LTE attach accept message is sent by the network to UE.
It is the response for ATTACH REQUEST.
ATTACH REJECT will be sent by MME if ATTACH ACCEPT cannot be accepted by network.
Appropriate cause will be updated in “EMM cause value”.
IE’s available in Attach Reject:
Attach reject message identity
ESM message container
If the attach procedure fails due to Default EPS bearer Setup failure or an ESM procedure failure, this details, MME will include in “ESM message container” IE of ATTACH REJECT message.
In this case “EMM Cause value” value will be #19 “ESM Failure”
The Attach Reject will be included in dlInformationTransfer as below:
dlInformationTransfer+Attach Reject+PDN Connectivity Reject
Reference: 3GPP TS 24.301