Control Unit Image Number Not Available to ESCON Link Service

MORE INFORMATION

The Polaris ESCON link service version 2.14 (with the v1.12 version of the Polaris ESCON configuration DLL) supports the ability to use the Control Unit image number. Here is some additional background describing what the control unit image number is used for:

The control unit image is an ESCON feature. It allows a physical control unit to support up to 16 logical control units. The CU image corresponds to the "CUADD" parameter in the CNTLUNIT IOCP macro or the Logical Address in HCD.

The Polaris ESCON link service supports a maximum of 250 devices over a maximum of 16 CU images. The change to SNA Server makes it easier for customers to configure SNA Server to use ESCON. Currently, in order to allow the customer to use multiple CU images, the customer must configure the link service with the devices they need on each CU image. Then they use the number in the channel address box of the connection to specify an index into a table made from the link service configuration. Here's an example:

Actual configuration:

   CU Image  Base Device Address  Num Devices
      0               20              4
      2               40              2
      3               20              4
				

Channel addresses to input into SNA Server configuration:

   Channel Address   CU Image - Device Address
        00                  0 - 20
        01                  0 - 21
        02                  0 - 22
        03                  0 - 23
        04                  2 - 41
        05                  2 - 42
        06                  3 - 20
        07                  3 - 21
        08                  3 - 22
        09                  3 - 23
				

This makes configuration more difficult. With this new feature, customers still must configure the link service as before because the ESCON channel adapter needs to know at initialization (ie. when SNA Server is started) which devices it will be using. But in the connection properties the user can put in address 20 CU image 0 instead of just address 00 and so on.

The link service configuration has a check box which allows the user to choose to use the new method. This is not done automatically. The link service has to be told which of the two methods is in use since the field that contains the CU image was 0 for old version of SNA Server, but 0 is a valid CU image. Regarding the use of the Polaris ESCON link service, this change will not affect users who don't want or need to change their link service configuration. They will still be able to update the link service and driver to newer version with having to redo any part of their configuration.