Determining MSC link buffer sizes for non-bandwidth mode

In non-bandwidth mode, you do not need to account for responses when determining the size of MSC link buffers.

You also do not need to account for the bandwidth header that is sent with each message when bandwidth mode is used.

In non-bandwidth mode, each send of the buffer contents includes only a single message. Responses are not sent as separate objects in the buffer contents.

Buffer work fields are not included in the contents sent from the buffer.

The TCP/IP link type does not support non-bandwidth mode.

Table 1. Byte format of MSC BUFMS buffers in non-bandwidth mode
Link type Buffer work fields Buffer header Response Message VTAM® work area
CTC 96 6 Not applicable Size of message data, as shown in the 01 or 03 log record, minus 124 bytes Not applicable
MTM 96 6 Not applicable Size of message data as shown in the 01 or 03 log record minus 124 bytes Not applicable
TCP/IP Not applicable Not applicable Not applicable Not applicable Not applicable
VTAM 96 23 Not applicable Size of message data as shown in the 01 or 03 log record minus 124 bytes 196

To calculate buffer sizes to use with MSC links in non-bandwidth mode:

Procedure

  1. Include 96-bytes for the buffer work field.
    The buffer work field is always the first 96 bytes of each buffer. The buffer work field is not sent.
  2. Include the bytes for the buffer header as determined by your link type:
    • For CTC links, include 6 bytes for the buffer header.
    • For MTM links, include 6 bytes for the buffer header.
    • For VTAM links, include 23 bytes for the buffer header.
  3. For VTAM links only, include 196 bytes for the VTAM work area.
    The data in the VTAM work area is not sent with the messages.
  4. Calculate and include the size of the messages that are sent.

    In non-bandwidth mode, the size of the message that is sent is the message size shown in the X'01' and X'03' log records minus 124 bytes. In non-bandwidth mode, certain message prefixes are not sent with the message data and instead are rebuilt by the receiving IMS system.

    You can check the sizes of messages sent by issuing the QUERY MSLINK NAME(linkname) SHOW(STATISTICS) and look at the Hi_Msg_Send_SZ and Avg_Msg_Send_SZ values.

    The Hi_Msg_Send_SZ and Avg_Msg_Send_SZ values are derived from the actual sizes of the messages sent by subtracting 124 bytes from the recorded size in the X'01' and X'03' log records. The sizes in X'01' and X'03' log records include the prefix data and user data segments of each message. The X'01' and X'03' log records are mapped by the QLOGMSGP macro.