Communication networks provide communicative connections between two or more devices, enabling the devices to exchange information with each other. Information, referred to generically as “data,” may include streaming media data, such as voice or video, and packet data, such as email and text files, to name just a few examples. Communication networks may employ various means to transport data between communicating devices. For example, in the public switched telephone network (PSTN) voice data are typically transported as digitized samples of input from a telephone or similar device, each sample being transmitted as it is generated. As another example, in a packet network, such as Internet Protocol (IP) network (e.g., the Internet), data are assembled into packets that are then transmitted between communicating devices. As a further example, voice data may be transmitted in a packet network as voice over IP (VoIP), wherein sequential groupings of digital samples are transported in IP packets. Other forms of transport are known as well.
The communication path between any two or more devices generally includes one or more logical links and one or more physical links. Logical links may identify endpoints of a connection, for example, and may be established or supported over one or more underlying physical links. For instance, a logical link could comprise a communication path between two computers wherein the path is defined according to endpoints that are specified by symbolic addresses of the computers. The logical link could, in turn, be established over physical connections between two or more network routers or switches. Other arrangements are possible as well.
Physical links correspond to the actual physical connections over which physical signals are transmitted, the physical signals carrying the information that comprise the communicated data. For instance, an analog phone and a PSTN central office (CO) switch may be physically connected by a two-wire loop circuit, in which case information is carried in signals comprising analog electrical signals. As another example, two PSTN switches may be physically connected by an optical link, in which case the signals may comprise digital data transported as optical signals. A further example is the physical connection between two network routers or switches, as mentioned above. These are just a few examples of physical links, and others are possible.
A device that acts to communicatively connect two or more physical links in a network is referred to generally as a network switch. That is, a network switch translates the physical signals input on one link to the output transmitted on another link, in addition to other functions that may be carried out, including determining the output link(s) to which a particular input should be translated. Exemplary network switches include PSTN switches, Ethernet switches, and asynchronous transfer mode (ATM) switches, among others.
In a switch that translates between two or more different types of physical links, the physical translation of an input signal to an output link may give rise to a partial reflection of the input signal back toward the source of that signal, in addition to the intended transmission of the signal on the output link. For example, the circuitry that performs the translation may suffer an impedance mismatch between the different types of links. Such reflected or leaked signal is referred to as echo because the source of the input signal receives a distorted version of the signal echoed back from the switch, the echoed signal being delayed by the round-trip transmission time between the source and the switch. The echoed signal may then combine with any signal transmissions intend for the source, thereby distorting or polluting those transmissions. The portion of the translation path in the switch on which echo is generated is called the echo path.
Echo may be most familiarly encountered in voice telephone calls in circuit-switched networks because of the common use in circuit telephony switches of a two-to-four wire device known as a hybrid that can be a source of echo. And also because when echo is present in a voice call, it is generally obvious and annoying to the party that hears it. However, other communications carried in circuit-switched networks, such as fax transmissions and computer modem transmission, may also be subject to echo. Moreover, echo in communication networks may arise in any network switch in which the characteristics of the physical translation between different physical types of input and output signals cause reflection or leakage. Consequently, echo may affect communications in other types of networks besides circuit-switch telephony networks, for instance. In any type of network, the impact of echo on the effected communications may range from annoying to debilitating.