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DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols
Sierra Wireless, Inc. 5-16
5. The data packet is directed by the network routers to the appropriate
port on the designated server. If the server or the data path becomes
inactive between the time the connection is established and the time
data transmission starts, the MAS is informed by an error message
being returned.
If the central site is the client establishing the TCP session, the modem at
the remote site must be set up in TCP sever mode. The MAS can wait for
a RING message and respond with an
ATA command, or its modem can
have auto answer active. Refer to Auto Answer setup, p. 8-11, for details
on activating this option.
Unlike UDP, with TCP a session is established by a three-way handshake
between the client and the server, not by the initial datagram. The client
IP Address is part of the incoming session establishment packet. When
the TCP handshake is received by the TCP server, the modem first
validates the source address (if the Friends Only option was specified).
If the source address is acceptable or if Friends Only is not active then a
RING result code is issued.
•
If auto answer is active the CONNECT message is issued. If not,
then an ATA must be issued by the MAS within 60 seconds or the
tentative connection will be dropped.
•
The modem extracts the source address (IP Address and port
number) from the handshake packet for use in constructing the
response data packets and also stores it in S-Register 53. This data
can be retrieved by the end-user application, if needed, with an
ATS53? The address data cannot be viewed with the AT&V
command:
ats53?
198.225.189.21.4000
•
The modem enters online mode.
If the source address is not acceptable, the connection is refused.
In either client or server mode:
1. When the connection is established a CONNECT message is
received.
2. The MAS continues to read from (server) or write to (client) the
serial port to exchanges data packets with its session partner.
3. If data is being sent, it accumulates until a data forwarding condition,
as described in
Data forwarding, p. 8-8 occurs:
•
The DART 200 then attaches the TCP header and sends the
packet onto the network
•
The packet is directed by the network routers to the appropriate
port on the designated server
? ?
NOTE:
Time-outs are not a
consideration if auto
answer is enabled.
? ?
NOTE:
The address remains in
S-Register 53 until the
session is ended by an
ATHn command.