The term virtual route pacing applies to the monitoring and flow control mechanisms that are used for virtual routes. On a network-wide basis, subarea nodes (VTAM® and NCP) continuously monitor the amount of traffic in the network. If congestion occurs, these subarea nodes can limit the amount of data they send over the affected virtual routes until the congestion clears. The monitoring of data flow and limiting of data occurs automatically, requiring no action by users or domain operators.

Activation of a virtual route includes initializing the pacing count. This count, the virtual route window size, indicates the number of path information units (PIUs) that are allowed to flow on a virtual route before the subarea node receiving the PIUs authorizes the sending of more data. The set of PIUs that can be sent is called a window. This value fluctuates between a minimum and a maximum value depending on network congestion. (Network congestion is determined by how many messages are queued.) You can affect how VTAM does virtual route pacing through the PATH definition statement or the VR pacing window size calculation module. The minimum and maximum values can be specified on the NCP PATH definition statements and the VTAM PATH definition statements that define the virtual routes.