Shared CTC unit addresses

IOCP calculates the number of unit addresses for a shared ESCON CTC channel path by multiplying the number of unit addresses defined with the UNITADD keyword on the CNTLUNIT statement by the number of unit address ranges for the physical control unit. (See Shared CTC unit address ranges.) The maximum number of unit addresses for a shared ESCON CTC channel path is 512. Because the maximum number of both unit addresses and devices for a shared CTC channel path is 512, you cannot define 512 devices on a shared CTC channel path if one of its control units has more than one unit address range.

For example, if your configuration has 8 LPs and a CTC channel path is shared by 4 LPs in its candidate list, each device assigned to the shared CTC channel path has a device candidate list that defaults to 4 LPs. The physical control unit assigned to the shared CTC channel path has 4 unit address ranges. The maximum number of unit addresses on the control unit is 128. Multiplying this number by the 4 unit address ranges gives 512. Therefore, the maximum number of devices that you can define is only 128 because this configuration first reaches the maximum of 512 unit addresses on the shared CTC channel path.

You can also control the number of devices per shared CTC channel path by specifying a device candidate list for each device assigned to a control unit. However, the number of unit address ranges for a control unit is based on the number of unique LPs in all the device candidate lists of the devices assigned to the control unit.

You can increase the number of unit addresses and devices you can define on a shared CTC channel path up to the maximum of 512 by decreasing to 1 the number of unit address ranges on each control unit assigned to the CTC channel path.