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To Be or Not to Be?

Originally published: Spring 1996

Hamlet uttered this question when he faced the dilemma of deciding between two equally unpleasant choices. This is a state that you, as a VTAM System Administrator, may be facing as well. With increasing session traffic burdens (one of our clients maintains a network of 100 VTAM domains!), it often seems you must decide between flexibility / management and network performance.

This can result in a difficult tradeoff between:

  • Using communications software for managing session traffic, which offers flexible control but weakens system performance, or -
  • Predefining a tightly structured network that guarantees improved system performance but does allow you to make timely, economical modifications.

Traffic Routing Requirements

Furthermore, you must consider two requirements when routing session traffic:

  1. Locating the desired network resources with minimal effort and overhead:
    • By predefining the resources. VTAM allows you to pre-define cross-domain resources (that can define the owning domain of the resource) and adjacent system services control points (SSCPs) so that a search for the requested resource is quick and efficient. However, maintaining these definitions becomes increasingly burdensome as the number of resources grows. It can be especially difficult if those resources are remote from the host or are under the control of a separate organization that may dynamically re-configure such resources.
    • By dynamically searching. Use VTAM to dynamically search for resources by forwarding a request to all known connected VTAM domains (which may in turn forward such a request to its adjacent nodes) until it successfully locates the requested resource. And while this method guarantees maximum network reconfiguration flexibility, it can result in very high overhead.

  2. Rerouting the traffic between the session partners across the optimal communication path for network performance.

    VTAM allows you to define a preferred communication path for a set of session partners using the virtual route definitions in the Class of Service (COS) table entry, which is specified in the Logon mode entry for the session. However, except for very simple networks, this approach can also become unmanageable. At best, it provides limited control because it does not allow for dynamic re-routing based on prevailing traffic loads for the available routes.

    If session traffic routing is controlled by system defaults you cannot control or predict the impact on system performance.

Using an SME: Dynamic Control over Search Routes and Communication Paths

The only alternative to these inefficient approaches is to implement a VTAM Session Management Exit (SME) that gives you dynamic control over the selection of search routes and communication paths. The Network Center's Select Component enables you to control the selection by VTAM of:

  • Gateway Path
  • Adjacent Link Station
  • Adjacent SSCP
  • Network Virtual Route

Dynamic selection delivers optimal network performance without sacrificing flexibility or to incurring an impossible maintenance burden.

You can get both network performance and minimal maintenance without suffering a trade-off! Hamlet should've been so lucky...

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