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Telephones Circuits, also referred to as telephone lines are physically wired links between the telephone and the service provider's local exchange. Historically, this was done with a pair of conductors called a local loop or last mile over what commonly referred to as the plain old telephone system (POTS). The two conductors are called tip and ring in reference to their location on a phone jack in the old days when connections were made manually on switchboards. Today such connections are improved and advanced using more digital loop carrier (DLC) or even fiber optic transmission.

 

The ring conductor has a direct current potential of approximately -48 Volts direct current (DC), relative to the tip conductor when on the hook, with no reference to ground to prevent hum from nearby noise sources like the main power conductors.  When the phone rings, approximately 90 Volts of 10-40Hz alternating current (AC) is present on the line. 

 

Modern telephone circuits carry audio and broadband data services. This has led to the integration of the DLC system with digital subscriber line (DSL) in a single interface called a digital subscriber line access multiplexer (DSLAM).  Optical Fiber is also used to connect telephone subscribers. Fiber in loop (FITL) performs the same function as DLC and can extend to the curb, or directly into each subscriber's premises.

 

Analog telephone networks are still widely used but in many countries the digital services that bypass the last mile of wired connection are replacing it. Wired analog systems are expensive for phone companies to maintain, and as the predominant user base is now made up of the elderly and people in rural areas, companies are investing in more modern alternatives. Voice over Internet protocol (VoIP) and the bundling of services by telecommunication companies over fiber cable, cellular and even satellite networks provide cheaper higher quality options.

 

Many people today have chosen to move away from wired telephone circuits and use cellphones and wireless broadband services, bypassing the wired connection completely. As few as 20% of households in the US still have land line telephone connections! Disruptive innovations like '4G' and the future ‘5G’ wireless mobile networks (predicted to be capable of data rates up to 1Gbit/s) will further erode the concept of a telephone circuit.