Glenayre Paging Link
Paging Modes: AFSK Paging Link - POCSAG - Glenayre Paging Link - Motorola PURC Tone Remote Control - FLEX - ReFLEX - Golay Paging (GSC) - ERMES
Glenayre's Paging Link. A way to upkeep a linked paging transmitter when no data is broadcast. Glenayre C2100/C2000 model control systems were likely to use this. Some transmitters in the United States still use this.
Transmitters may transmit modes such as V.29 QAMQuadrature Amplitude Modulation for this reason, if heard on an uplink/downlink frequency. Some may even utilize this mode with no data at all, thus being constant QAMQuadrature Amplitude Modulation noise. It also lets the receiver end know that the signal (or license) is "there" (hopefully somewhere, on the band) for other uses (either by a Morse ID, frequency/modulation, etc).
HDSDR/sdr# will help you with these IQs.
A different example from Indiana Paging Network from an AFSKAudio Frequency-Shift Keying POCSAG transmitter of theirs. Transmitter waits 10-20 seconds before the noise is transmitted:
Another example, except with constant v.29 noise on 454.425 in Chicago, IL. Has a crappy sounding Morse ID as the txmitter is from 1998.