-T
`Ethernet systems.
`
`[0031] FIG. 3 illustrates a further example of a PoE
`system in which the data transmission scheme of the present
`invention may be implemented. This example shows a
`midspan PSE arrangement, where a PSE is located within a
`link segment distinctly separate from and between the Media
`Dependent Interfaces (MDis). Midspan PSEs can be com(cid:173)
`patible with 1 OBase-T and 1 OOBase-TX Ethernet arrange(cid:173)
`ments.
`
`In the PoE system shown in FIG. 3, the PSE 12 is
`[0032]
`provided in the midspan power insertion equipment 28
`inserted into Ethernet cabling between the switch/hub 16
`and the powered end station 18. The PSE 12 provides power
`to the PD 14 in a common mode by applying voltage
`between two spare pairs 24. The data pairs 22 are used for
`differential-mode Ethernet data communications between
`the switch/hub 16 and the powered end station 18.
`
`

`

`US 2006/0109728 AI
`
`May 25,2006
`
`3
`
`[0033] As defined in the IEEE 802.3af standard, the PSE
`12 and PD 14 participate in the PD detection procedure,
`during which the PSE 12 probes a link to detect a PD. If the
`PD is detected, the PSE 12 checks the PD detection signa(cid:173)
`ture to determine whether it is valid or non-valid. The valid
`and non-valid detection signatures are defined in the IEEE
`802.3af standard. While the valid PD detection signature
`indicates that the PD is in a state where it will accept power,
`the non-valid PD detection signature indicates that the PD is
`in a state where it will not accept power.
`
`If the signature is valid, the PD has an option of
`[0034]
`presenting a classification signature to the PSE to indicate
`how much power it will draw when powered up. A PD may
`be classified as class 0 to class 4. A PD of class 1 requires
`that the PSE supplies at least 4.0 W, a PD of class 2 requires
`that the PSE supplies at least 7.0 W, and a PD of class 0, 3
`or 4 requires at least 15.4 W.
`
`[0035] Based on the determined class of the PD, the PSE
`applies the required power to the PD. When the PD has
`completed detection procedure and received power, it is
`required by the IEEE 802.3af protocol to draw a minimum
`current of 10 rnA for at least 7 5 ms, followed by an optional
`low-current period of up to 250 ms. The low-current period
`allows the PD to conserve power. Therefore, the PSE waits
`at least 250 ms before recognizing the minimum current
`condition of the PD.
`
`In accordance with the present disclosure, this 250
`[0036]
`ms window is utilized for providing common-mode data
`transmission between the PSE 12 and the PD 14. The
`common-mode data transmission involves an information
`exchange between the PSE 12 and PD 14 over the common(cid:173)
`mode path used for power transmission in a PoE system.
`
`[0037] Hence, in the PoE arrangement shown in FIG. 1,
`the data transmission between the PSE 12 and PD 14 may be
`carried out in a common mode over two twisted data pairs
`22. In the PoE arrangements shown in FIGS. 2 and 3, the
`data transmission between the PSE 12 and PD 14 may be
`performed in a common mode over two twisted spare pairs
`24. The common-mode data transmission of the present
`invention does not interfere with transmission of Ethernet
`data performed in a differential mode, and does not affect the
`802.3 af power signaling protocols.
`
`[0038] As shown in FIG. 4, a common-mode data trans(cid:173)
`mission system of the present invention includes a line
`voltage generator 122 and a PD load detector 124 associated
`with the PSE 12, and a voltage detector 142 and a variable
`load 144 associated with the PD 14. The line voltage
`generator 122 enables the PSE 12 to produce two distinctive
`values of a line voltage representing first and second logic
`levels of data to be transmitted to the PD 14. In the PoE
`arrangement shown in FIG. 1, the line voltage is applied
`between the center-taps of the isolation transformers 26 used
`to couple the differential Ethernet data signals to the data
`pairs 22 at the switch/hub side. On the PD side, the voltage
`detector 142 determines the line voltage provided between
`the center-taps of the isolation transformers 28 to detect the
`respective logic levels of the data received from the PSE 12.
