JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Data Systems, CDPD

Channel Sniffing and Channel Hopping

Contributed by Yi-Bing Lin

With channel sniffing and channel hopping, CDPD allows M-ESs to use idle voice channels without interrupting the cellular system.

Channel Sniffing

An Base Station MDBS periodically scans the channels within its radio coverage area and generates a candidate list of available channels for CDPD traffic. This list is then forwarded to the MDIS. The MDIS collects all channel lists from the associated MDBSs, and determines the CDPD channel streams based on its knowledge of the voice channel allocation algorithm.

During the monitoring phase, the MDBS detects the channel usage through the cellular transmit path by using a forward power monitor called sniffer. Since every cell may contains as many as 60 voice channels, it is critical that the MDBS finds the available channels in real-time.

Emergency Hop

With the channel monitoring mechanism, the MDBS should be able to change channels before a voice assignment is made on the current CDPD channel. This action should be completed in 40 msec. When the MDBS detects that the CDPD channel is about to be assigned for a voice call by the cellular system, it performs an emergency or forced hop by switching the channel without informing its M-ESs. When an M-ES loses contact with the forward link, it searches the likely-hop channel list (broadcast by the MDBS) to re-establish the radio link.

Planned Hop

The MDBS may periodically perform channel switching (referred to as timed or planned hop ) to avoid channel sealing or channel stealing. When the cellular system notices interference on a channel, the channel is sealed and becomes unavailable to a voice user. Since the cellular system cannot recognize CDPD, it may seal a channel used by CDPD, and the channel will not be used by the cellular users. If so, CDPD steals the channel from the cellular network, which violates the rule that CDPD should not affect the voice system. To avoid sealing of a CDPD channel, the MDBS uses timed hop to switch a CDPD channel stream periodically (the hop period is on the order of 10 seconds). In a timed hop, the MDBS broadcasts a control message to all M-ESs using the channel, and instructs them to move to a new channel (if any). Channel hopping may not be performed if dedicated channels are assigned for CDPD use.

JPL's Wireless Communication Reference Website Yi-Bing Lin (Author) and Jean-Paul M.G. Linnartz (Ed.), 1996.