GE NX-10 Installer Manual page 184

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The GPRS LED indicates that the module is connected to the GPRS network. This
LED turns off when the module is no longer connected to the GPRS network, for
example, when it uses the voice channel or CSD.
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The REP LED indicates that TCP/SMS reporting is taking place.
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The PSU ERR LED indicates that there is a problem with the 3.8 V Switched Mode
Power Supply Unit (the modem power source is too low).
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The GSM LED indicates that the module is using the CSD channel.