`In the PoE arrangements shown in FIGS. 2 and 3, the line
`voltage produced by the line voltage generator 122 is
`applied in a common mode between the spare pairs 24 at the
`PSE side, and is detected by the voltage detector 142
`between the spare pairs 24 at the PD side.
`
`[0039] The variable load 144 enables the PD 14 to draw
`from the PSE 12 two distinct values of a load current
`representing first and second logic levels of data to be
`transmitted to the PSE 12. The PD load detector 124 allows
`the PSE 12 to measure the PD load current in order to detect
`the respective logic levels of the data received from the PD
`14. In the PoE arrangement shown in FIG. 1, a path for the
`PD load current is provided in a common mode between the
`data pairs 22, whereas in the PoE arrangements shown in
`FIGS. 2 and 3, the PD load current path is provided in a
`common mode between the spare pairs 24.
`
`In accordance with a common-mode data transmis(cid:173)
`[0040]
`sion protocol of the present disclosure, an information
`exchange may be initiated by the PD 14 that transmits
`desired information to the PSE 12. For example, the PD 14
`that has just received power may transmit a 64-bit serial
`number to the PSE 12 to identifY itself. To provide the data
`transmission, the PD 12 may modulate its load current above
`and below a threshold level.
`
`[0041] For instance, as discussed above, the PD 14 is
`required by the IEEE 802.3af protocol to draw a minimum
`current of 10 rnA for at least 75 ms, followed by a low(cid:173)
`current period of up to 250 ms. Therefore, the PD 14 may
`draw a load current greater than 10 rnA for 75 ms to satisfy
`the IEEE 802.3af requirement, and then alternately draw a
`load current either greater than 10 rnA or less than 10 rnA in
`a pattern that corresponds to the digital data to be transmitted
`to the PSE 12. For example, a load current greater than 10
`rnA may correspond to a logic "1" and a load current less
`than 5 rnA may correspond to a logic "0".
`
`[0042] By modulating its quiescent current above and
`below the 10 rnA threshold level during the 250 ms low(cid:173)
`current window, the PD 14 may transmit a serial data stream
`to the PSE 12. The number of bits transmitted in the 250 ms
`window is proportional to the data rate. The upper limit of
`the transmitted bits represents the carrying capacity of the
`common-mode data transmission channel. For example, at a
`300 baud data rate, up to 75 bits may be transmitted in the
`250 ms window.
`
`In accordance with an alternative embodiment of
`[0043]
`the invention, the PD 14 may transmit data during a window
`defined by an overload time limit T ovLD established by the
`IEEE 802.3af standard, which specifies that if the output
`current I Port of a PSE exceeds an overload current detection
`range IcuT for longer than the overload time limit T ovLD•
`the PSE should remove power from a Power Interface (PI).
`The minimum value of the overload time limit is 50 ms.
`During this time period, the PD 14 may transmit data by
`modulating the load current above or below IcuT·
`
`[0044] The PSE 12 may acknowledge receipt of the data
`from the PD by briefly dropping the line voltage to a level
`lower than the nominal 48\1, and then restoring the voltage
`to its nominal level. Then, the PSE 12 may carry out the
`transmission of data to the PD 12 by alternately providing a
`line voltage at two distinct levels. Unlike the data transmis(cid:173)
`sion from the PD 14, where the number of transmitted bits
`is limited by the 250 ms window, the PSE 12 may transmit
`data for an indefinite period of time.
`
`[0045] For example, after providing power to the PD 14,
`the PSE 12 may maintain a line voltage above the 44V
`minimum level required by the IEEE 802.3af standard. After
`
`

`

`US 2006/0109728 AI
`
`May 25,2006
`
`4
`
`receiving data transmitted from the PD 14, the PSE 12 may
`modulate the line voltage between 48V and 38V to represent
`a logic "1" and a logic "0" of data to be transmitted to the
`PD 14.
`
`IEEE 802.3af-compliant PDs are required to stay
`[0046]
`operating until the line voltage is reduced to at least 37V to
`allow for voltage drop in the interconnection cable when the
`PD is drawing the maximum current. If during the data
`transmission from the PSE, the PD keeps its load current
`low, for example, at a 10 rnA level, the voltage drop across
`the cable will be minimal compared to the voltage drop at
`the maximum 350 rnA load current level. As a result, the
`38V line voltage provided by the PSE to represent the logic
`"0" will remain above the 37V maximum turn-offvoltage at
`the PD 14.
