Question: What does Cause No. 99 "information element non-existent or not implemented" indicate in a mobile radio interface Layer 3 specification?
Option 1: The receiving equipment has encountered a mandatory information element that it cannot process.
Option 2: The sending equipment has received a message with an unrecognized information element identifier, but the element is required for processing.
Option 3: The equipment sending the cause has detected a syntax error in the received message.
Option 4: The receiving equipment has encountered an information element that it does not recognize or support, but it can still process the message.
Explanation: Cause No. 99 indicates that the equipment sending this cause has received a message with an unrecognized information element identifier, but the element is not required for processing the message.