User's Guide

User’s Guide CDPD Operation
2110059 Rev 1.0 Proprietary and Confidential Page 49
ATH1 will force de-registration of the modem even if auto-registration is enabled. The modem
will remain de-registered. Although auto-registration remains enabled, the modem will not
resume auto-registration until the register is reset (+WS173=1) or the modem itself re-enters
CDPD mode (a modem reset or power cycle on CDPD-only modems).
NOTE: There is a PAD Inactivity Timeout in register S30 that can also close a session. This
timer is disabled by default (set to 0) but can be set to a value from 1 to 20 minutes to prevent the
modem from holding an idle session open. If this mechanism closes the session the modem will
send the message “NO CARRIER” and return to command state. The Carrier Detect (DCD)
signal will also go inactive indicating the session is closed.
5.7.3.4. Opening a TCP Server Session
Server sessions are those in which your modem answers a “call” from another system.
A detailed discussion of this type of session follows the outline of steps to open a TCP Server
Session. (AT is omitted for brevity):
1. +WS46=4 to place the modem in CDPD mode (only required on the SB320).
2. +S31=0 to disable logic sleep.
3. +WS175=0 to disable CDPD sleep.
4. +WS45=1 to select TCP packet service.
5. Optionally assign a channel with +WPCHAN=n where n is the channel to use.
6. Optionally verify the channel acquisition with +WPRSSI looking for the second value in the
return string to be 3.
If the modem is using auto-registration (+WS173=1) skip to step 9.
7. +WS197=n where n is the index (1-4) of the NEI to register, to select the NEI
8. +WPREG to register on the CDPD network. The modem will reply as noted in the section
above on manual registration.
9. +WS56? to query the registration status. Repeat this command until it returns a value of
1 (registered) or until the host chooses to timeout, failing the connection.
10. +WS212=<port> where <port> is the port number (1025-4999) that the modem will listen
to. The modem will only accept packets addressed to this port. The factory default is 2100.
11. S0=1 sets the modem into auto-answer mode. The modem will remain in command state
until a TCP handshake is completed with a calling client.
The session is not actually opened until a TCP connection handshake is negotiated with an
accepted IP address on the listening port. In server sessions the modem will check all incoming
packets for the source IP address. If Friends Only is enabled, the originator’s IP address is
checked against entries in the Friends List. If no match is found the handshake will fail and the
host will not be advised of any activity. This is handled entirely within the modem.
If the originator’s IP is accepted the session is opened and the host is notified with the messages
“RING” and “CONNECT <ip>” (where <ip> is the address of the client station). The
CONNECT message provides a “caller ID”. At this point the modem changes from command
state to data state. Incoming packets are then presented to the host and the modem is ready to
accept host reply data for packet assembly and transmission to the client.
Detailed Discussion
The first four steps simply place the modem in CDPD TCP mode. The modem’s sleep features
are disabled to prevent delays in TCP acknowledgements, which would cause excessive re-
transmissions and possibly loss of connection. The modem’s packet service is set in register
+WS45 (Packet Service) by step 4. Setting this value to 1 will enable the modem’s TCP stack.
This can be done before or after registration on the network so auto-registration can occur before
this step.