`
`[0047] Therefore, the PD 14 must keep its load current at
`a low level during the data transmission from the PSE 12. If
`a PD draws a large amount of power in a normal operation
`(for example, in the case of a wireless access point that
`includes a high-power radio transmitter), it may need to
`delay turning on the majority of its circuitry until the data
`transmission is complete.
`
`[0048] The common-mode data transmission protocol of
`the present invention may be used, for example, by a
`high-power PSE capable of providing higher power than the
`IEEE 802.3af standard requires. Using data transmission to
`a PSE, the PD may identify itself as a device requesting
`higher power than a regular PSE is required to provide. If the
`PSE receiving the PD data is a high-power PSE, it replies to
`the PD data by sending data acknowledging the request for
`higher power, and provides the power requested by the PD.
`
`If a PD that uses the common-mode data transmis(cid:173)
`[0049]
`sion protocol is connected to a legacy PSE that does not
`understand the common-mode data transmission, the legacy
`PSE continues to provide power to the PD but does not
`respond to the data transmitted by the PD. In this case, the
`PD recognizes that the PSE does not support the common(cid:173)
`mode data transmission protocol, and modifies its operation,
`if appropriate, to work with a legacy PSE.
`
`[0050] Similarly, if a PSE that incorporates the common(cid:173)
`mode data transmission circuitry applies power to a PD and
`does not receive any data from the PD, the PSE recognizes
`that the respective PD is a legacy PD, which does not
`support the common-mode data transmission protocol, and
`that the PD may immediately begin to draw full load current,
`which will disable the PSE transmission protocol. In this
`case, the PSE refrains from transmitting data to the PD. As
`long as the PSE does not lower the line voltage below the
`44V minimum level, the PD is able to continue its normal
`operations.
`
`[0051] FIGS. 5 and 6 illustrate exemplary implementa(cid:173)
`tion of the common-mode data transmission system of the
`present disclosure. As shown in FIG. 5, when the PD 14 has
`to transmit data to the PSE 12, the transmit data supplied by
`a PD controller 140 controls the variable load 144 supplied
`by the 48V voltage provided by the PSE 12 to enable the
`operation of the variable load 144 in two modes. The
`controller 140 may be a data processing unit external or
`internal with respect to the PD 14.
`
`[0052] For example, in response to a logic "0" in a data
`sequence to be transmitted to the PSE 12, the variable load
`
`144 operates in a first operating mode, where its quiescent
`load current Iq is maintained at a level less than 5 rnA. In
`response to a logic "1" in the data sequence to be transmitted
`to the PSE 12, the variable load 144 is switched by a switch
`S1 into a second operating mode, where an additional load,
`for example, a resistor, is connected across the line, in order
`to draw an additional load current Ib from the PSE 12. In the
`second operating mode, a total asserted load current Iq+Ib
`may be at a level higher than 10 rnA.
`[0053] The PD load current detector 124 in the PSE 12
`may comprise a resistor R1 and a comparator 126. The
`resistor R1 is arranged for converting the load current
`asserted by the PD 14 to a voltage representing the load
`current value. The voltage across the resistor R1 is compared
`by the comparator 126 with a reference voltage vref.l to
`detect a logic level of the data received from the PD 14. The
`value of the reference voltage Vref.1 and resistor R1 are
`pre-selected to distinguish the logic levels of data received
`from the PD 14. For example, R1=0.5 Ohm, and Vref.l =5
`m V. The received data may be supplied to a PSE controller
`130, which may be a data processing unit external or internal
`with respect to the PSE 12.
`[0054] As shown in FIG. 6, when the PSE 12 transmits
`data to the PD 14, the PSE controller 130 supplying transmit
`data controls a switch S2 to switch the line voltage generator
`122 between two operating modes. For example, in response
`to a logic "0" in the transmit data, the line voltage generator
`122 operates in a first operating mode to produce a 38V line
`voltage. In response to a logic "1", the line voltage generator
`122 is switched into a second operating mode, where an
`additional 1 OV voltage is connected to its output to produce
`a 48V line voltage.
