Mainframe Access from Multiple Platforms Using TN3270

Introduction

As UNIX systems gain acceptance in large organizations, a need to connect them to existing legacy systems and applications usually becomes an end user requirement. The reasons for the legacy access vary. Organizations may be creating Distributed Computing Environments or exchanging large data base files. However, the most common request from users in these mixed UNIX and mainframe environments is to have terminal access to the mainframe applications. This white paper explores a variety of methodologies and implementations for multi-platform access to mainframe applications using SNA, TCP/IP, Telnet and TN3270.

Traditional Mainframe Access

The mainframe integration of systems that employ non-SNA networking protocols requires that at some point they conform to the defined set of rules that comprise IBM's System Network Architecture. To understand how the integration is achieved, it helps to know a little about SNA.

In large mainframe-based organizations, the traditional method for terminal access to mainframe applications, and the printing of the resultant application data, is accomplished through the use of IBM's System Network Architecture (SNA). SNA can be defined loosely as a set of networking protocols, administration software, and hardware devices that provide physical delivery of those protocols.

The actual network protocols are defined within SNA as Logical Units, or LU's. Each LU provides a different service and they are divided into five basic categories: LU0,1,2,3 and 6.2. LU0 is a general purpose LU for the development of specialized applications. LU1 and LU3 are defined to carry printer data to system and network printers. LU2 defines how terminal related data will be formatted and transmitted. And, LU6.2 is an inter-program communications LU. LU6.2 is also known as APPC (Advanced Program to Program Communications) and is the network underpinning of a wide variety of functions including terminal connectivity as well as file transfer capabilities.

The combination of hardware devices and their associated administration software in a traditional SNA network also have definitions that govern what purpose they serve within the network. The physical devices and their software that transport the protocols and manage and control the resources directly attached to them are defined as Physical Units or PU's. Physical units are divided into four areas, or types. They are PU Types 2, 2.1, 4, and 5. Each LU in an SNA network is assigned to a PU that manages and controls it.