What is a four wire circuit?

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A four-wire circuit is defined as two pairs of two communication wires that allow the transmission of signals in both directions at the same time. Simultaneous communication is called a full-duplex system, where two people can speak and be heard at the same time. Many residential and commercial data lines use these circuits to optimize data transmissions. An electrical circuit to supply 120 and 240 volt power to an appliance using two wires carrying power or current, as well as a neutral and ground, is also called a four-wire circuit.

Normal home telephone equipment uses two-wire circuits for voice calls. Conversations can take place in both directions, but typically only a few phones can be connected to a single two-wire line. Data communications would be limited on this type of circuit.

Adding two more wires to create a four-wire circuit allows the system to send data to a computer or modem in one direction with two wires and receive data from the device with the other two wires. Data speeds are greatly improved with less errors and signal loss. This system is called a full-duplex line, common since the 1980s for data systems such as Integrated System Digital Networks (ISDN). As the demand for data speeds increased, digital systems expanded to digital Ethernet systems, which is a pair of four-wire, or eight-wire, circuits that can handle both voice and data communications.

A digital data system that can operate on a two-wire line is the Digital Subscriber Line (DSL). This system uses voice and data communication over a single pair of wires, transmitting two different frequencies. Typically, a filter is installed on any phone to prevent distortion of the higher frequency data signal. Due to signal losses over distances, DSL may not be available in areas that are too far from the telephone company’s transmission equipment.

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Until the end of the 20th century, communication systems were built with copper wire coated with non-conductive insulation. As the Internet grew and data demands increased, networks became increasingly limited and transmission speeds compromised. Network companies began converting wired networks first into coaxial cables and then into optical fibers capable of carrying more data using laser light transmitted through fiberglass. Home and business users can subscribe to Internet service from different companies and bring cable or fiber optic technology directly into the building.

The four-wire circuit was replaced by cable and fiber optic systems to transmit data to users. Inside the building, however, the four-wire circuit in single or multiple circuits continued to be widely used in the 21st century. Wireless data networks can be used, but companies or individuals concerned about data security may still choose a wired network to minimize the risk of data theft.

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