A four-wire circuit is defined as two pairs of two communications wires that permit 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 home and business data lines use these circuits to optimize data transmissions. An electric circuit to provide both 120- and 240-volt power to an appliance using two hot or current-carrying wires, as well as a neutral and ground, is also called a four-wire circuit.
Normal home phone equipment uses two-wire circuits for voice calls. Conversations can take place in both directions, but normally only a few phones may be hooked up to a single two-wire line. Data communications would be limited in 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 speed increased, digital systems expanded to Ethernet digital systems, which is a four-wire circuit pair, or eight wires, that can handle voice and data communications.
A digital data system that can operate on a two-wire line is a Digital Subscriber Line (DSL). This system uses voice and data communication over a single wire pair by transmitting two different frequencies. Normally a filter is installed on any telephone to prevent distortion from the higher frequency data signal. Due to signal losses over distances, DSL may not be available in areas that are too far away from telephone company transmitting equipment.
Until the late 20th century, communications systems were built with copper wires sheathed in non-conductive insulation. As the Internet grew and data demands increased, networks were increasingly limited and transmission speeds were compromised. Network companies began converting wire networks first to co-axial cable, then to fiber optics capable of carrying more data using laser light transmitted through glass fiber. Home and business users could 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 into 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.