Question: What does Cause No. 97 "message type non-existent or not implemented" indicate in mobile radio interface Layer 3 specification?
Option 1: The equipment has received a message with an invalid protocol discriminator.
Option 2: The equipment has received a message with a valid but unsupported protocol version.
Option 3: The equipment has encountered an internal error while processing the message.
Option 4: The equipment has received a message with a message type it does not recognize or implement.
Explanation: Cause No. 97 indicates that the equipment sending this cause has received a message with a message type it does not recognize either because this is a message not defined, or defined but not implemented by the equipment sending this cause. This means the equipment is unable to process the message due to lack of recognition or implementation of the specific message type.