Sierra Wireless DART 200 CDPD Modem - Manual (Page 64)

DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols
Sierra Wireless, Inc. 5-10
• The modem enters the online mode
• The source address is saved
• The datagram is passed to the MAS
In the answering modes already discussed:
1. The modem accepts incoming UDP datagrams.
2. The modem strips off the UDP header, and passes the packets
on to the attached device for processing.
3. Responses are assembled and transmitted as described in steps 1
and 2 for UDP clients.
4. The modem continues in this mode, accepting and transmitting
data until the MAS explicitly moves the modem to command
mode by issuing an escape sequence (+++), or by dropping DTR
with a non-zero &D option specified.
5. Now, the MAS can return to online mode with the ATO
command, or drop out of the CONNECTED state with the ATH
command. Issuing a hang up command (ATHn) causes the
current source (S-Register 53) and destination address to be
reset, and the UDP server to automatically listen for the next
datagram. At this point, both passive and active connection
requests will be honored.
A MAS functioning as a server receives calls from any client device.
However, it can only save one source address, the one that was
extracted from the last datagram received, for sending any required
response. For this reason, if multiple clients are expected server
modems must use receive locking to delay accepting subsequent
datagrams until a response is sent to the source of the prior datagram.
Datagrams subsequent to the first are placed in a FIFO queue for
later processing.
In both client or server modes, the modem extracts the source
address (IP Address and port number) from the first packet received
and stores it in S-Register 53. This information can be retrieved by
the end-user application, if needed, with an ATS53?, for example:
ats54?
198.225.189.21,4000
This address data cannot be viewed with the AT&V command. The
data remains in S-Register 53 until the session is ended by an ATH
command (client mode), or a datagram is received from a different
source (server mode).
? ?
NOTE:
The DART 200 has
approximately 6K of
buffer space available
for storing queued
datagrams.