TCP/IP Tutorial and Technical Overview - IBM Redbooks

A TN3270 server must support these characteristics during initial client/server
session negotiations. Binary transmission and end of record options can be sent
in any order during the TN3270 negotiation. Note that TN3270 does not use any
additional options during the TN3270 negotiation; it uses normal Telnet options.
After a TN3270 connection is established, additional options can be used. These
options are TELNET-REGIME, SUPPRESS-GO-AHEAD, ECHO, and
TIMING-MARK.
Terminal type option is a string that specifies the terminal type for the host, such
as IBM 3278-3. The -3 following 3278 indicates the use of an alternate screen
size other than the standard size of 24x80. The binary transmission Telnet option
states that the connection will be other than the initial NVT mode. If the client or
server want to switch to NVT mode, they send a command that disables the
binary option. A 3270 data stream consists of a command and related data.
Because the length of the data associated with the command can vary, every
command and its related data must be separated with the IAC EOR sequence.
For this purpose, the EOR Telnet option is used during the negotiation.
Other important issues for a TN3270 connection are the correct handling of the
ATTN and SYSREQ functions. The 3270 ATTN key is used in SNA environments
to interrupt the current process. The 3270 SYSREQ key is used in SNA
environments to terminate the session without closing the connection. However,
SYSREQ and ATTN commands cannot be sent directly to the TN3270 server
over a Telnet connection. Most of the TN3270 server implementations convert
the BREAK command to an ATTN request to the host through the SNA network.
On the client side, a key or combination of keys are mapped to BREAK for this
purpose. For the SYSREQ key, either a Telnet Interrupt Process command can
be sent or a SYSREQ command can be sent imbedded into a TN3270 data
stream. Similarly, on the client side, a key or combination of keys are mapped for
SYSREQ.
There are some functions that cannot be handled by traditional TN3270. Some of
these issues include:
► TN3270 does not support 328x types of printers.
► TN3270 cannot handle SNA BIND information.
► There is no support for the SNA positive/negative response process.
► TN3270 cannot map Telnet sessions into SNA device names.
13.1.8 TN3270 enhancements (TN3270E)
The 3270 structured field allows non-3270 data to be carried in 3270 data.
Therefore, it is possible to send graphics, IPDS printer data streams, and so
on. The structured field consists of a structured field command and one or more
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