The percentage of CSA usage below 16 megabytes. The VTAM address space uses a nominal amount of CSA below the 16MB line. Common storage areas are used by VTAM and other resources in your network to maintain buffers and control blocks in the CSA for z/OS systems. For performance reasons, VTAM offers you some control over its use of common storage areas with the start options. Isolate the cause of the problem using diagnostic procedures outlined in the IBM z/OS Communications Server SNA Network Implementation Guide.