User's Guide

The Operational View SB300 Series
Page 38 Proprietary and Confidential 2110059 Rev 1.0
aware of both the wake-up delay and that the first character to the modem was lost. Automatic
wake-ups to check the network will still occur so no incoming network traffic is lost.
5.6. Friends Only
Friends Only is a feature available when either the UDP or TCP PAD is enabled in a server
session, but is not used with the SLIP protocol.
Friends Only permits reception of packets to be restricted (for security reasons) to a predefined
group of IP addresses (friends). Friends Only mode is controlled by entering IP addresses in a
Friend Only table. If the table is not empty, the feature is enabled and the comparison of packet
source addresses against the list stored in the modem will be made.
This subject is discussed in further detail in both the UDP and TCP packet services sections
below.
5.7. Packet Services
SB300 Series modems support three connection protocols in CDPD:
• SLIP (PPP is coming in a future firmware release)
• UDP PAD
• TCP PAD
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.
5.7.1. 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.
5.7.1.1. Opening a SLIP Client Session
A detailed discussion of a SLIP session follows the outline of 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.