User's Guide

The Operational View SB300 Series
Page 44 Proprietary and Confidential 2110059 Rev 1.0
When a packet does arrive for the host, the modem will first send the messages “RING” and
“CONNECT <ip>” (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”.
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.
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 5.8 for a discussion of these services.
NOTE – Reply Addressing in UDP Server Sessions
The source address (IP address and port number) of the last accepted packet is saved
in a temporary variable for use as the current destination address for all packets being
sent by the modem during the current server session.
This can lead to some conflict on busy systems. If a packet arrives from source A and
is currently being serviced by the host when a packet arrives from source B, a reply
intended for A will be sent to B.
A future firmware revision will implement a BUSY response which will avoid this
problem. For now, this situation cannot be avoided in UDP server sessions. The only
remedy is to use the TCP PAD to establish distinct point-to-point communication links.
The host will only know the identity of the calling station from the “caller ID” provided in the
CONNECT message. Once in data state the incoming packets could be from any acceptable
station (based on Friends Only activation) and matching the listening port.
The normal behaviour would have the server receive a message, issue a reply, and then close the
session. Auto-answer will be re-enabled so the client can send another packet if needed which
will appear to the host as a new session with “caller ID”.
An alternative would have all clients use an agreed protocol in which the client includes station
identification data within the packet. It is the user’s responsibility to supply this level of protocol.