Section: 8.4 Unknown or unforeseen message type
Content: If a mobile station receives an RR, MM or CC message with message type not defined for the PD or not implemented by the receiver in unacknowledged mode, it shall ignore the message. If a mobile station receives an RR, MM or CC message with message type not defined for the PD or not implemented by the receiver in acknowledged mode, it shall return a status message (STATUS, MM STATUS depending on the protocol discriminator) with cause # 97 "message type non-existent or not implemented". If a mobile station receives a GMM message or SM message with message type not defined for the PD or not implemented by the receiver, it shall return a status message (GMM STATUS or SM STATUS depending on the protocol discriminator) with cause # 97 "message type non-existent or not implemented". If the network receives an MM message with message type not defined for the PD or not implemented by the receiver in a protocol state where reception of an unsolicited message with the given PD from the mobile station is not foreseen in the protocol, the network actions are implementation dependent. Otherwise, if the network receives a message with message type not defined for the PD or not implemented by the receiver, it shall ignore the message except that it should return a status message (STATUS, MM STATUS, GMM STATUS or SM STATUS depending on the protocol discriminator) with cause #97 "message type non-existent or not implemented". NOTE: A message type not defined for the PD in the given direction is regarded by the receiver as a message type not defined for the PD, see 3GPP TS 24.007[ Mobile radio interface signalling layer 3; General Aspects ]  [20]. If the mobile station receives a message not compatible with the protocol state, the mobile station shall ignore the message except for the fact that, if an RR connection exists, it returns a status message (STATUS, MM STATUS depending on the protocol discriminator) with cause #98 "Message type not compatible with protocol state". When the message was a GMM message the GMM-STATUS message with cause #98 "Message type not compatible with protocol state" shall be returned. When the message was a SM message the SM-STATUS message with cause #98 "Message type not compatible with protocol state" shall be returned. If the network receives a message not compatible with the protocol state, the network actions are implementation dependent. NOTE: The use by GMM and SM of unacknowledged LLC may lead to messages "not compatible with the protocol state".