The SNA services provide primary and secondary SDLC protocol support. The SNA services support leased lines that can be point-to-point or multi-point. In multi-point, SDLC operates in one of three ways:
The SNA services do not support half-duplex at the physical level. However, at the SDLC protocol level, each port used for SNA SDLC can be half- or full-duplex.
The group poll allows a primary to poll multiple secondaries on a single line for data. A Passport 4400 unit SNA SDLC port can act as a secondary, receiving group polls from the Network Control Program (NCP). It provides all the secondaries in the group. It can also be a primary issuing group polls, or even act as the NCP.
If an SNA SDLC port operates as primary on a multi-drop line, then it can support multiple groups on the line. A station can be in only one group.
Here is a typical SNA SDLC network configuration:
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