Tip and ring
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"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side (positive) and Ring is the battery (negative) side of a phone circuit. The ground side is common with the telco's central office; the battery side carries -48 volts of DC voltage when in an "idle" or "on hook" state. The combination of tip and ring, then, makes up a normal phone line circuit, just as a car's battery needs both connections leads to have a complete electrical circuit. To ring the phone to alert to an incoming call, about 90 volts of 20 Hz AC current is superimposed over the DC voltage already present on the idle line.
In the early telephone years of rotary dial, tip and ring wire reversal was of little consequence. Then came DTMF or Touch Tone. Because the tone generator is electronic, tip and ring had to be in the correct order at any given phone jack in order to be able to make outgoing calls with a Touch Tone phone. If they were reversed in polarity, there would still be a dialtone and calls could be received, but not dialed out. In most phones manufactured in the late 20th century, a diode bridge eliminates that problem. Today, tip and ring reversal is mostly immaterial, except for special circuits including DID (Direct Inward Dialing) trunks, T-1 lines, and ground start lines where the field side ("terminal") equipment--a company's PBX switch, for example--can only function correctly with correct tip and ring polarity.
See POTS and PSTN for more details on telephone networks.
[edit] Origin of the term Tip and Ring
The terms Tip and Ring originated from the telephone operators' phone plug, which were used in the early days of telephony with operators handling customer calls. The names of the wires are derived from the part of the plug to which they are connected. Tip and Ring are often abbreviated as T and R in wiring diagrams. However, when so abbreviated, T and R are often mistakenly interpreted as Transmit and Receive.
While the ring is connected to -48 volts, the tip is connected to ground for safety. This is because when the telephone operator is handling the plug, the tip is more likely to touch something other than intended. With the tip as ground, nothing dangerous will happen.
[edit] Negative 48 Volts
The voltage applied to the telephone wires is related to the distance at which the telephone can be operated. Higher voltage can signal to greater distance. High voltage, however, presents a hazard to both customers and technicians. Thus 48 volts is a compromise between safety and distance. Usually central office battery is adjusted to between 50 and 52 volts, but the nominal voltage is still used in documents and designations.
In the middle 20th century, long loops in many rural areas of North America used range extenders, which operated at 100 or 130 volts to ensure reliable signaling. Some rural switching systems were designed to apply range extenders internally, while for other lines the extender was an external applique.
Originally, the voltages were positive. Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage, the copper is protected from corrosion.
[edit] Color code
When simple on-premises wiring is color coded, two-wire telephone plugs or the first pair of a multipair plug commonly have the tip wire coded green and the ring red. In four wire plugs, the second pair are black tip and yellow ring. For a third pair, orange and blue. For larger numbers of wires, more complex schemes are used.
Telco technicians often used the phrase "red-right-ring" to remember that the red wire connects to the right-side post in the wall jack and to the ring on the plug. Sometimes "rough" or "ridge" was added for jumper wires with a tactile code.
