LTE Attach Procedure Interview Questions 31-35

Q31. What is UE Capability Request? What are the Information Elements present in UE Capability Request?

Ans. if the ue-CapabilityRequest includes 'eutra':
include the UE-EUTRA-Capability within a ue-CapabilityRAT-Container and with the rat-Type set to 'eutra';
UE capability request also called UE capability enquiry which is
* sent by network to ue
* and it’s a rrc message.
Important information elements are:
    ueCapabilityEnquiry
        ControlInfo {
            CnfFlag false
            FollowOnFlag false
            }
        RoutingInfo {
            RadioBearerId {
                Srb 1
            }
        }
        TimingInfo {
            Now true
        }
    CellId eutra_Cell1
        criticalExtensions {
            c1 {
                ueCapabilityEnquiry_r8 {
                    ue_CapabilityRequest {
                    }
                }
            }
        }
    rrc_TransactionIdentifier 0

Q32. Who send Capability Request message to UE, Why Capability Request is transmission in Attach Procedure 2 times?

Ans. Network sent capability enquiry (request) message to ue in downlink direction.
*    2nd capability enquiry is sent by network to know the ue other RAT technologies capability information that’s way a network sent a another ue capability enquiry.
Note:

    > if the ue-CapabilityRequest includes 'eutra-nr':
        >> include the UE-EUTRA-NR-Capability within a ue-CapabilityRAT-Container and with the rat-Type set to 'eutra-nr';

    > if the ue-CapabilityRequest includes 'geran-cs' and if the UE supports GERAN CS domain:
        >> include the UE radio access capabilities for GERAN CS within a ue-CapabilityRAT-Container and with the rat-Type set to 'geran-cs';
    > if the ue-CapabilityRequest includes 'geran-ps' and if the UE supports GERAN PS domain:
        >> include the UE radio access capabilities for GERAN PS within a ue-CapabilityRAT-Container and with the rat-Type set to 'geran-ps';
    > if the ue-CapabilityRequest includes 'utra' and if the UE supports UTRA:
        >> include the UE radio access capabilities for UTRA within a ue-CapabilityRAT-Container and with the rat-Type set to 'utra';
    > if the ue-CapabilityRequest includes 'cdma2000-1XRTT' and if the UE supports CDMA2000 1xRTT:
        >> include the UE radio access capabilities for CDMA2000 within a ue-CapabilityRAT-Container and with the rat-Type set to 'cdma2000-1XRTT';
* submit the UE Capability Information message to lower layers for transmission, upon which the procedure ends;

Q33. How many Default and Dedicated Bearer a UE can establish in LTE?

Ans. It’s depend on how many P-Gw connects to different different PDN. For example if 1 P-Gw connects to 2 PDN’s than 2 default should be active and dedicated based on service using by ue.
If 2 P-Gw connects to 2 PDN’s than also 2 default bearer should be active and how many dedicated bearer’s is based on services are using be ue.
Note: conclusion of this is simple No. Of  GTPC Tunnel b/w SGW and PGW= No. Of default bearer= No. PDNs.
1 P-Gw can be connect with 1 or more PDN but reverse not possible.

In Below Diagram White Lines refer to GTPC Tunnels and Purple Lines refers to GTPU Tunnels

Q34. What happens to default bearer which is present earlier if UE don’t have DL/UL data?

Ans. Default bearer will be present on S5/S8 interface between S-Gw to P-Gw and here UE moves into the ECM Idle state in which Radio signalling and S1 signaling released but MME knows the Location of UE . Once any downlink or uplink data is ready for transmission then MME activate the ERAB bearer from ue to enodeB to S-Gw.

Q35. Suppose UE have only partial Default Bearer present and suddenly P-Gw receive Downlink Data then how MME re-activate default bearer?

Ans. When downlink data comes from P-Gw to S-Gw, the S-Gw buffered the downlink data into its buffer and request to MME to re-activate the E-RAB in E-UTRAN side. The MME first send the paging msg to eNB so that eNB again established the RRC connection and UE comes into ECM and RRC connected state than for DL MME sent Initial context setup request  where GUTI and NAS PDU with S-Gw S5/S8 GTPU TEID Tunnel detail is present and in the Initial Ue context setup accept and Attach Accept eNB provides its eNB S1-GTPU TEID Tunnel for receiving the DL Data. For UL Data UE start Service request procedure(Before this BSR and SR already happened) to tell MME that I have something for sending than here also MME have to exchange the  eNB TEID and SGW TEID details than ENB forward the data to SGW.