The Telnet USS function provides the user with a USSMSG10 logon screen similar to the logon screen used by native SNA terminals. The Telnet USS function supports sending USSMSGs to the client, receiving and parsing USSCMDs from the client, and using a translation table defined in the USS table.
Telnet supports both 3270-format and SNA character stream (SCS)-format USS tables. The SCS USS table name is optional on the USSTCP mapping statement. If an SCS name is provided, the SCS USS table is used for all TN3270E connections.
The CLEAR key is handled differently by the client depending on whether a 3270-format or SCS-format USS table is used. The CLEAR key has a special function for 3270-format USSMSGs. Pressing the CLEAR key sends the CLEAR key data stream to Telnet. If REFRESHMSG10 is specified or used by default, pressing the CLEAR key refreshes the screen with USSMSG10 for any USSMSG other than USSMSG10. If you press the CLEAR key while at USSMSG10, the screen is cleared and the cursor is placed near the upper left corner (row 1, column 2). If you press the CLEAR key a second time, the screen refreshes with USSMSG10. If NOREFRESHMSG10 is specified, pressing the CLEAR key always clears the screen of any USSMSG and places the cursor in the upper left corner (row 1, column 1). For SCS-format USSMSGs, the CLEAR key does not provide the same special function. For SCS, the client processes the CLEAR key by clearing the screen and placing the cursor at the upper left corner (row 1, column 1) with no data sent to Telnet.
USS data traffic is also handled differently depending on whether a 3270-format or SCS-format USS table is used. If a 3270-format USS table is used with a TN3270E connection that has negotiated BIND-IMAGE, a Telnet-generated Bind/Unbind encapsulates the USS traffic. If the connection is TN3270, BIND-IMAGE is not negotiated, or the SCS-format USS table is used, Telnet does not encapsulate the USS traffic in a Bind/Unbind. If a 3270-format USS table is used, the Telnet data type for all traffic is 3270-DATA. If an SCS-format USS table is used, the Telnet data type for all traffic is SSCP-LU-DATA. Regardless of USS table format, if the SYSREQ function is supported on a TN3270E connection and SYSREQ is received by Telnet, Telnet will accept a LOGOFF command in SSCP-LU-DATA format. If the client sends any command other than LOGOFF, COMMAND UNRECOGNIZED is returned in SSCP-LU-DATA format. If the client sends a second SYSREQ, Telnet reverts back to whatever the session state was prior to receiving the first SYSREQ.
USSCMD parsing also includes checking for INTERPRET table entries that might provide more function than USS tables alone can provide. Sample USS tables are in TCP/IP data sets SEZAINST(EZBTPUST) and SEZAINST(EZBTPSCS). A sample INTERPRET table is in TCP/IP data set SEZAINST(EZBTPINT). The 3270 format USS sample has been assembled, linked, and loaded into the product data set. The tables can be used by coding the USSTCP and INTERPTCP mapping statements in BEGINVTAM. For example, the statements below will map the sample tables to the client at IP address 1.1.1.1. See Mapping Objects to Client Identifiers for mapping details.
USSTCP EZBTPUST,EZBTPSCS 1.1.1.1
INTERPTCP EZBTPINT 1.1.1.1A new table can be created at any time and link-edited. Customized USS and INTERPRET tables can be created to change messages, commands, and translation tables. For example, messages can be changed to have non-English text or to have different syntax. Commands can be changed to accept different syntax or to have different default values. A VARY TCPIP,tnproc,OBEYFILE command will cause Telnet to load the new table with the new profile being processed. Any new connection using the new profile will be assigned the new table. Telnet also supports dynamic updating of same-name USS or INTERPRET tables. The VARY TCPIP,tnproc,OBEYFILE command adds the new version of the table to the new profile. New connections use the new copy associated with the new profile while old connections continue to use the old copy associated with the old profile.