发现很多人咨询这个问题，其实在协议中是这样写的：
3GPP: 24008 R8（不同版本可能不同）：
L.1 Mapping of NAS procedure to RRC establishment cause(Iu modeonly)
When MMrequests the establishment of a RR connection, the RRC establishment cause usedby the MS shall be selected according to the CS NAS procedure as specified intable L.1.1.
Table L.1.1/3GPP TS 24.008: Mapping ofCS NAS procedure to establishment cause
CS NAS procedure
RRC Establishment cause (according 3GPP TS 25.331 [23c])
Originating CS speech call
Originating Conversational Call
Originating CS data call
Originating Conversational Call
CS Emergency call
Emergency call
Call re-establishment
Call re-establishment
Location update
Registration
IMSI Detach
Detach
MO SMS via CS domain
Originating Low Priority Signalling
Supplementary Services
Originating High Priority Signalling
Answer to circuit switched paging
Set equal to the value of the paging cause used in the reception of paging in the RRC layer
Answer to paging for CS fallback
Terminating Conversational Call
SS part of Location services
Originating High Priority Signalling
When GMMrequests the establishment of a PS signalling connection, the RRC establishmentcause used by the MS shall be selected according to the PS NAS procedure asspecified in Table L.1.2.
Table L.1.2/3GPP TS 24.008: Mapping ofPS NAS procedure to establishment cause
PS NAS procedure
RRC Establishment cause (according 3GPP TS 25.331 [23c])
GPRS Attach
Registration
Routing Area Update – for the case of ‘Directed Signalling Connection Re-Establishment (see chapter 4.7.2.5.)
Call Re-Establishment
Routing area Update – all cases other than ‘Directed Signalling Connection Re-Establishment
Registration
GPRS Detach
Detach
Request to re-establish RABs
Either ‘Originating Conversational Call’ or ‘Originating Streaming Call’ or ‘Originating Interactive Call’ or ‘Originating Background Call ‘ – depending on the Traffic Class in QoS of the "most demanding" RAB. (see Note 1)
Request to establish a PS signalling connection for MBMS
MBMS reception or MBMS p-t-p RB request
Activate PDP Context
Either ‘Originating Conversational Call’ or ‘Originating Streaming Call’ or ‘Originating Interactive Call’ or ‘Originating Background Call ‘ – depending on the Traffic Class in QoS of the "most demanding" RAB. (see Note 1) –
If Traffic Class in QoS is not ‘Conversational Class’ or ‘Streaming Class’ or ‘Interactive Class’ or ‘Background Class’ but is ‘Subscribed Traffic Class’, then ‘Originating Subscribed traffic Call’ shall be used.
Modify PDP Context
Originating High Priority Signalling
Deactivate PDP Context
Originating High Priority Signalling
MO SMS via PS domain
Originating Low Priority Signalling
SS part of Location services
Originating High Priority Signalling
Answer to packet paging
Set equal to the value of the paging cause used in the reception of paging in the RRC layer
NOTE 1: For classification of "most demanding" Traffic Class the following ranking order applies: "Conversational" followed by "Streaming" followed by "Interactive" followed by "Background", where "Conversational" is the most demanding Traffic class in terms of being delay sensitive.
In chosing the "most demanding" Traffic Class all already active PDP Context together with the PDP Context to be activated shall be considered
NOTE: The RRC establishment cause may be used bythe network to prioritise the connection establishment request from the MS athigh load situations in the network.
