Direct Current Signaling

This form of signaling relies on direct current (DC) to signal the end switch or office. DC signaling indicates transition state changes by toggling on or off the flow of DC. These end office switches use current detectors to identify changes in state. DC signaling is used in the following two signaling arrangements:

• Subscriber Loop—This is a simple form of DC signaling between the subscriber and the local end office. When a subscriber goes off-hook, DC (-48V) flows across the line or loop between the telephone and the local end office switch. Line cards in the local office are equipped with current detectors to determine when a connection is being requested. When a subscriber goes on-hook, the capacitor in the telephone blocks the flow of current.

Similarly to off-hook, the change in DC signals to the end office switch that the call was terminated. In this case, the same pair of wires is used to provide the voice and signaling path.

• recEive and transMit (E&M)—This trunking arrangement uses a form of DC signaling to indicate state changes on trunks or tie-lines. With E&M, two leads—one called "E" and the other called "M"—are dedicated to signaling. You can detect the toggling of E&M leads by applying either ground (earth) or a voltage potential (magneto). This form of signaling is covered in the "E&M Signaling" section later in this chapter.

DC signaling has some limitations. Signaling is limited to the number of states you can represent by DC, for instance. Also, when you use the same pair of wires for voice and signaling, the lines or trunks are kept busy even when the two subscribers are not connected.

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