Internet telephony, also known as Voice over IP (Internet Protocol), or VoIP, is the routing of voice conversations over the Internet or any other IP network. In VoIP, voice data flows over a general-purpose packet-switched network, instead of the conventional dedicated, circuit switched voice transmission lines used with conventional telephony, also known as plain old telephone service (POTS).
Telephones require electrical power to operate. In POTS, a combined voice signal and power signal is transmitted over twisted pair cable between a telephone and a line card at a public telephone exchange. Unlike POTS, where the voice signal and power signal are combined and require only two wires, Ethernet uses four pairs. In a 10Base-T (10 Mbps) or 100Base-T (100 Mbps) Ethernet system one pair is used for the transmit signal, a second pair carries the receive signal and there are two unused or spare pairs. To provide a similar ease of use for VoIP phones as for conventional phones, configurations have arisen that provide power to a VoIP phone from a power source over an Ethernet connection. Power over Ethernet (PoE), or IEEE standard reference 802.3af, allows the electrical power necessary for the operation of a powered device to be carried by data cables rather than by separate power cords. This minimizes the number of wires that must be used in order to install the network and eliminates the need for AC outlets and AC/DC adapters for each powered device, resulting in lower cost, easier maintenance and greater installation flexibility.
There are two main types of PoE devices: endspan and midspan. An endspan device is generally a network switch that transmits the data signal and provides power. It resides at the end of a link. Endspan devices can provide power in either of two ways: “phantom” feed devices provide power over the active or signal (transmit and receive) wire or line pairs; or power can be sourced on the unused or spare pairs. A midspan device fits in between a switch and a powered device, and can be mounted adjacent to the Ethernet switch in an equipment rack or located near the peripheral end device such as an IP Phone. A midspan device, or power adapter, typically supplies power on the unused wire pairs and simply passes the data signal through without modification—it does not include any transmit or receive functionality. A midspan power adapter is typically a stand-alone device, making it suitable for use with a network switch that does not support PoE. Furthermore, since midspans are less expensive than endspans, midspans are a cost-effective way of adding PoE on a port by port basis to an existing network. As such, IP phones are conventionally powered by a midspan power adapter where adding endspan PoE devices is cost prohibitive.
FIG. 1 illustrates a system 100 in which a conventional IP phone 102 is coupled to a known midspan power adapter 104, which is in turn coupled to a network switch 106. The midspan power adapter 104 includes, or is otherwise connected to or in electrical communication with, a power source 108. The power source 108 in FIG. 1 is connected between device connector wire pairs 4,5 and 7,8. The power source 108 is typically 48 volts DC, and can comprise one or more batteries, or an uninterruptible power supply (UPS). More typically, and as shown in FIG. 1, the power source comprises an AC adapter which can plug into a typical wall outlet. The AC adapter can include a power supply 110, a transformer 112, and a wall plug 114. Connection between the IP phone 102, the midspan power adapter 104, and the network switch 106 is conventionally enabled by cabling 116, such as Category 5, or CAT5, cabling. Different categories of cabling can be used for different equipment, such as 10/100/1000 Mbps Ethernet, also known respectively as 10/100/1000 Base-T, with 1000 Mbps Ethernet also being known as Gigabit Ethernet.
Midspan power adapters typically feed the signal wires directly through a first set of wires, identified as wires 1 to 3 and 6 in FIG. 1. Power is injected on the phone (or powered device) side from the power source 108 via spare pairs, shown as pairs 4,5 and 7,8 in FIG. 1. On the switch side, the spare wires are usually unterminated. Cutting the spare pairs to insert power, and not properly terminating the pairs significantly changes the common mode impedance between each pair within the cable. This discontinuity alters the balance and symmetry of the cable, causing an increase in radiated emissions when a midspan power adapter is used with an IP phone. Thus the powered device may exceed emission limits when in operation. The design intent for powered Ethernet devices, such as IP phones, is to have minimal electromagnetic emissions. Discontinuities such as those encountered in midspan PoE adapters can significantly elevate emission levels.
It is possible to provide power to an end device without breaking or cutting the spare pairs inside the midspan power adapter. However, there is no way of knowing what the spare pairs are connected to at the network switch 106. For example, if a resistive load is attached at the network switch 106, placing power on the spare pair could destroy the load. For this reason, a break in the spare pairs is generally provided in a midspan power adapter. Unfortunately, this break also causes an increase in the amount of radiated energy.
Several methods exist for containing unwanted emissions. One approach is to add a common mode filter clamp on an Ethernet cable to be used with a powered device. This involves customers placing chokes on cables which often does not occur, and makes cable management difficult since the chokes can be bulky. Alternatively, a choke could be moulded onto the cable, but this can be costly and standard replacement cables cannot be used. Another approach is to add common mode chokes within the powered device either as discrete parts or embedded into the Ethernet connectors. A further approach is to use shielded cable throughout any links containing a midspan power adapter. Shielded cable, however, is more expensive and not as commonly used as the Unshielded Twisted Pair Category 5 Ethernet cable specified for use with IP Phones today. Rewiring a building tends to be cost prohibitive.
In general, these approaches merely add extra filtering and cost to each IP phone, rather than fixing the source of the problem. The problem lies in the midspan power adapter and the radiated energy it causes.
It is, therefore, desirable to provide a midspan power adapter that can power an IP device without adding significant radiated energy to the system.