User's Guide

The Operational View SB300 Series
Page 46 Proprietary and Confidential 2110059 Rev 1.1
If auto-answer is enabled, the modem will then open the session. If DTR is off (indicating the
host is unavailable), the packet is buffered. At this time, there is no DTR flow control. Any
incoming packets received while DTR is off are held in the modem buffers until one arrives at a
time when DTR is on. At that point, all buffered packets are forwarded to the host.
If auto-answer is disabled, the modem will buffer the packet and wait for the host to issue the
Answer – Manual (A) command.
Upon opening the session, the modem will issue the CONNECT <ip> message (where <ip> is the
address of the calling station) to the host to indicate the transition from command state into data
state. The CONNECT message provides a “caller ID”. Note that quiet mode (Q) will suppress
the word CONNECT but not the IP caller id.
The incoming packet is then presented to the host and the modem is ready to accept host reply data
for packet assembly and transmission to the network.
NOTE
Packets are buffered at the modem. If manual answer is being used, and several
connection attempts are ignored, the packet buffers may overflow. When a session is
opened, all buffered packets are forwarded to the host.
It is possible to have buffered packets from multiple source clients but only the last
packet received can be answered.
The onus is on the user to implement host protocols when using UDP to ensure data is
not lost or corrupted.
Detailed Discussion
The first two steps simply place the modem in CDPD UDP mode. The modem’s packet service is
set in register +WS45 (Packet Service) by step 2. Setting this value to 0 will enable the modem’s
UDP stack. This can be done before or after registration on the network so auto-registration can
occur before this step.
The registration process can be more closely controlled with steps 3 through 6. Steps 3 and 4 can
be used in auto-register as well as manual register settings.
The listening port (+WS212) must be set to the desired port number to communicate over. Both
the client and server must share this port number.
In a server session, the host does not originate transmissions, but rather waits for service requests
from a calling client. The modem does not enter data state until the first packet is received.
Receiving
Incoming packets are accepted from the network only if the IP of the sender is on the Friends List
or the list is empty (Friends Only disabled) and the destination port matches the listening port set
in register +WS212. Any packets arriving from an IP other than those accepted are ignored.
When a packet arrives, the IP address and port of the sender is stored as the address to send
packets to. See Transmitting for additional details and issues on this.
If the modem has been placed in command state but is still in on-line condition, incoming data
will be buffered. There is an automatic remote flow control between the modem and the network
to prevent overflow.
Transmitting
Once in data state, the modem uses its PAD management services to assemble the data coming
from the host into packets. See Section 5.9 for a discussion of these services.