LTE attach accept message is sent by the network to UE.
It is the response for ATTACH REQUEST.
Similar to ATTACH REQUEST, ATTACH ACCEPT will be included in RRC CONNECTION RECONFIGURATION message.
Including a message inside another message is called as piggy backed message.
The result of EPS attach is indicated in “EPS Attach Result” IE of the RRC Connection Reconfiguration message.
The “EPS Attach Result” IE type can be “Combined EPS/IMSI attach” or “EPS only”.
Here “Combined EPS/IMSI attach” means EPS and non-EPS services or for EPS and “SMS only” services.
If it is “EPS only”, then non EPS or “SMS Only” has failed, if the UE has requested in ATTACH REQUEST message.
IE’s present in Attach Accept are:
EPS attach result
ESM message container
GUTI
TAI list
EMM cause
Timers include: T3412 value
Additional update result
EPS attach result:
The content of this IE, we have seen above.
ESM message container:
It holds ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST + ATTACH ACCEPT. Here the default bearer is activated as part of attach procedure.
GUTI:
TAI list:
The purpose of the Tracking area identity list information element is to transfer a list of tracking areas from the network to the UE.
EMM cause:
The purpose of the EMM cause information element is to indicate the reason why an EMM request from the UE is rejected by the network. In our case the reason for non-EPS services is not successful, will be updated.
Timers include: T3412 value
Additional update result
The purpose of the Additional update result information element is to provide additional information about the result of a combined attach procedure or a combined tracking area updating procedure.
0 0 no additional information
0 1 CS Fallback not preferred
1 0 SMS only
1 1 reserved
Reference: 3GPP TS 24.301
