Session-level pacing

Session pacing can be used to influence performance between session partners. Using high-pacing window sizes or no-session pacing for interactive sessions and using lower-session pacing window sizes for batch sessions is a widely used approach to begin tuning an SNA network. For example, a VTAM® application driving a low-speed printer needs to use small session pacing window sizes to keep one session partner from overwhelming the other with data. Using higher (or no) pacing window sizes for interactive sessions than for batch sessions favors the interactive users in the network containing both batch and interactive traffic. For batch, if the session partners and network can support it, higher-pacing window sizes can be used to improve throughput.

Session-level pacing involves each session-layer component in a session path. Session pacing occurs in session stages. Each stage is independently paced, allowing different pacing window sizes to be used for each stage.

There are two types of session pacing:

Fixed: With fixed session-level pacing, the pacing window size is constant. The amount of data that can be transmitted or received between session partners is maintained as a constant window size.
Adaptive: With adaptive session-level pacing, the window size can vary depending on the resources available at each session endpoint. After each window of data is transmitted, the receiver informs the transmitter how large the next window can be.

Adaptive pacing will be used whenever possible. A session path between two LUs may have a mixture of pacing types along the route.

See the SNA Technical Overview for additional information about session-level pacing concepts.