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Chapter 10
Designing for Mainframe Connectivity


ü Discuss the hierarchical and connection-oriented nature of SNA.
ü Describe the use of gateways to attach Token Ring devices to an SNA network.
ü Explain how LLC2 and SDLC sessions are established.
ü Describe reasons for integrating SNA technology with internetworking technology.
ü Examine a client’s requirements and recommend SNA internetworking solutions.
ü Construct SNA designs that replace legacy communications equipment with multiprotocol routers.
ü Build redundancy into SNA internetworks.
ü Design remote source-route bridged SNA internetworks in full-and partial-mesh configurations.
ü Choose the appropriate place to do priority queuing or custom queuing for SNA.

One can easily imagine the mainframe sharing a line from author Samuel Clemens (Mark Twain)—“The report of my death was an exaggeration.” For years, experts predicted the demise of the heavy iron, and while servers have definitely impacted sales of these traditional necessities, it is clear that mainframes will exist in modern networks for some time.

The mainframe was initially designed to be a central processing point in the corporation, sharing resources with hundreds of users on dumb terminals—workstations that could not function without a host. This chapter will focus more on SNA and the evolution from front-end processors and cluster controllers to 3270 terminal emulators than on the mechanics of dumb terminals and the intricacies of the protocol itself. This includes the integration of the mainframe into the modern network design.

Mainframe Overview

It’s best to begin at the beginning, and in mainframe networks that requires an understanding of the traditional dumb-terminal configuration and the protocols that were, and still are, used.

As shown in Figure 10.1, traditional mainframe networks typically incorporate four basic components. These include the host, a front-end processor (FEP), a cluster controller, and dumb terminals. Such installations communicate using SNA. The FEP is responsible for handling all user communications, which frees the host for processing. Under this configuration, the FEP runs the Network Control Program (NCP) and the host typically runs the Virtual Telecommunications Access Method (VTAM) program.

FIGURE 10.1  The traditional mainframe installation

SNA divides each component in the network into one of three logical elements, called Network Addressable Units, or NAUs. These are:

  Logical units (LU)
  Physical units (PU)
  System services control points (SSCP)

These components interact with the data-flow control, transmission control, path control, and data-link control layers of the SNA protocol. Designers must keep in mind that SNA was never designed to operate on the reliable high-speed, variable-delay links found in modern networks. Rather, the protocol was designed for consistent, low-latency, low-delay connections, and sessions can be lost with only the slightest variation. A recurrent theme in SNA is the fact that longer, more complex paths through the network demand greater attention to timers and latency than other protocols, such as IP.

The LUs are further divided into two subcategories. Primary LUs (PLUs) are associated with host applications, while secondary LUs are associated with the end user.

PUs are the actual devices used in communications. However, this component of SNA is responsible for communication with the SSCP as well as the control and monitoring of the physical systems.

The SSCP is part of the VTAM program on the host system. It is responsible for controlling all sessions with the mainframe. These sessions may be divided into domains, creating logical groupings of devices.

SNA is generally considered a hierarchical networking technology. This is more due to the control placed on the domain by the host than the physical and logical design of the topology. The host computer, which is usually the mainframe, groups PLUs and the various host systems. These systems are usually referred to by their individual names, including CICS (Customer Information Control System) and TSO—which are both applications that run in regions on the mainframe. VTAM and SSCP are found at a lower layer of the hierarchy—VTAM and SSCP map closer to an operating system than to applications. One of the benefits of mainframe systems is the isolation between different operations in the machine.

The physical layer of the mainframe is called the channel. This is typically an ESCON (Enterprise System Connect) connection; however, bus and tag is also used. ESCON connections operate at 17MBps (megabytes per second), which is greater than Fast Ethernet in the non-mainframe environment. While they are not as fast as the SuperHPPI (High Performance Parallel Inter-face, capable of 800 MBps) standard and other high-bandwidth technologies, designers must keep in mind that ESCON connections are very efficient and that mainframe data typically involves very small, 2-thousand-byte transfers. While large file transfers do occur, they usually use tape and other high-capacity media.

The FEP is a Type 4 node in SNA, contrasted with the Type 5 designation given the host. This function is typically provided with a 3745 communication controller, which can connect to the network via a Token Ring adapter, or TIC (Token Ring interface coupler). The Type 4 device connects to cluster controllers (devices that provide sessions to dumb terminals) or logical units via SDLC or Token Ring. Ultimately, connections are established between two logical units, which require connections to be established between the SSCP-LU and SSCP-PU. The LU is a logical unit, whereas the PU is a physical unit.

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