LTE Attach Procedure Interview Questions 11-20

Q11. Why Security Mode Command are using 2 times in Attach Procedure?

Ans. 1st times security mode command use to activate NAS level security for signalling messages transmitting between UE to MME via enodeB. After Authentication procedure.2nd time it’s use to activate AS level security to protect signalling message between UE to EnodeB. Before RrcConnectionReconfiguration message.
Note: 1st SMC message is NAS message so ue receive it to in DlInformationTransfer from enodeb carry’s SecurityModeCommand.
But 2nd SMC message is enodeB generated message, ue receive it to in SecurityModeCommand message.

Q12. What is the difference between Attach Request, Attach Accept and Attach Complete? And in which messages these information's is present?

Ans. Attach Request is nothing but initial ue attach request, pdn connectivity request etc. message’s present in RrcSetupComplete.Attach Accept carry’s activate defaut EPS bearer context request which is present in RrcConnectionReconfiguration message.Attach Complete carry’s activate default eps bearer context accept present in RrcConnectionReconfigurationComplete message.Note: all these above message or nas messages are piggyback with RRC messages. Click here more info

Q13. What is difference between Radio Bearer and EPS Bearer?

Ans. Radio Bearer’s are present from UE to enodeB for example (SRB’s and DRB’s). in Rel-9 there are upto 32 Radio Bearer’s present. Information present in:

     RadioResorceConfigDedicated
       srb_ToAddModList {
           srb_Identity 2
           }
       drb_ToAddModList {
           drb_Identity 1
           }
EPS bearer present between UE to P-Gw. Upto 0-15 (Total 16) eps bearer can be possible in LTE Rel-9. Information present in:
   RadioResorceConfigDedicated
       drb_ToAddModList {
           eps_BearerIdentity 5
           }

Q14. How many Radio Bearers are uses in LTE?

Ans. As per LTE Rel-9 there are 32 radio bearer’s can be possible but a UE can activate upto 11 radio bearer at a time SRB0,1,2 and DRB1-8. Total 11 radio bearer’s.

Q15. How many EPS Bearers are uses in LTE?

Ans. As per LTE rel-9 upto 16 eps bearer can be possible.

Q16. Which type of Attach Procedure UE will do to utilize IMS Services?

Ans. < PDN Connectivity Request : Additional APN >

   NAS EPS session management messages: PDN connectivity request (0xd0)
   0011 .... = PDN type: IPv4v6 (3)
   .... 0001 = Request type: initial request (1)
   Access Point Name
           Element ID: 0x28
           Length: 4
           APN: ims // Usually UE request PDN with specific APN name
   Protocol Configuration Options
behalf of that ue send attach request to IMS pdn.

Q17. What are the information elements present in Initial Context Setup and which message is using?

Ans. UE receive Initial context setup message in RrcConnectionReconfiguration. carry’s following ie’s.

ME-UE-S1AP-ID
ENB-UE-S1AP-ID
UE-Aggregate-Maximum-Bitrate
E-RAB-To-Be-Setup-List-Ctxt-SUReq
UE-Security-Capabilities
Security-Key
Trace-Activation
Handover-Restriction-List
UE-Radio-Capability
Subscriber-Profile-ID-for-RFP
CS-Fallback-Indicator
SRVCC-Operation-Possible
CSG-Membership-Status
Registered-LAI
GUMMEI-ID
MME-UE-S1AP-ID-2
Management-Based-MDT-Allowed
Management-Based-MDT-PLMN-List
Additional-CS-Fallback-Indicator
Masked-IMEISV
Expected-UE-Behaviour
ProSe-Authorized
UE-User-Plane-CIoT-Support-Indicator
V2X-Services-Authorized
UE-Sidelink-Aggregate-Maximum-Bit-Rate
Enhanced-Coverage-Restricted
NR-UE-Security-Capabilities
CE-ModeB-Restricted

Q18. Once UE send Attach Request toward EnodeB then how EnodeB identified the particular MME?

Ans. Enodeb and MME perform some globle procedure messages before performing ue relevant actions. Both entities works on network topology and it’s operator jobs when they implement any entity into their network.And when ue receive a paging message it have the informations that when ue do attach procedure, which mme need to fulfill ue actions.
Ue sent rrcsetupcomplete message with registered mme information to help enodeb to find out the mme information.

Q19. Attach Failure Causes?

Ans. EMM cause information element

Cause value (octet 2)
Bits
8 7 6 5 4 3 2 1

0 0 0 0 0 0 1 0 IMSI unknown in HSS
0 0 0 0 0 0 1 1 Illegal UE
0 0 0 0 0 1 1 0 Illegal ME
0 0 0 0 0 1 1 1 EPS services not allowed
0 0 0 0 1 0 0 0 EPS services and non-EPS services not allowed
0 0 0 0 1 0 0 1 UE identity cannot be derived by the network
0 0 0 0 1 0 1 0 Implicitly detached
0 0 0 0 1 0 1 1 PLMN not allowed
0 0 0 0 1 1 0 0 Tracking Area not allowed
0 0 0 0 1 1 0 1 Roaming not allowed in this tracking area
0 0 0 0 1 1 1 0 EPS services not allowed in this PLMN
0 0 0 0 1 1 1 1 No Suitable Cells In tracking area
0 0 0 1 0 0 0 0 MSC temporarily not reachable
0 0 0 1 0 0 0 1 Network failure
0 0 0 1 0 0 1 0 CS domain not available
0 0 0 1 0 0 1 1 ESM failure
0 0 0 1 0 1 0 0 MAC failure
0 0 0 1 0 1 0 1 Synch failure
0 0 0 1 0 1 1 0 Congestion
0 0 0 1 0 1 1 1 UE security capabilities mismatch
0 0 0 1 1 0 0 0 Security mode rejected, unspecified
0 0 0 1 1 0 0 1 Not authorized for this CSG
0 0 0 1 1 0 1 0 Non-EPS authentication unacceptable
0 0 1 0 0 1 1 1 CS domain temporarily not available
0 0 1 0 1 0 0 0 No EPS bearer context activated
0 1 0 1 1 1 1 1 Semantically incorrect message
0 1 1 0 0 0 0 0 Invalid mandatory information
0 1 1 0 0 0 0 1 Message type non-existent or not implemented
0 1 1 0 0 0 1 0 Message type not compatible with the protocol state 0 1 1 0 0 0 1 1 Information element non-existent or not implemented
0 1 1 0 0 1 0 0 Conditional IE error
0 1 1 0 0 1 0 1 Message not compatible with the protocol state
0 1 1 0 1 1 1 1 Protocol error, unspecified

Any other value received by the network shall be treated as
0110 1111, "protocol error, unspecified".

Q20. Detach call flow(Procedure)?

Ans. UE initiated detach procedure:

by the UE to detach for EPS services only;
by the UE to disconnect from the last PDN it is connected to;
by the UE in CS/PS mode 1 or CS/PS mode 2 of operation to detach for both EPS services and non-EPS services or for non-EPS services only via a combined detach procedure;

Network initiated detach procedure:

by the network to inform the UE that it is detached for EPS services or non-EPS services or both; and
by the network to disconnect the UE from the last PDN to which it is connected.