LTE: Attach Request

The UE includes ATTACH REQUEST (initial-NAS) message within RRC CONNECTION SETUP COMPLETE message. The IE dedicatedInfoNAS in RRC CONNECTION SETUP COMPLETE includes this initial-NAS message

The IEs (some of them are explained below) in ATTACH REQUEST message include EPS attach type, EPS mobile identity, UE network capability, ESM message container, Old P-TMSI signature, Additional GUTI, Last visited registered TAI, DRX parameter, Old location area identification, Additional update type, Voice domain preference and UE's usage setting etc…

The IE EPS Attach Type indicates the purpose of the attach procedure. The value can be EPS attach (to attach for EPS services only) or combined EPS/IMSI attach (to attach for both EPS and non-EPS services) or EPS emergency attach (to attach for emergency bearer services)

The IE EPS Mobile Identity can be GUTI, IMSI or IMEI. The UE shall include GUTI if it holds a valid GUTI (either native GUTI or mapped GUTI). If there is no valid GUTI available, the UE shall include the IMSI as EPS Mobile Identity IE. If the UE is attaching for emergency bearer services and does not hold a valid GUTI, P-TMSI or IMSI as described above, the IMEI shall be included in the EPS mobile identity IE

The purpose of the ESM message container IE is to enable piggybacked transfer of an ESM message within an EMM message (This may contain IEs for example, EPS bearer identity, procedure transaction identity, PDN connectivity request, PDN type, Request type, ESM information transfer flag, EIT (ESM information transfer), Protocol Configuration Options (Configuration Protocol, DNS Server Address Request etc..)

The IE Additional Update Type provides additional information about the type of request for a combined attach procedure. Bit1 value 0 means that there is “no additional information” and 1 means that “SMS only”

Voice domain preference and UE's usage setting IE shall be included if the UE supports CS fallback and SMS over SGs, or if the UE is configured to support IMS voice, but does not support 1xCS fallback. This IE provides the network with the UE's usage setting and the voice domain preference for E-UTRAN. Refer to 3GPP TS 24.008, section 10.5.5.28 for detailed information

         The IE Last Visited Registered TAI shall be included if the UE holds a valid last visited registered Tracking Area Identity (TAI)

Reference: 3GPP TS 24.301 and 3GPP TS 24.008

Example: Attach Request Message

The decoded content of  ESM message container IE is presented below