User's Guide

The Operational View SB300 Series
Page 40 Proprietary and Confidential 2110059 Rev 1.1
The modem’s packet service is set in register +WS45 (Packet Service). This setting can be made
before or after registration on the network. It should not be changed once the modem starts a
session (is on-line). If a change to packet service is required, the user must close the current
session (go off-line), change the setting, and then establish a new session.
Use of each of the services is described in sub-sections below. For more on protocols generally
see Section 4.4 above.
5.8.1. Serial Line Interface Protocol (SLIP)
SLIP is one method for encapsulating packet data over dedicated and/or switched serial lines. It is
a useful and reliable way to allow mixes of hosts and routers to communicate with one another in
various combinations.
When operating in SLIP mode, the modem’s internal Packet Assembly / Disassembly (PAD)
services are bypassed and the modem acts as a router passing data between the host’s protocol
stack, and the CDPD network. All protocol headers are the responsibility of the host as is the
SLIP framing to the packets. The modem will parse the SLIP frame character to distinguish
packets and to identify the data state escape sequence.
This protocol is useful if the host application must have multiple sessions active concurrently. An
example would be a police car, which may need simultaneous connections to a local dispatcher, a
state motor vehicle bureau, and a federal database.
Note that the SLIP protocol does NOT support auto-answer. If the modem was in auto-answer in
another packet service, it will be turned off (S0=0) when SLIP is selected. Attempts to set the
auto-answer register (S0) to a value other than 0 will return the ERROR result code.
5.8.1.1. Opening a SLIP Client Session
A detailed discussion of a SLIP session follows the outline below of the steps to open a SLIP
Client Session. (AT is omitted for brevity):
1. +WS46=4 to place the modem in CDPD mode (only required on the SB320).
2. +WS45=3 to select SLIP 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 use for this registration.
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 network connection.
8. O to start the session. The modem will reply with CONNECT at which point you are in data
state.
The modem is now ready to pass host packets to the network. Any incoming packets, from any
originator, will be passed to the host. This allows the host to send and receive packets to multiple
destinations asynchronously provided the host can manage multiple socket connections.
Detailed Discussion
The first two steps simply place the modem in CDPD SLIP mode. The modem’s session state is
set in register +WS45 (Packet Service) by step 2. Setting this value to 3 will place the modem in
SLIP mode. This can be done before or after registration on the network so auto-registration can
occur before this step.