Transmission impedance for four-pair midspan powering in a power over ethernet application

A midspan power sourcing equipment (PSE) for operation with power over Ethernet (PoE). The midspan PSE provides powering over wire pairs that are also used for data communication. To ensure compatibility with legacy Ethernet devices, the ports used for transmission of data are designed with a low effective impedance at a frequency of operation.

BACKGROUND

1. Field of the Invention

The present invention relates generally to Power over Ethernet (PoE) and, more particularly, to transmission impedance for midspan powering in a PoE application.

The IEEE 802.3af and 802.3at PoE specifications provide a framework for delivery of power from power sourcing equipment (PSE) to a powered device (PD) over Ethernet cabling. In this framework, various PDs can be deployed such as voice over IP (VoIP) phones, wireless LAN access points, network cameras, computing devices, etc.

In the PoE process, a valid device detection is first performed. This detection process identifies whether or not it is connected to a valid device to ensure that power is not applied to non-PoE capable devices. After a valid PD is discovered, the PSE can optionally perform a Layer 1 power classification.

In one configuration, power can be sourced directly from data terminal equipment such as an Ethernet switch. This type of network device is referred to as an endspan device. In another configuration, power can be supplied by a midspan device.FIG. 1illustrates such a midspan configuration, which includes a medium dependent interface (MDI) link between Ethernet switch110and PD130. Midspan configurations can be used for various systems such as 100BASE-TX and 1000BASE-T systems. As illustrated, midspan PSE120is placed in the middle of the MDI link for the insertion of power to be supplied to PD130. An advantage of such a midspan configuration is the usage of legacy Ethernet switches that do not have PoE capabilities.

In one midspan configuration that supports 100BASE-TX, data is transmitted using pins1(TX+),2(TX−),3(RX+), and6(RX−) of the Ethernet cable. The remaining pins4,5,7, and8, which are representative of the two unused pairs of the Ethernet cable, can be used by midspan PSE120for the transmission of power to PD130. In an alternative midspan configuration, power is not transmitted on the two unused pairs, but transmitted on the two signal pairs used for data transmission.

Midspan PSE design guidelines have not been tightly specified beyond a general recognition that the cabling channel performance should be maintained by the midspan PSE when it is inserted into an MDI link. What is needed therefore is a mechanism that ensures that a midspan PSE maintains proper compatibility with various network elements such as legacy Ethernet devices.

SUMMARY

Transmission impedance for midspan powering in a PoE application, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.

DETAILED DESCRIPTION

As noted, one type of midspan PoE configuration can be designed to use the two unused wire pairs for the transmission of power to the PD (referred to as Alternative B).FIG. 2illustrates an example of such a midspan configuration. As illustrated, the two unused wire pairs are terminated at midspan equipment220. The remainder of the link of the two unused wire pairs is then used to transmit power from PSE222to PD230. As power is inserted onto the link by midspan equipment220, Ethernet switch210need not have PoE capabilities. Ethernet switch210can therefore represent legacy Ethernet equipment. In general, a PD is capable of receiving power from either wire pair in 802.3af and two-pair powering in 802.3at.

In a four-pair Ethernet cable, two signal wire pairs can be used for data transmission. In one implementation such as 100BASE-TX, pins1(TX+) and2(TX−) are used for the transmission (TX) portion of the link, while pins3(RX+) and6(RX−) are used for the receiving (RX) portion of the link. Pins4,5,7, and8represent the two unused wire pairs, which are not used in the portion of the link between Ethernet switch210and midspan equipment220.

At midspan equipment220, power is inserted using PSE222. Specifically, PSE222provides power to PD230using the two unused wire pairs (pins4,5,7, and8), which are terminated at midspan equipment220. The power carried on the two unused wire pairs is then extracted at PD230to power load232.

As further illustrated inFIG. 2, the two signal wire pairs used for data transmission (pins1,2,3, and6) are passed through midspan equipment220onto PD230. This pass-through connection can be implemented as an undisrupted channel connection between Ethernet switch210and PD230. In other words, the characteristics of the wired channel connection between Ethernet switch210and PD230can be left largely undisturbed as the data transmissions pass through midspan equipment220.

FIG. 3illustrates an alternative midspan powering configuration that uses the two data transmission signal wire pairs for powering, instead of the two unused wire pairs. Thus, in the context of the example ofFIG. 2, the two signal wire pairs represented by pins1,2,3, and6can be used for both data transmission and powering. In this alternative midspan powering scheme, the channel connection between Ethernet switch310and PD330is disturbed to accommodate the insertion of power by PSE326.

Consider, for example, the signal wire pair represented by pins1and2. In this signal wire pair, data is transmitted from Ethernet switch310and received at midspan equipment320. After passing through transmit/receive (TX/RX) coupler322, the data is then transmitted to PD330. Similarly, for the signal wire pair represented by pins3and6, data that is transmitted by PD330is received at midspan equipment320. After passing through TX/RX coupler324, the data is then transmitted on to Ethernet switch310. In one conventional midspan equipment example, the TX/RX couplers are embodied using a magnetic element such as a transformer.

The disruption in the channel connection between Ethernet switch310and PD330allows for the application of DC power on a side of the TX/RX coupler that is isolated from the other side. The transmitted power can then be extracted at PD330through the taps of the corresponding transformers in PD330. The extracted power is then used to drive load332.

One of the consequences of having a disrupted channel connection between Ethernet switch310and PD330is the impact on the end-to-end channel connection. In a non-disrupted channel connection, only a single transformer pair is used at the two ends of the link (i.e., Ethernet switch310and PD330). When a disruption occurs at the midspan equipment, two separate segments in the link are created. On the transmit side, a first segment is represented by the transmission transformer in switch310and a first side of TX/RX coupler322, and a second segment is represented by the second side of TX/RX coupler322and the receiving transformer in PD330.

