User's Guide

The Operational View SB300 Series
Page 40 Proprietary and Confidential 2110059 Rev 1.0
The modem will enter command state but remain registered and in on-line condition (the session is
kept open). If data arrives at the modem while the session is open and the modem is in command
state, the data is buffered as described above.
An alternative to the escape sequence is to pre-set the DTR Options (&D) to either 1 or 2 and then
switch DTR off to escape data state. If the option is set to 1 (&D1), the modem will enter
command state the same as if the escape sequence was issued; it will remain registered with the
current session still open. If the DTR Option is 2 (&D2) then the modem will close the session,
de-register, and return to command state. Note that if auto-registration is enabled, the modem will
switch off DCD (closing the current session) but will remain registered.
If the session has been kept open, the host can re-enter data state by issuing the On-line command
(O). The modem will respond with CONNECT and data state is restored.
5.7.1.2. Closing a SLIP Session
To end a SLIP session, escape data state as described above. If the &D2 DTR option is used then
the session is closed.
If the escape sequence or the &D1 DTR option is used then you must hang-up with either H0 or
H1 to close the session. Both of these will close the current session. If the modem is set to
manual registration, it will also be de-registered by both of these commands.
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).
5.7.2. 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.
5.7.2.1. 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.