LTE: Attach Complete

The ATTACH COMPLETE message is sent by the UE to the network in response to an ATTACH ACCEPT message

When the UE receives the ATTACH ACCEPT message combined with the ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message, it shall forward the ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message to the ESM sublayer. Once the ESM sublayer indicates that the default EPS bearer context has been activated, the UE shall send an ATTACH COMPLETE message together with an ACTIVATE DEFAULT EPS BEARER CONTEXT ACCEPT message contained in the ESM message container IE to the network

If the ACTIVATE DEFAULT BEARER CONTEXT REQUEST message combined with the ATTACH ACCEPT is not accepted by the UE due to failure in the UE's ESM sublayer, then the UE shall not send ATTACH COMPLETE message. Instead, it shall initiate the detach procedure by sending a DETACH REQUEST message to the network. Further UE's behavior is implementation specific

Reference: 3GPP TS 24.301

Example: ATTACH COMPLETE Message

LTE: Attach Reject

If the UE initiated ATTACH REQUEST cannot be accepted by the network, the MME shall send an ATTACH REJECT message to the UE including an appropriate EMM cause value

If the attach procedure fails due to a Default EPS bearer Setup Failure or an ESM procedure failure or operator determined barring is applied on Default EPS Bearer Context Activation during attach procedure, the MME shall combine the ATTACH REJECT message with a PDN CONNECTIVITY REJECT message contained in the ESM Message Container IE. In this case the EMM Cause value in the ATTACH REJECT message shall be set to #19 "ESM failure"

If the attach request is rejected due to NAS level congestion control, the network shall set the EMM cause value to #22 "congestion" and optionally assigns a back-off timer T3346

Reference: 3GPP TS 24.301

Example: ATTACH REJECT Message

LTE: Attach Accept

The ATTACH ACCEPT message is sent by the network to the UE to indicate that the corresponding ATTACH REQUEST has been accepted. Typically, the ATTACH ACCEPT message is piggy-backed on RRC CONNECTION RECONFIGURATION message

The result of the EPS attach is indicated in the IE EPS Attach Result. The result can be either “EPS only” or “combined EPS/IMSI attach”. The result "combined EPS/IMSI attach" indicates that the attach request for EPS and non-EPS services, or for EPS services and "SMS only" have been successful. The result "EPS only" indicates that the attach request for EPS services (only) has been successful but attach for non-EPS services or "SMS only" (if requested by the UE in the ATTACH REQUEST) has failed

When the default bearer is activated as part of the attach procedure, the MME shall send the ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message together with ATTACH ACCEPT message. The IE ESM Message Container is used for this purpose. The network may also initiate the activation of dedicated bearers towards the UE by invoking the dedicated EPS bearer context activation procedure

The GUTI Reallocation may be part of the attach procedure. If the ATTACH REQUEST message includes the IMSI or IMEI, or if the MME considers the GUTI provided by the UE is invalid, or if the GUTI provided by the UE was assigned by another MME, the MME shall allocate a new GUTI to the UE. The MME shall include in the ATTACH ACCEPT message the new assigned GUTI together with the assigned TAI list

The IE TAI List indicates a list of tracking areas for which the UE doesn't need to perform a tracking area updating procedure when entered one of these TAs (in the list). The TAIs in a TAI list assigned by an MME to a UE belongs to the same MME area

The MME includes EMM Cause IE when IMSI attach for non-EPS services is not successful during a combined EPS/IMSI attach procedure

The purpose of the EPS network feature support IE is to indicate whether certain features are supported by the network. It can indicate the following features whether supported or not: IMS voice over PS session indicator (IMS VoPS), Emergency bearer services indicator (EMC BS), Location services indicator in EPC (EPC-LCS), Location services indicator in CS (CS-LCS), Support of EXTENDED SERVICE REQUEST for packet services (ESRPS)

The purpose of the Additional update result IE is to provide additional information about the result of a combined attach procedure. 00-no additional information; 01-CS Fallback not preferred; 10-SMS only; 11-reserved 

Reference: 3GPP TS 24.301

Example: ATTACH ACCEPT Message

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

LTE: System Information Block Type 7

Direction: E-UTRAN => UE
Signalling Radio Bearer: N/A
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: DL-SCH

The SystemInformationBlockType7 (SIB7) contains inter-RAT cell re-selection information only for GERAN. It includes cell re-selection parameters for each frequency. SIB7 also contains cell reselection priority information 

Reference: 3GPP TS 36.331

Example: SystemInformationBlockType7 message

LTE: System Information Block Type 6

Direction: E-UTRAN => UE
Signalling Radio Bearer: N/A
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: DL-SCH

The SystemInformationBlockType6 (SIB6) contains information relevant only for inter-RAT cell re-selection i.e. information about UTRA frequencies and UTRA neighbouring cells relevant for cell re-selection. It includes cell re-selection parameters which are common for an UTRA frequency.

Reference: 3GPP TS 36.331

Example: SystemInformationBlockType6 message

LTE: System Information Block Type 5

Direction: E-UTRAN => UE
Signalling Radio Bearer: N/A
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: DL-SCH

The SystemInformationBlockType5 (SIB5) contains neighbor cell related information for inter-frequency cell-reselection i.e. the information about neighbor E-UTRA frequencies

SIB5 includes neighbor cell list, carrier frequency, cell reselection priority, threshold used by the UE when reselecting a higher/lower priority frequency than the current serving frequency etc. It also contains a list of blacklisted inter-frequency neighbouring cells

Reference: 3GPP TS 36.331

Example: SystemInformationBlockType5 message

LTE: System Information Block Type 4

Direction: E-UTRAN => UE
Signalling Radio Bearer: N/A
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: DL-SCH

The SystemInformationBlockType4 (SIB4) contains intra-frequency neighboring cell information for intra-LTE intra-frequency cell reselection, such as neighbor cell list, and black listed Cell list

Reference: 3GPP TS 36.331

Example: SystemInformationBlockType4 message