Systems Network Architecture (SNA)

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Definition of Systems Network Architecture (SNA) in The Network Encyclopedia.

What is SNA (Systems Network Architecture)?

SNA stands for Systems Network Architecture, is a set of IBM mainframe networking standards and protocols introduced in 1974. Systems Network Architecture (SNA) originally defined a centralized architecture with mainframe hosts controlling terminals, but it has also been adapted for peer-to-peer communication and distributed client/server computing environments.

SNA includes services for configuring and managing system resources within an IBM mainframe networking environment.

IBM SNA Architecture
IBM SNA Architecture.

How SNA Works?

SNA has seven protocol layers and is similar but not identical to the Open Systems Interconnection (OSI) reference model, whose development it influenced. The SNA protocol suite includes the following:

  • Synchronous Data Link Control (SDLC) protocol: For data-link layer control of the flow of frames within an SNA network. SNA also supports IEEE 802.5 and 802.2 token passing with Logical Link Control (LLC). 
  • Network Control Program (NCP): For routing, segmentation, and framing functions. NCP usually runs on the host or on the front-end processor. 
  • Virtual Telecommunications Access Method (VTAM): For sequencing, flow control, error recovery, and session management functions. You use VTAM to implement Network Accessible Units (NAUs), which control the flow of data, in an SNA network. 
  • Advanced Peer-to-Peer Networking (APPN): Enables SNA connections between two hosts, such as a PC host accessing an application running on a mainframe host using Advanced Program-to-Program Communications (APPC) sessions. You use APPN to implement Physical Units (PUs) and Logical Units (LUs), which are forms of NAUs that control communication processes for hosts and terminals. LUs represent SNA end nodes such as connections by users or applications, and two LUs communicate by using associated PUs, which are hardware devices or terminals. A number of types of LUs and PUs are used in an SNA networking environment. 
  • NetView: A network management program for configuring, controlling, troubleshooting, and usage accounting of SNA networks. 

Before data can be transferred over SNA, a session must be established between an LU on the client and an LU on the host. For example, a Microsoft Windows NT–based or Windows 2000–based server running Microsoft SNA Server can connect to a mainframe host by using SNA. SNA Server provides connectivity between Windows and SNA environments by providing an SNA gateway running on a Windows NT–based or Windows 2000–based server. Windows clients can then connect to the SNA mainframe host by going through the SNA Server gateway.

By using LU 6.2, which is a peer-to-peer protocol, the Windows NT–based server running SNA Server or the mainframe host can initiate the user session. Clients on a Windows NT-based or Windows 2000-based network can then access data stored on the host, including data stored in structured or unstructured AS/400 or Virtual Storage Access Method (VSAM) files, DB2 database tables, and transaction processing monitors.

Non-SNA architectures

Non-SNA architectures such as Token Ring networks can interface with SNA networks using Service Points (SPs).

SNA Distribution Services (SNADS)

SNADS stands for SNA Distribution Services, which is the email messaging transport protocol for IBM’s Systems Network Architecture (SNA). SNADS is a mainframe host-based messaging system that is commonly used in SNA networking environments. Microsoft Exchange Server includes a connector for enabling messaging connectivity between SNADS mail systems and Exchange servers.

You can use Microsoft SNA Server to provide the underlying network connectivity for this connector to function. The SNADS connector allows Exchange Server to leverage the functionality of existing host-based messaging systems such as AS/400 and System 3x during migration to a distributed client/server-based environment.