Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 10
Planned channel hops occur at a time specified by the CDPD network. For example, the CDPD
network management function may configure the CDPD base station to use only a specific
channel for a fixed period of time and then hop in a round-robin fashion to another of the radio
channels used within a specific sector. In this case, the CDPD base station (known as the Mobile
Data Base Station, or MDBS) directs the modem (the CDPD subscriber device) to the new
channel to be used for CDPD activity. If not within another sector, the same channel may be
acquired.
Forced channel hops occur when cellular voice activity is detected on a channel currently
carrying CDPD packet-switched data traffic. In this case, the CDPD base station initiates a
change of frequency to a new channel available within that sector that does not have any cellular
voice activity on it.
The base station in the CDPD system uses both planned and forced hops to switch CDPD
subscribers between the unused cellular voice channels to avoid interference between CDPD and
cellular voice traffic. While this channel-hopping activity is in progress, the CDPD network
maintains the data link connection between the CDPD subscriber and the CDPD network even
though the physical radio link between the CDPD subscriber and the CDPD network changes
radio channels over time. This channel hopping activity is transparent to the CDPD subscriber.
Figure 3-3 shows an example of simultaneous cellular voice and CDPD data use on a sector
supporting three radio channels.
AMPS Voice
Unused Capacity
Channel Usage
CDPD Data
PH
PH
PH
FH
PH
FH
PH = Planned Channel Hop
FH = Forced Channel Hop
Time
Channel 1
Channel 2
Channel 3
Figure 3-3: How cellular voice and CDPD coexist in a three-channel sector
In this example, a single CDPD data link is supported within the three-channel sector. Of the six
channel hops shown in the example, four are planned and two are forced. The first forced channel
hop is used to avoid the cellular voice activity that occurs on channel 2, and the CDPD data link is
maintained by the CDPD base station, forcing the subscriber device to hop to channel 3.
3.1.3. Dedicated CDPD Channels
CDPD data traffic was originally expected to be infrequent short bursts of data typical of telemetry
or credit-authorization applications. In the early days, most CDPD systems used shared channels
only with voice having priority.
It quickly became apparent that during busy periods there were few if any channels available for
CDPD to switch to—the CDPD system was “blocked.” A number of applications, including
public safety and credit card authorization, could not tolerate blockage of the system. The carriers
solved the problem by reserving one or more channels on most CDPD cell sites for CDPD traffic
only.
Although giving up a voice channel for CDPD meant less voice revenue, carriers did not want to
hinder the growth of CDPD by forcing it to use only the available extra space on voice channels,
although such service can still be found in some rural areas.