Chapter 5. Diagnostics
181
Channel Bank: Line Loopback
Line loopback is set in response to receiving the “Line Loop Set” code on a span's input, or via
software. Pulse Code Modulation (PCM) arriving at the input is looped back to the opposite
span's output. Figure 5-9 illustrates PCM looping back from span A’s output, as a result of a Line
Loop Set code received by span B’s input. PCM is also passed on to the LIU B input, even
though the LIUs are not in the PCM path. T1 line receivers and line buildouts are kept in the
PCM loop to maintain the proper T1 line levels. Figure 5-9 shows the path of the line loopback
signal.
Figure 5-9. Channel Bank: Line Loopback
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