1. Essentials Guide
  2. Oracle USM Supporting the IMS Core
  3. S-CSCF Selection Based on Capabilities
  4. Selection Process without SLRM

Selection Process without SLRM

The capabilities-oriented S-CSCF selection algorithm on the Oracle CSM and Oracle USM S-CSCF include selections based on mandatory and optional capabilities information received from HSS and the configured S-CSCF Capabilities Database.

The general approach to selection within this scenario include the following principles:

  • Only S-CSCFs with all mandatory capabilities can be selected.
  • The process gives priority to the S-CSCF with the most optional capabilities.
  • The process gives priority to the local S-CSCF.
  • The system attempts to spread assignments to remote S-CSCFs of the same priority.

The capabilities-oriented S-CSCF selection algorithm uses the following high-level steps within the I-CSCF function to arrive at a selection:

  1. Determine that the capabilities algorithm is required:
    1. No server-name in the LIA or UAA.
    2. Capability list exists.
    3. Assigned S-CSCF flag is not set.
    4. Mandatory/Optional Capabilities received in UAA/LIA.
  2. Identify potential S-CSCFs, which must support all mandatory capabilities:
    1. Ensure the S-CSCF capabilities database is configured.
    2. Build capable S-CSCF list. This list contains all S-CSCFs from the S-CSCF capabilities database that support the Mandatory capabilities.
    3. Ensure that the capable S-CSCF list is not empty. If the capable S-CSCF list is empty, return an error to the UE.
  3. Ensure that the I-CSCF is not SLRM.
  4. Complete capabilities selection process using optional capabilities as criteria:
    1. An S-CSCF has the most optional capabilities.

      (If so, forward.)

    2. The local S-CSCF can take on more users, has all mandatory capabilities, and has most optional capabilities.

      (If so, forward locally.)

    3. Use round robin to select the S-CSCF that has most optional capabilities.

      (If so, forward.)

  5. Forward message:
    1. Forward to selected S-CSCF.
    2. Remove selected S-CSCF from capabilities list.
    3. If there is an error, for example, the SIP response requires a re-assignment, check the assigned flag.
    4. If the assigned flag is set, return to the top.

      If the assigned is not set, return to the step that checks whether the capable S-CSCF list is empty.

    5. If the capable S-CSCF list is empty, return an error to the UE.

      If the capable S-CSCF list is not empty yet, perform capabilities selection process using optional capabilities as criteria again.