TN3270 Enhanced (TN3270E) connections support full-screen 3270 emulation that is sometimes referred to as TN3270 Extended. Do not confuse TN3270E function with the IBM® 327x device types that end in -E (for example, 3278-2-E). In these cases, the E indicates that the terminal supports Extended field attributes such as color and highlighting and is not related to Telnet functions.
Telnet is often used as the primary method of connection between client workstations and the SNA mainframe environment. To make this form of remote connection as seamless as possible, Telnet terminal emulation simulates actual SNA terminals as closely as possible. To accomplish this, RFC1647 and RFC2355 (both known as TN3270E) add the ability to specify device names at connection time, add support for printer devices, and add additional SNA functions. An Internet draft, RFC 2355 Extensions, adds Contention Resolution and SNA Sense code support.
- Device name specification
- Telnet assigns LUs based on the LU mapping statements supplied. Clients are assigned a device name (Telnet LU name) based on those statements. However, a TN3270E client can optionally specify that a particular device name be assigned, or it can specify that a device name from a pool of LUs be assigned. If the specified device name is allowed for this client based on the LU mapping statements and the LU is available, Telnet assigns the specified device name. If the specified device pool is allowed for this client based on the LU mapping statements and an LU within the pool is available, Telnet assigns a device name from the specified pool. Otherwise, the request is rejected with an appropriate reason code, and the connection is dropped. See Mapping Objects to Client Identifiers for additional LU mapping information.
- 328x printer support
- Many Telnet clients emulate 328x class printers
(device type IBM-3287-1). Most support both SNA Character Stream (SCS)
as an LU1 and 3270 data stream as an LU3. The support of each is negotiated
at connection time. When connected in TN3270E mode, Telnet supports
these emulators in a manner similar to terminal LUs. Telnet can be
configured to initiate a session at connection time or to open an
ACB to let the application initiate the session. The bind initiating
each session is sent to the client, and the bind informs the emulator
which data stream to expect. The VTAM® application
perceives the Telnet LU to be an an actual 3287-class printer and
sends the SCS or 3270 data to the Telnet LU. Telnet passes the data
on to the client, which prints the data. Telnet printer support allows
you to use a single product, Telnet, to control both SNA terminals
and SNA printers.
Some Telnet client printer emulators can request to be associated with a terminal device name by specifying the terminal device name during connection negotiation. Using printer association, users can connect their Telnet terminals to an application and then have Telnet assign an associated printer device name based on the terminal name. To associate printers with terminals, Telnet must have a printer device pool of LUs defined and a terminal device pool of LUs defined with each having the same number of device names. For more information, see Associated printer function.
- Additional negotiated TN3270E support
- Responses and SysReq functions are supported by most clients that
support TN3270E connections. Contention Resolution and SNA Sense support
are newer and less prevalent.
- Responses - The client or host VTAM application
can request that it receive definite, exception, or no response. Client
responses to application requests provide more accurate response information
for application-based monitoring tools compared to TN3270 connections.
When the client negotiates responses and the SNA application
sends data with a definite response, the Telnet Server disables the
DELAYACKs function at times.
For TN3270 connections, Telnet must
intercept response requests from the host and respond on behalf of
the client, incorrectly reducing measured response time. Telnet monitoring
will provide accurate response time information for either TN3270E
or TN3270 connections. - SysReq function - The user can request that a current session be dropped by entering LOGOFF (in upper, lower, or mixed case) after pressing the SysReq key. If LUSESSIONPEND is not mapped to the client, the connection will be dropped. Otherwise, a USSMSG10 screen is sent to the client. If, instead of entering LOGOFF, the SysReq key is pressed a second time and if the application supports LUSTAT 082B (presentation screen is lost), the previous screen is resent to the client emulator.
- Contention Resolution - Improves communication between the client
and host VTAM application about
which owns the send state. Contention Resolution includes the following
items:
- Keyboard Restore Indicator (KRI) - Whenever the host VTAM application sends End Bracket (EB), Telnet sends a KRI to the client. This directly notifies the client that the keyboard can be unlocked. Without the KRI indicator, Telnet would have to make sure a WCC with the unlock keyboard flag set is sent to the client. The KRI flag is set regardless of whether the keyboard restore flag in the WCC byte is set.
- Start Data Indicator (SDI) - When the host VTAM application sends change direction or end bracket, Telnet sends the SDI to the client. This allows the client to know exactly when data can be sent to Telnet.
- BID - A BID sent from the host VTAM application is forwarded to the client instead of being intercepted and handled by Telnet. This allows the client to manage the BID process itself.
- Signal Indicator - A signal that is received from the host VTAM application is forwarded to the client. When the client responds to the signal, Telnet sends a change direction indicator to the host VTAM application.
- SNA Sense Support - Allows the client to include SNA sense codes in a response message. The client retains the option of letting Telnet map the errors to an appropriate sense code by not turning on the SNA-Sense-Code indicator in the response message.
- Responses - The client or host VTAM application
can request that it receive definite, exception, or no response. Client
responses to application requests provide more accurate response information
for application-based monitoring tools compared to TN3270 connections.