Transmission Control Queue (TCQ) and Index (TCX)

The TCQ index (TCX) is also a VSAM RRDS file. It consists of a record that is a map for controlling the allocation of space in the TCQ.

The control interval (CI) size of the TCQ is from 1,536 to 30,720 bytes. Each Process defined in the TCQ occupies one or more CIs, depending on the number of statements within the Process. Because TCQ access characteristics prohibit secondary allocation, be sure to allocate enough space for the maximum number of potential Processes when you define the TCQ.

The TCX controls space use and access to the TCQ. The TCX average and maximum record sizes should be defined as the CISIZE - 7. To maximize the number of usable TCQ CIs, allocate a record size of 30,713 and a CI of 30,720 to the TCX since it occupies only one track. Sterling Connect:Direct® will use the number of CIs in the TCQ (rounded down to a multiple of 8) or 65536, whichever is smaller.

The largest TCQ that can be allocated and used is 2621 cylinders, on 3390 DASD device, with a CI size of 30,720 and a record size of 30,713. This size can accommodate 65,520 Processes in the queue if no single Process exceeds 30,713 bytes in its internal format.

The following formula shows the number of CIs that can be controlled:

Maximum number of TCQ CIs = ((TCX Max Record Size - 12) / 2) * 8

In the default TCX definition, the definition of the TCX CI Size is 1,024 bytes; maximum record size is 1,017. Substituting the default maximum record size of 1,017 bytes in the formula results in 4,016 TCQ CI records, as shown in the following calculation:

Maximum number of TCQ CIs = ((1,017 - 12) / 2) * 8 = 4,016

Using the default TCX definition, you can define the TCQ with up to 4,016 records, thereby allowing the TCQ to hold up to 4,016 Processes, depending on the number of statements in each Process. However, the default definition for the TCQ specifies 1,000 TCQ CIs but actually holds only about 500 simple one-step Processes).