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IBM's Systems Networking Architecture (SNA) provides a complete structure for transferring data between a variety of computing platforms. Originally developed to provide for economical communications between mainframe computers and display (dumb) terminals for inquiry/response applications, the architecture has expanded and developed to support peer-to-peer and newer client/server networking needs. The architecture is extremely well-defined and encompasses all of the data communications functionality required to send data accurately over a wide variety of media. At the frame layer, SNA is usually supported across trunk facilities using the Synchronous Data Link Control (SDLC) protocol. An SNA message structure consists of a Request/Response Unit of information to be transferred, preceded by a Response/Request Header (RH) to form a Basic Information Unit (BIU). This BIU is then preceded by a Transmission Header to form a Basic Transmission Unit (BTU). The BTU is placed in an SDLC frame for transit across a wide area network. With all the different FID types, PU types, LU types, it's clear that IBM meant for SNA to be so daunting that you'd always need them to keep it running for you, that's where it's a good fit, it's extremely reliable. Click here to see a representation of the SNA/SDLC protocol structure.

SNA/SDLC Protocol Structure

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