Using the MAXBUF operand, you can specify the number of VTAM® I/O buffers allocated for inbound data transfer. You need to set MAXBFRU on both the NCP HOST definition statement and the VTAM GROUP or LINE definition statements for channel-attached NCPs or SNA controllers. When VTAM creates a read channel program, it must always have MAXBFRU number of I/O buffers available for reading from the channel-attached device. VTAM always attempts to hold in reserve MAXBFRU number of I/O buffers for the next read from the channel-attached device.

VTAM and NCP do not support segmenting PIUs across channel connections between them. Therefore, ensure MAXBFRU times IOBUF size accommodates the largest PIU that NCP can send to VTAM. NCP informs VTAM the maximum size PIU that VTAM can send in its XID2 (this is based on TRANSFR*BFRS-18). VTAM also honors the MAXDATA keyword coded on the PCCU definition and will send PIUs to NCP no larger than the smaller of the MAXDATA keyword or what VTAM received from NCP in the XID2. Therefore, you should ensure that MAXDATA and TRANSFER*BFRS-18 is larger than the largest PIU VTAM can send to NCP.

In addition, choose MAXBFRU so that more messages can be coattailed. The MAXBFRU value determines when an attention interrupt is signaled to the host. MAXBFRU=5 is a good starting point. There should be enough buffers available to transfer all the messages likely to arrive in the DELAY time period. (The use of MAXBFRU=5 assumes one inbound message fits in one I/O buffer. More details on I/O buffer size are given in the following paragraphs.) You can use tuning statistics to verify that MAXBFRU is sufficient for coattailing. Using the example in Figure 3, for instance, MAXBFRU could have been set to 2. Instead, it was set to 6 to maximize coattailing.