Section: 4.7.1.8a Establishment of the PS signalling connection (Iu mode only)
Content: In order to route the NAS message to an appropriate SGSN, the MS NAS provides the lower layers with the routing parameter according to the following rules: a) if the TIN indicates "P-TMSI" or "RAT-related TMSI", and the MS holds a valid P-TMSI, the MS NAS shall provide the lower layers with the P-TMSI; b) if the TIN indicates "GUTI" and the MS holds a valid GUTI allocated by an MME, the MS NAS shall provide the lower layers with the P-TMSI mapped from the GUTI (see 3GPP TS 23.003[ Numbering, addressing and identification ]  [10]); c) if the TIN is not available and the MS holds a valid P-TMSI, the MS NAS shall provide the lower layers with the P-TMSI; or d) if the TIN is not available and the MS holds a valid GUTI allocated by an MME, but no valid P-TMSI, the MS NAS shall provide the lower layers with the P-TMSI mapped from the GUTI (see 3GPP TS 23.003[ Numbering, addressing and identification ]  [10]). When an ATTACH REQUEST message, or a ROUTING AREA UPDATE REQUEST message after a routing area change, is sent to establish a PS signalling connection, the MS NAS also provides the lower layers with the DCN-ID according to the following rules: a) if a DCN-ID for the PLMN code of the selected PLMN is available in the MS, this DCN-ID the MS NAS shall provide this DCN-ID to the lower layers; or b) if no DCN-ID for the PLMN code of the selected PLMN is available but a Default_DCN_ID value is available in the MS, as specified in 3GPP TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [135] or in USIM file NASCONFIG as specified in 3GPP TS 31.102[ Characteristics of the Universal Subscriber Identity Module (USIM) application ]  [112], the Default_DCN_ID value MS NAS shall provide the Default_DCN_ID value to the lower layers.