|3GPP| ENDC attach procedure

This is a note for 5G NSA attach procedure.
TS 36.331: E-UTRA; Radio Resource Control (RRC); Protocol specification
TS 24.301: Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS)
TS 38.331: NR; Radio Resource Control (RRC)

The following is included the messages and parameters of RRC and NAS.


— : control plane
— : user plane

– 5G gNodeB does not connect to the MME
– gNodeB connect to the LTE eNodeB
– Receive eNodeB’s requests to activate and deactivate 5G bearers

Spec. Guide

1. Discover NR Coverage

2. LTE RRC Connection Request

  • (UE -> NW) RRCConnectionRequest: is used to request the establishment of an RRC connection
  • (UE <- NW) The RRCConnectionSetup: is used to establish SRB1(Signalling radio bearer).
  • (UE -> NW) RRCConnectionSetupComplete: is used to confirm the successful completion of an RRC connection establishment.
  • Note: SRB: transfer RRC sub-layers control messages, DRB transfer user’s network packets

3. LTE RRC Connection Setup Complete


<Attach Request>
1. TS 24.301: UE Network Capability:
UE sets the DCNR bit in octet 9 to signal that it supports EN-DC dual connectivity
2. MS network capability (TS 24.008, MS network capability)
ms network capability
Dual connectivity of E-UTRA with NR capability = 1

<UECapabilityEnquiry> (TS36.331: 6.2.2 Message definitions)
ue-CapabilityRequest: eutra-nr, nr

<UECapabilityInformation> (TS36.331: 6.2.2 Message definitions)
1. ue-CapabilityRAT-ContainerList
rat-Type eutra-nr
2. rf-Parameters {
supportedBandListNR {

4. LTE RRC Connection Reconfiguration

<RRCConnectionReconfiguration> 38.331 – 5.5 Measurement
1. Confirm the successful completion of an RRC connection reconfiguration
2. Carries the Attach Accept NAS message.
3. This message included the measure target: measObject measObjectNR-r15
and report timing: reportConfigToAddModList: eventId eventB1-NR-r15

<Attach accept>
If RestrictDCNR = 0, UE is not restricted to use DCNR.
If RestrictDCNR = 1, UE is restricted to use DCNR. Even UE has NR capability.

5. NR RRC Connection for ENDC

  • Initiate measurement of 5G NR channels specified in theMeasurement Objects received in the RRC Connection Reconfiguration.

It is including the information of NR
measResultNeighCells measResultNeighCellListNR-r15 :
pci-r15 xxx
rsrpResult-r15 xx,
rsrqResult-r15 xx

(TS 38.331 6.3.2 Radio resource control information elements: RadioBearerConfig)
nr-Config-r15 setup
keyToUse secondary

secondaryCellGroup (TS 36.331: Connected mode mobility)


– The control plane of ENDC depends on eNB, therefore, NAS attach is similar to LTE.
– RRC:
  UE capability included NR rat
  RRC Connection Reconfiguration: for NR
Measurement report for NR
NR RRC connection NR radio bearer
– NAS:
  Attach request: the bit of UE Network Capability
Attach accpet: the bit of RestrictDCNR