C H A P T E R37
Introduce the SNA protocol, used primarily by mainframe systems and terminals.
• Describe the structures and functioning of this protocol, from its introduction in the early 1970s to
its current form.
• Describe IBM peer-based networking.
• Describe the basic information unit (BIU) format.
• Describe the path information unit (PIU) format.
IBM Systems Network Architecture Protocols
IBM networking today consists of essentially two separate architectures that branch, more or less, from
a common origin. Before contemporary networks existed, IBM’s Systems Network Architecture (SNA)
ruled the networking landscape, so it often is referred to as traditional or legacy SNA.
With the rise of personal computers, workstations, and client/server computing, the need for a peer-based
networking strategy was addressed by IBM with the creation of Advanced Peer-to-Peer Networking
(APPN) and Advanced Program-to-Program Computing (APPC).
Although many of the legacy technologies associated with mainframe-based SNA have been brought
into APPN-based networks, real differences exist. This chapter discusses each branch of the IBM
networking environment, beginning with legacy SNA environments and following with a discussion of
APPN. The chapter closes with summaries of the IBM basic-information unit (BIU) and
path-information unit (PIU).
IBM-based routing strategies are covered in a separate chapter. Refer to Chapter 41, “IBM Systems
Network Architecture Routing,” for details about IBM routing protocols.
Traditional SNA Environments
SNA was developed in the 1970s with an overall structure that parallels the OSI reference model. With
SNA, a mainframe running Advanced Communication Facility/Virtual Telecommunication Access
Method (ACF/VTAM) serves as the hub of an SNA network. ACF/VTAM is responsible for establishing
all sessions and for activating and deactivating resources. In this environment, resources are explicitly
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