`[0055] The voltage detector 142 in the PD 14 may com(cid:173)
`prise a comparator 146, and a voltage divider composed of
`resistors R2 and R3 selected to divide the supplied line
`voltage by a predetermined value. The comparator 146
`compares the voltage at the output of the voltage divider
`with a pre-determined reference voltage vref.2 to detect a
`logic level of data received from the PSE 12. The value of
`the reference voltage Vref.l and resistors R2 and R3 are
`pre-selected to distinguish the logic levels of the data
`received from the PSE 12. For example, R2=35 kOhm,
`R3=1 kOm and Vref_2=1.2V. The received data may be
`supplied to the PD controller 140.
`[0056] The foregoing description illustrates and describes
`aspects of the present invention. Additionally, the disclosure
`shows and describes only preferred embodiments, but as
`aforementioned, it is to be understood that the invention is
`capable of use in various other combinations, modifications,
`and environments and is capable of changes or modifications
`within the scope of the inventive concept as expressed
`herein, commensurate with the above teachings, and/or the
`skill or knowledge of the relevant art.
`[0057] The embodiments described hereinabove are fur(cid:173)
`ther intended to explain best modes known of practicing the
`invention and to enable others skilled in the art to utilize the
`invention in such, or other, embodiments and with the
`various modifications required by the particular applications
`or uses of the invention.
`[0058] Accordingly, the description is not intended to limit
`the invention to the form disclosed herein. Also, it is
`intended that the appended claims be construed to include
`alternative embodiments.
`
`

`

`US 2006/0109728 AI
`
`May 25,2006
`
`5
`
`What is claimed is:
`1. A Power over Ethernet (PoE) system, comprising:
`
`a Power Sourcing Equipment (PSE) for providing power
`to a PoE link, and
`
`a Powered Device (PD) coupled to the PoE link for
`receiving the power from the PSE,
`
`the PSE having data communication circuitry configured
`for providing data communication with the PD.
`2. The system of claim 1, wherein the PD is configured for
`supporting data communication with the PSE.
`3. The system of claim 2, wherein data communication
`between the PSE and PD is provided in a common mode
`between two pairs of lines in an Ethernet twisted pair cable.
`4. The system of claim 3, wherein the PSE provides power
`to the PD in the common mode between the two pairs of
`lines in the Ethernet twisted pair cable.
`5. The system of claim 4, wherein an Ethernet system
`associated with the PoE system provides data transmission
`differentially via the two pairs of lines in the Ethernet
`twisted pair cable.
`6. The system of claim 2, wherein the PD is configured to
`initiate data communication with the PSE after the PD
`receives power from the PSE.
`7. The system of claim 6, wherein the PD is configured to
`initiate data communication with the PSE during a low(cid:173)
`current period defined by the IEEE 802.3af Standard.
`8. The system of claim 2, wherein the PD is configured to
`initiate data communication with the PSE to identify itself.
`9. The system of claim 2, wherein the PD is configured to
`transmit to the PSE data to determine whether the PSE is
`capable of providing power higher than an amount of power
`required by the IEEE 802.3af Standard.
`10. The system of claim 9, wherein the PSE is configured
`to transmit to the PD data confirming ability to provide
`power higher than an amount of power required by the IEEE
`802.3af Standard.
`11. The system of claim 2, wherein the PD comprises a
`variable load for drawing from the PSE a first value of
`current in a first operating mode and a second value of
`current in a second operating mode, the first and second
`values of current correspond to respective logic levels of a
`data signal to be transmitted to the PSE.
`12. The system of claim 11, wherein the PD further
`comprising a voltage detector for detecting first and second
`voltage levels corresponding to respective logic levels of a
`data signal transmitted from the PSE.
`13. The system of claim 12, wherein the voltage detector
`includes a comparator for comparing a voltage level of a
`signal received from the PSE with a reference voltage to
`detect a logic level of the data signal transmitted from the
`PSE.
`14. The system of claim 2, wherein the PSE comprises a
`voltage generator for producing first and second voltage
`levels corresponding to respective logic levels of a data
`signal to be transmitted to the PD.
`15. The system of claim 14, wherein the PSE further
`comprises a current detector for detecting a value of curren