SNA nodes

An SNA node is a set of hardware and associated software components that implement network functions.

A data communication network can be described as a configuration of nodes and links. Nodes are the network components that send data over, and receive data from the network. Node implementations include processors, controllers, and workstations. Links are the network components that connect adjacent nodes. Nodes and links work together in transferring data through a network.

SNA nodes differ based on the architectural components and the set of functional capabilities they implement. Nodes with different architectural components represent different node types. In SNA, four types of nodes exist:

Type 5 (T5): A T5 node is located only in the mainframe. The software that implements the T5 node is the SNA component of the Communications Server. The SNA component in z/OS is also referred to as VTAM (Virtual Telecommunications Access Method).
Type 4 (T4): A T4 node is a communication controller attached to peripheral nodes through communication lines or LAN, to another communication controller through communication lines, or to a mainframe through an ESCON or a parallel channel. IBM uses special hardware and software to implement the T4 Node. The software is IBM's network control program (NCP) and the hardware is the IBM 3745 or 3746 device. The Communication Controller of Linux (CCL) is a software package that replaces the 3745 or 3746.
Type 2.0 (T2.0): A T2.0 is a peripheral node that attaches to the communication controller or the mainframe. T2.0 is an alias for the IBM 3174 display controller, which attaches 3270 displays and printers and is connected through a communication line to the T4 node or through a channel to the T5 node. Additional devices that implement T2.0 node are banking branch controllers and retail store controllers.
Type 2.1 (T2.1): A T2.1 is a peer-oriented peripheral node that attaches to a mainframe, a communication controller, or another peripheral node. A T2.1 node is called a low entry networking (LEN) node.

The links connecting two subarea nodes, either type 5 to type 5, or type 5 to type 4, or type 4 to type 4 are called transmission groups.

Note: Many of the SNA node types have been replaced by up-to-date hardware and software.

The T2 node was replaced by a workstation (Windows or Unix) that implements software called "3270 emulation" and the banking and retail controller by Windows, Unix, or Linux-based servers.

The 3745 and 3746 hardware is nearing its end of life. The migration to TCP/IP in the backbone reduces the number of lines in the 3745 and 3746. The OSA and the routers can implement most of the functions of the 3745 and 3746 at much lower cost.

One of the alternatives to the 3745/3746 hardware is IBM's Communication Controller of Linux (CCL) software package implemented on the mainframe. CCL uses OSA for NCP (OSN) and routers.

Figure 1 shows the SNA nodes in a subarea network and the connections between the various nodes.

Figure 1. SNA nodes connectivity

Nodes that perform different network functions are said to act in different network roles. It is possible for a given node type to act in multiple network roles. A T4 node, for example, can perform an interconnection role between nodes at different levels of the subarea network hierarchy. When a T2.0 or T2.1 is connected directly to T4 node, the T4 node performs a boundary function. When interconnecting nodes in different subarea networks, the T4 node performs a gateway function.

Every T5 node in a subarea network contains a control point, which in general manages the network resources. Management activities include resource activation, deactivation, and status monitoring.

Note: APPN nodes also implement a control point.

A control point's domain and the range of its capabilities depend on the type of node (APPN or subarea) in which it resides. Regardless of the node type, a control point performs the following common functions:

Manages resources
Monitors and reports on the status of resources