If a MS supports Phase 2 3GPP TS 24.010 error handling and the Phase 2 TS 24.080
extension mechanism it shall send the screening indicator to the network during layer 3 connection establishment. The value of the indicator shall indicate Phase 2. The sending of the screening indicator does not depend upon the invocation of any supplementary service.
At layer 3 connection establishment with the MS, the network shall check for the SS screening indicator and note, for the duration of the connection, whether the indicator was sent, and if sent, the value of the indicator.
On invocation of any network initiated SS operation (unless an SS version indicator has taken precedence over the screening indicator) the network shall check the screening indicator status. If the screening indicator was not sent, the network shall screen information sent to the MS, i.e. invoke the Phase 1 version of the operation or abort the invocation if only a Phase 2 version is available. If the screening indicator was received, indicating that Phase 2 error handling and extension mechanisms are supported at the MS, the network shall invoke the highest supported version of the operation toward the MS.
According to this version of the standards the highest version is Phase 2. However when the next version of standards is available, new services may also be invoked. If the MS does not support the service the error handling or extension mechanism will handle unrecognised information cleanly.
If in the future a new value is assigned to the screening indicator, new screening procedures may also be defined for networks of similar or higher capability. These procedures cannot be predicted and no definition is required in this version of the standards.
If the value of the screening indicator is unrecognised the network shall attempt to handle network initiated SS operations as if the MS had indicated the highest values supported by the network.
The indicator has been defined in such a way that it is ignored when received by a Phase 1 network therefore no Phase 1 procedures are described.