Sierra Wireless DART 200 CDPD Modem - Manual (Page 57)

DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols
PN1197-00 Revision 1.0 5-3
Even though a successful ATDP command receives a CONNECT
response a connection was not established. The ATDP merely passes
the IP Address and port number of the destination system to the
DART 200, where they are stored for use in constructing UDP
headers for the data packets that follow. If the communication
session is being set up between two DARTs, both have to execute a
UDP dial command to go into online mode so that communications
can begin.
If data transmission is initiated by the MAS:
1. The MAS writes application data blocks to the DART 200.
2. Data blocks are assembled until a data forwarding condition, as
described in Data forwarding, p. 8-8, occurs.
3. The DART 200 attaches the UDP header and checksum and sends
the packet to the network.
4. The datagram (packet) is directed by the network routers to the
appropriate port on the designated host. If the host or host port is
inactive the data is lost and the local application is not informed.
Good UDP application design includes packet sequence checking, and
acknowledgments to ensure that both the sender and receiver can
determine if all application data arrived successfully.
If the MAS is on the receiving end:
1. When expecting a message following the initial CONNECT message
or in response to an output message, the MAS reads continuously
until all expected data is received.
2. When the data block is received it should be checked against
preceding blocks for sequence errors before proceeding.
3. The DART 200 passes the data portion of the incoming datagram on
to the MAS after removing the source IP Address and port number
from the header for use as the destination address in the response (if
required). This address data is available to the MAS through
S-Register 53 with an ATS53? command; it cannot be viewed with
an AT&V.
The handling of network connectivity issues (such as loss of carrier), as
well as data delivery and sequencing, is the responsibility of the MAS.
Refer to Modes of operation, p. 8-3, for guidelines on handling network
errors.
Session termination may or may not be necessary. If the MAS originates
sessions, it must terminate the first session before starting a second.
Terminate sessions by the escape function followed by a hang-up
command (H), as described in Appendix F, DART AT Command Set. If
the MAS only receives calls (performs a server function), it can remain
in online mode waiting for a datagram (service request) from another
client.
? ?
NOTE:
A good error-checking
and acknowledgment
scheme is essential to
successful UDP
operation.
? ?
NOTE:
Even though a
successful ATDP
command receives a
CONNECT response a
connection was not
established.