TM's Blog for 3GPP Spec

當UE要送NAS Message給UTRAN時，RRC需要先建立一條Singalling Connection，NAS Message (如CM Service Request)才能藉由這個Connection傳送到網路端。而Initial Direct transfer procedure的目的，就是用來建立一條Uplink Signalling Connection。

1. 如果UE在Idle Mode下，(原則上)UE送出Initial Direct transfer後，後續要執行RRC Connection Establishment procedure。如果RRC Connection Establishment procedure失敗，則RRC需通知NAS並結束這個procedure

2. 如果UE的RRC State為CELL_PCH or URA_PCH，Initial Direct transfer procedure會將RRC State轉變成CELL_FACH

3. 當Initial Direct transfer Message成功傳送到網路端，經RLC確認後，這個procedure即結束。後續因為Uplink Signalling Connection已建立完成，UE即可透過Uplink Direct transfer傳送NAS Message

4. 訊息中有一參數: EstablishmentCause用來告知網路端建立Uplink Signalling Connection的目的。可能的值為:

(1) Originating Conversation Call

(2) Originating Streaming CallA

(3) Terminating Conversation Call

(4) Emergency Call

(5) Inter-RAT cell re-selection

(6) Registration

(7) Detach

(8) Call re-establishment

(9) …

5. CN domain identity: 標示這個Uplink Signalling Connection是for CS or PS domain