Sierra Wireless DART 200 CDPD Modem manual Telnet

DART 200 CDPD Modem User’s Guide 5 DART Supported Protocols

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.

Sierra Wireless, Inc. 5-28