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436 Chapter 12: SNA Technology

NOTE

Throughout this chapter, you'll see the terms APPN and subarea. In subarea SNA networks,
physical units need a VTAM to establish and maintain sessions. In APPN SNA networks, the
VTAM is not needed.

SNA Components

For SNA to work properly, it requires a means to provide services to move information through
the network between nodes and to allow the network to be managed and controlled. Those
means are called network addressable units (NAUs). These consist of physical units, logical
units, and system service control points. All of these components are discussed in the following
sections.

Network Addressable Units

NAUs are a major component of the SNA network. Think of NAUs as having unique addresses
and symbolic names. NAUs send and receive messages and communicate with other NAUs in
the SNA network. NAUs consist of physical units, logical units, and control points. They
communicate with each other via a path control network, as shown in Figure 12-2. The path
control network is responsible for reliable, ordered transfer of data between NAUs.

Figure 12-2

Path Control Network

Physical Units

Physical units (PUs) are depicted in Figure 12-3. They represent a set of functions. PUs are
comprised of software, hardware, and microcode; think of a PC or a 3174 front-end processor. Also,

Token

Ring

Token

Ring

New York

Sure is nice having only

one session to the DSPU!

IBM S/370

IBM 3745

FEP

DSPU

Richmond

Internet

300 dumb terminals

Sure glad I am not a PU

anymore. I donÕt have to

maintain that session

with the mainframe.

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