Sierra Wireless DART 200 CDPD Modem - Manual (Page 68)

DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols
Sierra Wireless, Inc. 5-14
TCP
Transmission Control Protocol (TCP) is an advanced transmission
protocol that adds substantial functionality to the underlying Internet
Protocol it is built on. Because of this structure it is often referred to as
TCP/IP. The advantage of TCP is reliability of data transmission;
achieved by using positive acknowledgments with retransmission if
required. The main disadvantage is the overhead needed to provide this
reliability under any and all conditions.
Unlike UDP, TCP offers both reliable and connected data transmission
service. Lost, delayed, duplicated, or out of sequence packets are
detected and automatically corrected. A logical point-to-point connection
is established and maintained throughout the communications session.
TCP is ideally suited for applications where speed of development is
critical, or for applications where the MAS is low on compute power,
available memory, or both. It requires very little other than session
establishment, sending data over the serial port, and session termination
to get a basic application operational.
The price for this reliability is the number of extra data packets used and
the time required to process them. Both need to be considered when
selecting an application protocol.
DART TCP capabilities
The DART software stack offers industry standard client-server
capability where the client originates communications and the server
waits for incoming requests. In server mode it includes auto answer
capability and an optional Friends Only mode. Friends Only mode
restricts the devices the server can receive calls from to help prevent
unauthorized access to applications or devices.
Friends Only mode operation
Friends Only mode permits communication with the modem to be
restricted (for security reasons) to a predefined group of addresses
(friends). Friends Only mode is controlled by S-Register 82, that enables
and disables the comparison of packet source addresses against a list
stored in the DART 200’s destination dial directory. Refer to the Modem
dial directory, p. 8-24 AT&Z command in Appendix F, DART AT
Command Set, for details. Only the IP Address portion of the addresses
in the dial directory is used with this feature. Bit 0 of S-Register 82
controls this mode. The default is to accept all incoming datagrams (bit 0
= 0)
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