SNA Terms and Concepts

Basic APPN Concepts

data. In ISR, once a session route has been established, all data on that session uses the same route. If part of the route fails, the session ends.

•In automatic network routing (ANR), available in network nodes that support APPN's High-Performance Routing (HPR) function, intermediate network nodes can dynamically reroute session traffic if part of the route fails. ANR does not provide intermediate session pacing or segmentation and reassembly.

ANR enables intermediate nodes to route session traffic much faster than is possible with traditional APPN ISR. However, ANR requires additional overhead at the RTP (Rapid Transport Protocol) endpoints. In routes with few intermediate nodes, an ANR route might actually be slower than an ISR route, due to processing time at the endpoints. For routes containing a larger number of intermediate nodes (hops), ANR routes are typically faster. The exact location of the break-even point depends on the efficiency of the RTP nodes.

Direct Connectivity

Direct connectivity enables session traffic to travel directly between two nodes without the need for an APPN network node to route the session. In general, sessions between directly connected nodes can exchange data more quickly than sessions for which data is routed through a network node. For nodes on a shared-access transport facility (SATF)—for example, for nodes on a token ring as shown in Figure 1-9—efficiency would be increased by defining links between every pair of nodes in your network. However, this can be a difficult task—the number of link stations is n × (n−1), where n is the number of nodes in the network.

An APPN network on a token ring is shown in Figure 1-9, “APPN Network Using a Shared-Access Transport Facility.”

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Chapter 1