Sierra Wireless DART 200 CDPD Modem - Manual (Page 82)

DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols
Sierra Wireless, Inc. 5-28
Telnet
The telnet capability within TCP/IP is designed to provide support for
ASCII terminals to communicate with text-oriented server applications
on host systems. As implemented in the DART 200, the telnet support
mode provides the capability for the modem to emulate such a terminal;
the default is the DEC VT100 but the actual type is negotiated with
(dictated by) the server.
A device (usually a PC, but potentially an ASCII terminal) attached to
the DART 200 and using telnet mode has its data stream translated to
that of the VT100 or whatever other terminal type was negotiated. This
usually involves not only translating a few control characters, but also
turning off the high order bit of each data character. This translation can
make it very awkward to use telnet mode for a typical telemetry
application. It is strongly recommended that telnet not be used for other
than the intended use; providing access to ASCII terminal application
servers.
Setup options
With the exception of the side preference, the DART 200 factory defaults
permit a telnet session to be established. However, certain profile
parameters need to be checked to ensure they work appropriately with
the remote host at minimum cost, as follows:
• Online Mode Echo (ATFn) - Controls the echoing of characters
when in online mode. The default for telnet mode is F1 (host echo)
When a telnet session starts, one of the items that the DART 200
negotiates with the host is whether the host or modem provides the
echo for characters entered at the modem end of the connection.
Most processes connected on the well-known telnet port (23) provide
the echo. However, in a CDPD telnet session where charges are
based on packet and data flow rather than on connect time, there is a
big incentive to minimize the flow of extraneous information; use the
F0
option (modem echo) to do this.
The tradeoff is one of cost versus the confirmation that the characters
arrived correctly, which is presumed if they echo back correctly from
the host. Also, the client has no knowledge of how certain characters,
such as a backspace (BS), should be handled locally since the remote
process is what determines if a BS is destructive or harmless.