VTAM® examines the HOST definition statement to determine the amount of data that VTAM must be prepared to receive from the NCP over the channel. The value VTAM uses is the MAXBFRU specification times the size of VTAM buffers (IOBUF size). In exchange identification (XID) processing, VTAM informs NCP of the maximum amount of data that it can receive from the NCP. NCP also informs VTAM of the maximum amount of data that it can receive. The value NCP uses in the XID process is the NCP buffer size (BFRS) times the value specified on the TRANSFR operand of either the BUILD or LINE definition statements. VTAM compares this value to the MAXDATA operand on the PCCU definition statement and returns the smaller of the two values.

Sample HOST definition statement

Following is a sample HOST definition statement:

VTAM116  HOST  BFRPAD=0,           BUFFER PAD FOR VTAM
               MAXBFRU=50,         INITIAL UNITS FOR INPUT
               SUBAREA=116,        HOST SUBAREA
               UNITSZ=512          BUFFER UNIT SIZE

Note: With NCP Version 4 Release 2 and later releases, the UNITSZ specification for an NCP is ignored by VTAM and is no longer used in channel programming. If specified, it should match the size specified for the IOBUF buffer pool start option, but it does not affect the success or failure of link establishment.

Specifying a value for MAXDATA

For NCP Version 4 Release 3 and later releases, the MAXDATA operand is redundant and need not be specified. MAXDATA is set to 65 535 by default. The maximum size of data transfers between VTAM and NCP is established during XID exchange. For NCP releases before 4.3, the value of MAXDATA can affect VTAM, NCP, and network performance. The following section provides guidelines for specifying a MAXDATA value.

The MAXDATA operand should be smaller than or equal to the size of the largest PIU the network will handle. Use the following criteria for choosing the value of MAXDATA:

• MAXDATA is related to the BFRS and TRANSFR operands of the BUILD definition statement. MAXDATA should not exceed the product of BFRS and TRANSFR, minus 18 (BFRS x TRANSFR - 18).
• MAXDATA should not exceed the size of the VTAM IOBUF buffer pool.
• MAXDATA should not exceed the product of the MAXBFRU and UNITSZ values for the NCP.
• If a MODIFY LOAD command will be issued for a local or remote NCP, the size of MAXDATA must be at least 2082, which is 2048 plus the size of the PIU header (34).
• Although MAXDATA applies only to the PCCU statement for a channel-attached NCP, its value should not exceed the capacities of any link-attached or cross-domain NCPs. An SNA path error can occur if MAXDATA exceeds the capacity of any NCP along the route taken by a message.

For more information about selecting the values for VTAM, see Tuning VTAM for your environment.