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Interception of low-power radio transmissions often requires field deployed devices. The locations dictated by the enviornment are barely equipped with power and telephone lines and almost never real Internet connections. PC controlled wideband receivers, such as the ICOM PCR1500/2500 devices, are ideal candidates for monitoring applications. Field deployment of PC's is however non-ideal and problematic. Remote monitoring facilities may be considered as a general resource of a parent enforcement agency to be assigned to other organizations on a situational basis. Such sharing of resources can be further improved by the use of today's technology.
The VOCAL Remote Radio Monitoring system supports dial-up access of field deployed radio receivers. Serial or USB communications with the acutal radio is carried in a Telnet-style connection from a central monitoring location to the field deployed radio. Audio is digitized, compressed and packetized using Internet stardards for VoIP. In this case SIP and RTP are used to initiate audio connections and for its delivery. Special radio tones (SELTONE, CTSS, etc) can be detected remotely and delivered as out-of-band indications within the RTP stream.
In an enhanced radio monitoring system, multiple listening posts may receive the radio audio from each location being monitored. In such a system, a radio monitor server communications with all remotely located radios and distributes the audio to each listening station. This configuration may use the dial-up networking connection or a public/private Internet connection (supported by the VOCAL Radio Monitor Ethernet). The figure below illustrates some of the deployments possible with the VOCAL radio monitoring system.
