User’s Guide Advanced Features
2110212 Rev 1.0 Page 55
Assign a Channel
The command +WPCHAN can be used to force the modem to lock on a particular channel,
typically for diagnostic purposes. When set to a specific channel, it overrides the setting of Side
Preference but does not override the settings in the SPNI list (both discussed below).
If you assign a channel and it is acquired, the modem will use that channel for registration
attempts. If registration fails on the assigned channel NO automatic attempts are made to locate
another channel. This is highly discouraged. Locking to a specific channel deviates from the
CDPD specification.
Changing the channel assignment while the modem is registered will not change the registration
status. The modem may lose connectivity if the new channel does not present sufficient signal or
fails to meet other requirements such as SPNI matching.
If the cellular carrier forces CDPD channel hopping (part of the CDPD specification) the modem
will follow the hops regardless of the user assigned channel.
To assign a channel for use by the modem issue the AT command +WPCHAN=n where n is the
channel number to assign. The modem will go to that channel. Subsequent use of +WPRSSI will
report the strength of signal and if the channel has been acquired.
Auto-locate a Channel
This is the default setting of the modem.
To have the modem locate a channel, issue the AT command +WPCHAN=0. The modem will
then scan for a channel when requested to register (or auto-register). RSSI requests will reflect the
status of the channel found. The channel number will also be available by querying register
+WS200 (Current Channel Number).
7.2.1.2. Side Preference The modem scans for channels based on a side preference set in register +WS174. The settings
are as follows:
1 – A side preferred,
2 – B side preferred,
3 – A side only,
4 – B side only.
The channel side preference is stored in a register not associated with a particular
entry in the NEI table. The preference will be used for all NEI registrations.
The service provider will have advised you of the channel side they use. Settings of 1 or 2 are
encouraged over settings of 3 or 4 if the modem is to be mobile. For “preferred” settings, the
modem will first scan for channels on the side specified, but if a suitable channel is not found, the
other side channels will also be scanned. Note that the modem may stay on the alternate side as
long as usable CDPD channels are found there; it does not periodically check for usable channels
on the preferred side.
7.2.1.3. SPNI Matching Once a suitable channel is found, the modem can optionally verify which carrier is providing
service on that channel.
If you want to restrict the modem to only using channels belonging to a user-spec i fi ed list of
service providers you can enter the associated SPNI values into a table. When the modem is
finding a channel, it will check the SPNI of each channel against entries in this table. If there is no
match, the modem will reject the channel and continue searching. You can use this option to
slightly improve the speed of the registration process if there is an alternate carrier in your
coverage area.