Following
information for reference only, please click product name
below to get more information.
Background
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.
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 predefined, thereby eliminating the
requirement for broadcast traffic and minimizing header overhead.
IBM SNA Architecture
IBM SNA-model components map closely to the OSI reference
model. The descriptions that follow outline the role of each
SNA component in providing connectivity among SNA entities.
• Data-link control (DLC)—Defines several protocols, including
the Synchronous Data Link Control (SDLC) protocol for hierarchical
communication, and the Token Ring Network communication protocol
for LAN communication between peers.
• Path control—Performs many OSI network-layer functions,
including routing and datagram segmentation and reassembly
(SAR).
• Transmission control—Provides a reliable end-to-end connection
service, as well as encrypting and decrypting services.
• Data flow control—Manages request and response processing,
determines whose turn it is to communicate, groups messages
together, and interrupts data flow on request.
• Presentation services—Specifies data-transformation algorithms
that translate data from one format to another, coordinate
resource sharing, and synchronize transaction operations.
• Transaction services—Provides application services in the
form of programs that implement distributed processing or
management services.
Note SNA does not define specific protocols for its physical
control layer. The physical control layer is assumed to be
implemented via other standards .
