Sidebar: IBM Mainframe Hardware
An IBM mainframe consists of many computing devices
Centered on these devices is the CPU. The existence
of these devices
allows the CPU to offload computing power relative to
Over the years, IBM developed a standard called "Systems
Architecture (SNA)" for these devices.
To understand SNA, it is necessary to understand the
term SNA User.
An "SNA User" is simply a person or application
SNA network to communicate with another user. SNA creates
or logical paths between users so they can communicate.
are called "Logical Units" (or LUs). An LU
is a logical port,
not a physical port, to which a user connects. The LU
through microcode that resides on the devices in the
An LU is identified by a number.
Along with LUs, SNA uses "Physical Units"
(or PUs) that
represent the actual devices on the SNA network. "SNA
are physical points in the SNA network that contain
one or more network
components. Since an SNA Node represents an actual physical
there is an SNA PU associated with the device.
There are two major types of SNA Nodes: "Peripheral
and "Subarea Nodes." Peripheral Nodes are
attached to Subarea
Nodes. A Peripheral Node only talks to the Subarea Node
to which it
is attached. Peripheral Nodes are also known as "Terminal
Controllers." User terminals are connected to these.
type of Cluster Controller is Type 2. The emulation
for these terminals is "3270 Terminal Emulation."
can be configured as either "CUT Mode," which
a single session, or "DFT (Distributed Function
providing up to five sessions over a single line. Unlike
Nodes, Subarea Nodes can talk with other Subarea Nodes
on the SNA
Network. There are two types of Subarea Nodes: Type
5 and Type 4.
Type 5 is usually called a "Host Node." It
refers to the CPU.
Type 4 is usually called a "Communications Controller
or Front-End-Processor (FEP). The communications controller
or many cluster controllers attached to them. For a