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Line signaling
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<p>Line signaling is a class of <a href="/facts/Telecommunications_signaling/6d4zgUj7">telecommunications signaling</a> <a href="/facts/Communications_protocol/5uady7cB">protocols</a>. Line signaling is responsible for <a href="/facts/Off-hook/WMN8tfTt">off-hook</a>, <a href="/facts/Ringing_signal/peCvzFn5">ringing signal</a>, answer, <a href="/facts/Ground_start/7Dzw5k6a">ground start</a>, <a href="/facts/On-hook/WMN8tfTt">on-hook</a> unidirectional supervision messaging in each direction from <a href="/facts/Calling_party/Dcw0V4hJ">calling party</a> to <a href="/facts/Called_party/TxXCTuQT">called party</a> and vice versa. After an off-hook, line signaling initiates <a href="/facts/Register_signaling/8G9w7A3j">register signaling</a> to accomplish the exchange of <a href="/facts/Telephone_number/Px4q5qFo">telephone numbers</a> of called party and in more modern line-signaling protocols, the calling party as well. While register signaling occurs, line signaling remains quiescent unless the calling party goes on-hook or an abnormal cessation of the call occurs, such as due to equipment malfunction or shutdown or due to <a href="/facts/Telecommunications_network/NFCX0STS">network</a> outage upstream in that call-attempt's series of spanned trunks. </p><p>Line signaling can be conveyed in a single <a href="/facts/DS0/wYRTr8Rl">DS0</a> channel of a <a href="/facts/Trunking/YazQ6Stv">trunk</a>. In modern <a href="/facts/Pulse-code_modulation/5gdfjtH3">PCM</a> telecommunications, line signaling is represented by the ABCD bits in DS0 #16 of an <a href="/facts/E-carrier/h0MXcgXN">E1</a> or a selected DS0 within a <a href="/facts/Digital_Signal_1/cghJPRpQ">T1</a>. </p><p>Line signaling can also be conveyed within the channels being supervised, as in the original T1 scheme which used one out of every eight bits for supervision, or with <a href="/facts/Robbed-bit_signaling/BaGvyf9r">robbed bits</a> of a <a href="/facts/Superframe/jUj3oBYv">superframe</a> format, or as <a href="/facts/In-band_signaling/KAgeV09X">in-band signaling</a>, e.g. as in L1 signaling where 2280 Hz tone pulses are used. </p>