gtps2m0aACF/SNA Data Communications Reference

Basic SNA Terms and Concepts

A network addressable unit (NAU) is a resource managed by the communications system. NAUs provide a window through which users access the communications system network. An NAU is the origin and/or destination for information transmitted through the network. Each NAU has a unique network name that is translated into a unique address.

Note:
Throughout this and related TPF publications, the terms node and NAU have the same meaning.

The communications program must formally connect two NAUs before data may be exchanged. This is referred to as a session. The four types of NAUs are:

An adjacent link station (ALS) is the term used to describe a connection between the TPF system and a channel-attached NCP. With TPF's support of PU 2.1, the NCP appears to TPF as an ALS. ALSs exchange identification (XID) to discover each others characteristics. XID processing is also done for 3174 APPN connections and closely follows PU 2.1 ALS processing.

With SNA CTC, a channel contact procedure is implemented between TPF and channel-attached T5 nodes; that is, an explicit PU5 to PU5 session exists. This PU-PU session is comparable to the PU-PU sessions supported between NCPs. A formal channel protocol between the physical units is used. The session establishment and termination is initiated by the T5 node at the direction of its SSCP.

The TPF system can connect to a PU 2.1 network as either a low-entry networking (LEN) node or an Advanced Peer-to-Peer Networking (APPN) node, but not both. There is a systemwide switch indicating the mode in which the TPF system is running: