NAS native SNA terminal support utilizes IBM's Qualified Logical Link Control (QLLC) transport protocol. Network QLLC devices can access NCP-based host applications as native SNA terminal equipment. Support is provided for PU type 1 terminal nodes (e.g., interactive 3767 hard copy terminals) and for PU type 2 controller nodes (e.g., 3274 cluster controllers).
As per the QLLC protocol, each PU is supported as a Switched Virtual Circuit (SVC) across the X25 access link. PIU's going to or coming from the LU's on each PU are effectively multiplexed across each SVC. This means, for example, that a single SVC can support 1 traffic for up to 254 3270 devices from a single 3274 controller.
Some QLLC Packet Assembler and Disassembler (QPAD) manufacturers have elected to support one LU connection per SVC by establishing a virtual PU call for each LU. Although this approach may not increase overall throughput for the real PU, it does allow different LU's on the same real PU to be routed to different hosts on the X25 network. Conversely, IBM's QPAD implementation dedicates all LU's on a PU to the host controlling the PU. LU's can only be rerouted after a host connection has been established via the Multiple System Networking Facility (MSNF). The non-IBM QPAD must, of course, appear to the PU's and LU's it controls as an SSCP for the purpose of activating the appropriate sessions and accepting terminal operator routing information. This type of QPAD implementation must also logically satisfy certain control sequences that it receives from NAS since these sequences will have been previously performed by the QPAD for the real PU. For example, NAS will send a QXID request as part of REQCONTACT processing to read the PU identification. Since the QPAD establishes one SVC for each virtual PU and real LU pair, the QXID response that is returned to NAS must be simulated by the QPAD for the virtual PU. NAS QLLC support employs the SNA programmed resource facility of NEO for its PIU boundary function processing. FID1 PIU's that flow from the host are converted to FID3 format for a PU type 1 SVC or to FID2 format for a PU type 2 SVC before being packetized for the QPAD. Conversely, packets received from a PU type 1 or 2 SVC are converted from FID3 or FID2 to FID1 format before they are passed to the host. Unlike the NEO support for ITI or DSP devices, NAS does not emulate the processing of control PIU requests (e. g., ACTPU, ACTLU, BIND) that are directed at QLLC PU or LU components. All PIU's are routed unaltered (except in the TH) to their respective destinations. The responses returned to host request PIU's are those from the QPAD, except in cases where the SVC has been cleared. In these cases, NAS will return appropriate error responses and/or generate an INOP PIU on behalf of the disconnected PU.
NAS also supports the older SNA PSH (Packet Switched Header) transport protocol. PSH uses flags in packet headers to transfer PU 1 control commands. QLLC uses qualified DATA packets ('Q-bit') to 1 transfer PU commands.