In the present invention, it is recognized that the conventional inclusion of a transformer as TX/RX couplers in midspan equipment320can lead to mismatches between a transmitter and receiver. For example, consider the inductance requirement that is specified by the 100BASE-TX specification for the transmit side. As the 100BASE-TX specification sets forth, the minimum inductance measured at the transmit pins shall be greater than or equal to 350 μH with any DC bias current between 0-8 mA injected. As would be appreciated, other inductance ranges (higher or lower) can also be specified for a given application. For example, a higher inductance range can be specified to accommodate higher DC bias currents that would be expected in an application such as 802.3at. In general, the specification of a minimal inductance in a transformer on the transmit side is designed to ensure compatibility with PHY receivers (e.g., 100BASE-TX) that expect to see the effective inductance on the channel such that the signal quality and BER of the link is not impacted.

In an undisrupted channel connection between Ethernet switch310and PD330, this requirement can be met by the inclusion of a transmitter transformer at Ethernet switch310that presents at least 350 μH at the transmit pins. This minimum inductance level can be specified to present sufficient inductance that would overcome killer patterns that cause baseline wander so that the receiver can recover. In other words, the inductor at the source slows down the edges.

For channel connections that are disrupted at midspan equipment320, however, this transmitter transformer inductance characteristic would not apply to far end PD330. Rather, the performance experienced by the receiver in far end PD330would be dictated by the characteristics of the side of TX/RX coupler322that was facing PD330. If the PD side of TX/RX coupler322is not properly designed, then the bit error rate (BER) can increase to unacceptable levels due to baseline wander. This issue is especially problematic when dealing with legacy equipment that does not have compensated physical layer devices (PHYs) at the receiving end.

It is therefore a feature of the present invention that the midspan equipment is designed to ensure compatibility with all legacy equipment whether or not power is inserted by the midspan equipment onto the two data transmission signal wire pairs. In one embodiment, this compatibility is enabled through the inclusion of a TX/RX coupler having a low effective impedance at a frequency range of interest (e.g., operation frequency for 100BASE-TX). By including such a TX/RX coupler design at the midspan equipment, powering on the signal wire pairs would present a sufficiently low impedance at the frequency range of operation, thereby preventing issues such as baseline wander. As would be appreciated, this effect would be felt regardless of the level of inductance in the transformer at the switch.

FIG. 4illustrates a simple example embodiment of a TX/RX coupler according to the present invention. As illustrated, TX/RX coupler400includes blocking capacitors C1, C2that enables DC isolation between the two sides of TX/RX coupler400. On the PD side of TX/RX coupler400, elements E1, E2are designed to create a low impedance effect at the frequency range of interest. As would be appreciated, the particular inclusion of resistive, capacitive, and/or inductive components in elements E1, E2would be implementation dependent. Here, it is significant that the PD side of TX/RX coupler400would appear as the far end of the link to a legacy receiving PHY at the PD. In general, the midspan equipment design on the transmit side is critical for ensuring proper compatibility with legacy equipment.

The midspan equipment design of the present invention enables the support of legacy PHYs on either end of the link. The legacy PHY can be included in either the switch or the PD. As such, the inclusion of a midspan TX/RX coupler having a low effective impedance at a frequency of operation enables the support of legacy Ethernet switches and PDs. In general, the principles of the present invention places an increased design constraint on midspan equipment. This is in contrast to conventional midspan equipment designs that are designed to insert power onto the two unused wire pairs. For those applications, no requirement for the transmitter transformers would be necessary as data communication and powering would occur on separate sets of wire pairs.

FIG. 5illustrates another midspan embodiment that can benefit from the principles of the present invention. In this embodiment, the midspan equipment is designed to inject power into a link that uses all four wire pairs for data transmission (e.g., 1000BASE-T). As illustrated, four TX/RX couplers521-524are included in midspan equipment520. These four TX/RX couplers521-524can be designed to replace four data transformers, thereby generating significant cost savings. While the power connections are not shown, PSE525can be designed to inject power into one or both (e.g., 802.3at) of the two wire pair groupings. Specifically, power can be injected through TX/RX couplers521,522and/or through TX/RX couplers523,524.

As would be appreciated, the principles of the present invention would apply regardless of whether power was applied to the signal wire pairs. In one scenario, the midspan equipment design can also support non-PD devices that have legacy PHYs. These non-PD devices would not receive power since they would not be discovered as valid PD devices.FIG. 6illustrates an example of such a scenario where PDs632,634and non-PD636are supported by midspan PSE620. Here, each of PDs632,634and non-PD636can be a link partner that is attached to midspan PSE620and switch610. If a TX/RX coupler in midspan PSE620is designed with a low effective impedance at the frequency of operation, then midspan PSE620can support a legacy PHY in non-PD636, regardless of whether Ethernet switch610could present an appropriate inductance level.

Finally, the principles of the present invention can also be applied to medium dependent interface crossover (MDIX) ports.

These and other aspects of the present invention will become apparent to those skilled in the art by a review of the preceding detailed description. Although a number of salient features of the present invention have been described above, the invention is capable of other embodiments and of being practiced and carried out in various ways that would be apparent to one of ordinary skill in the art after reading the disclosed invention, therefore the above description should not be considered to be exclusive of these other embodiments. Also, it is to be understood that the phraseology and terminology employed herein are for the purposes of description and should not be regarded as limiting.