The SNA services implement the upper layers of the SNA architecture. SNA Transport implements a type 2.0 (T2.0) node (formerly called a Physical Unit Type 2 [PU.T2]) and type 2.1 (T2.1) node (formerly called a Physical Unit Type 2.1 [PU 2.1]) for SNA/SDLC Link, a T2.0 node for SNA/X.25 Link, and a T2.0 and T2.1 for SNA/Token Ring Link. (Note that, at this time, LU 6.2 API/XL is the only SNA service that makes use of the T2.1 node capabilities.) SNA Transport also includes the Path Control and Transmission Control layers of SNA, as well as the Configuration Services and Address Space Manager functions. It provides a direct send/receive interface for the SNA services to establish LU-LU sessions with the remote system. So, the lower layers of SNA are implemented by running the services with an SNA link.
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