Determining MSC link buffer sizes for bandwidth mode

When MSC links run in bandwidth mode, you need to account for some additional bytes when determining the size of your link buffers.

In bandwidth mode each send of the buffer contents can include multiple responses and messages. However, if nothing else is on the output queue, a send of the buffer contents can include only one response or one message.

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

Table 1. Byte format of MSC BUFMS buffers in bandwidth mode
Link type Buffer work fields Buffer header Each response Each message VTAM® work area
CTC 96 128 96 Size of message data, as shown in the X'01' or X'03' log record, plus 144-byte bandwidth prefix N/A
MTM 96 128 96 Size of message data, as shown in the X'01' or X'03' log record, plus 144-byte bandwidth prefix N/A
TCP/IP 96 240 96 Size of message data, as shown in the X'01' or X'03' log record, plus 240-byte bandwidth prefix N/A
VTAM 96 128 96 Size of message data, as shown in the X'01' or X'03' log record, plus 144-byte bandwidth prefix 196
To calculate buffer sizes to use with MSC links in 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 that is in the front of each buffer sent. The size of the buffer header is determined by your link type:
    • For CTC links, include 128 bytes for the buffer header.
    • For MTM links, include 128 bytes for the buffer header.
    • For TCP/IP links, include 240 bytes for the buffer header.
    • For VTAM links, include 128 bytes for the buffer header.
  3. For VTAM links only, include 196 bytes for the VTAM work area.
  4. Include 96 bytes for responses.
    Each response is 96 bytes. The number of responses depends on the number of messages sent. Responses are inserted into the buffer first.
  5. Determine the size of the messages being sent.

    The size includes both the message prefix, data, and a 144-byte bandwidth prefix.

    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 sizes returned by the QUERY MSLINK command include only the message prefix and data, but not the bandwidth prefix. The sizes returned are the same as the type X'01' and type X'03' queue manager message sizes.

    The Hi_Msg_Send_SZ and Avg_Msg_Send_SZ values are derived from the actual sizes of the messages sent, as recorded 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.