User's Guide

User’s Guide CDPD Operation
2110059 Rev 1.1 Proprietary and Confidential Page 47
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.
5.8.2.5. Escaping Data State
Escaping a UDP server session is handled exactly the same way as a client session. See
Section 5.8.2.2 above for details.
5.8.2.6. Closing a UDP Server Session
Closing a server session is handled exactly the same way as a client session. See Section 5.8.2.3
above for details.
Provided the modem is set to auto-register and H1 is not used to close the session, the modem is
ready for the next UDP server session without further configuration. Auto-answer will remain
enabled and the modem registered on the network.
To disable auto-answer, you must set register S0=0. This will prevent the modem from starting
any further server sessions.
5.8.2.7. UDP Broadcast and Multicast
These services are not yet supported by the SB300 Series modems but are planned for future
firmware upgrades.
Broadcast and multicast are CDPD functions that support sending the same data to a large group
of IP Addresses with a single transmission to the CDPD network. This capability is implemented
through Mobile Data Intermediate System (MDIS) software. Check with your carrier to determine
if these capabilities are offered before planning to use it as part of your application.
Broadcast and multicast transmissions are point-to-multi-point and provide a one-way UDP
connectionless service. The operational considerations are:
• Protocol restrictions prevent broadcast or multicast messages from being encrypted.
• Broadcast or multicast data packets will not be received by a SB300 Series modem with an
active TCP PAD session.
• If a UDP PAD session is active the broadcast and multicast messages will be interspersed
with the application data packets and must be separated by the application.