NAS BSC 3270 support utilizes the international Display System Protocol (DSP) transport protocol. Device polling and selection is handled by the network Terminal Packet Assembler and Disassembler (TPAD) machines which act as substitute hosts for the real BSC 3270's. Data that comes from a 3270 is packetized by the TPAD before it is sent to the CCU. NAS un-packetizes the data before passing it to the host. Data that is destined for a 3270 is packetized by NAS before it is sent to the network. The TPAD then un-packetizes the data before passing it to the terminal via a selection sequence.
For EP based applications, DSP terminals are supported as native BSC 3270 devices. Data is passed to the host via standard POLL channel programs. Data is taken from the host via standard SELECTION channel programs. All channel sequences are processed by NAS to simulate the activity of a real BSC 3270 leased communications line.
For NCP based applications, DSP terminals are supported as SNA 3270 devices (LU type 2 for displays, LU types 1 or 3 for printers). NAS utilizes the SNA programmed resource facility of NEO for its PIU boundary function processing. NAS interprets each FID1 PIU and provides its translation to the appropriate DSP packet sequence. FMD PIU's are converted to DSP DATA packets during host transmit operations. DATA packets received from the DSP TPAD are converted to FMD PIU's for presentation to the host. NAS manages the entire hierarchy of SNA sessions (SSCP/LINK, SSCP/PU, SSCP/SLU and PLU/SLU). For data that flows in the PLU/SLU session, NAS provides complete bracket state management, RU chaining control, normal flow sequencing and both inbound and outbound pacing. NAS also supports the FDX, HDX-contention and HDX-flip/flop modes of operation.
The TYMNET CMT (Character Mode Translation) package may be used to support CMT compatible asynchronous devices as 3270 BSC devices. CMT uses the DSP X25 transport protocol. This allows CMT devices (PCs and selected block mode ASCII terminals) to communicate with host applications that support BSC or SNA 3270 terminals.
BSC 3270 traffic is multiplexed across the X25 access lines using the DSP transport protocol. Terminal attributes are passed to NAS in the CALL REQUEST packet for proper device mapping. Application switching occurs at call connect time based on a system select character entered by the terminal operator. Access to SNA hosts is provided by NAS/NEO logic. Note that only DATA packets pass between the CCU and the DSP TPAD. Device polling and selection are performed remotely.