User's Guide

The Operational View SB300 Series
Page 42 Proprietary and Confidential 2110059 Rev 1.1
H1 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).
5.8.2. User Datagram Protocol (UDP)
UDP is the most basic transport protocol provided by the modem. It is a thin protocol, adding
only a port specification to the underlying Internet Protocol (IP). It has the same features as IP,
that is a best effort, connectionless delivery service, with the chief benefit being minimum
overhead.
UDP is not considered reliable because packet delivery is not guaranteed. Packets can be lost,
duplicated, delayed, or delivered out of sequence. These conditions are not detected, and the
sender or receiver is not informed.
UDP is connectionless because sender and receiver are never logically connected. If the intended
receiver is not active and available the message is lost. It is a best effort delivery, because the IP
software makes an earnest attempt to deliver the packets, failing only if system resources are
overloaded or the underlying networks fail.
Application programs using UDP must accept full responsibility for handling the problems of
reliability, including message loss, duplication, delay, out-of-order delivery, and loss of
connectivity.
These problems are often treated casually by programmers. Testing done when using highly
reliable, low delay local networks may not expose potential failures. This explains why many
applications that use UDP work well in a local environment but often fail in dramatic ways on a
more global network.
5.8.2.1. Opening a UDP Client Session
Client sessions are those in which your modem initiates a conversation with a server.
A detailed discussion of this type of session follows the outline of steps to open a UDP Client
Session. (AT is omitted for brevity):
1. +WS46=4 to place the modem in CDPD mode (only required on the SB320).
2. +WS45=0 to select UDP packet service.
3. Optionally assign a channel with +WPCHAN=n where n is the channel to use.
4. 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 7.
5. +WS197=n where n is the index (1-4) of the NEI to register, to select the NEI
6. +WPREG to register on the CDPD network. The modem will reply as noted in the section
above on manual registration.
7. +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.
8. D<dest> where <dest> is the destination IP and port number to send your data to. If the
Dial Directory (quick dial) is being used, <dest> can be an index to the Friends Only list.
The entry there will be used to determine the IP address and port for the call. See the AT
Command Reference for details on this command.
The modem will reply with CONNECT at which point you are in data state.
The modem is now ready to accept host data for packet assembly and transmission to the network.