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.
