For NCP Version 4 Release 2 and later releases, the UNITSZ value specified in the NCP HOST definition statement is no longer used. If coded, the value is ignored. The value of VTAM IOBUF size is determined by NCP during XID exchange. For NCP releases before 4.2, the value of UNITSZ can affect VTAM, NCP, and network performance. This section provides guidelines for specifying this value.

The initial amount of storage you allocate for the I/O buffer, and the xpanno value you specify, depends on the bufsize specification. This, in turn, depends on the UNITSZ value specified in both the NCP HOST definition statement and the buffer pool start option for the I/O buffer. In the case of channel-attached cluster controllers, bufsize depends only on the buffer pool start option for the I/O buffer. The value in the VTAM start option is used by both VTAM and the NCP when they exchange information. The bufsize value entered as part of the buffer pool start option for the I/O buffer is interpreted by VTAM as the UNITSZ value used, except for channel-attached cluster controllers, where UNITSZ is IOBUF minus 16. The I/O buffer size, as it appears on the console after a DISPLAY BFRUSE operator command, is greater than the bufsize that was specified in the buffer pool start option because VTAM adds control information to each I/O buffer.