Type of Dual Connectivity

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Today, we will learn about different type of Dual Connectivity between NR and LTE. If you are interested in learning more about 5G please click here.



What is MR-DC?


MR-DC stands for Multi-Radio Dual Connectivity.
UEs which support multiple Tx/Rx can be configured to utilize resources from two different nodes. One of these nodes could provide NR access and the other one could provide either E-UTRA or NR access. One node will act as the Master Node (MN) and the other as the Secondary Node (SN).

MR-DC with the EPC (legacy LTE Core):


Legacy LTE (E-UTRAN) supports MR-DC through EN-DC (E-UTRA-NR Dual Connectivity).

1) EN-DC


DUT (device) shall connect to one eNB that is acting as a master-node (MN) and one en-gNB acting as a secondary-node (SN). The master-node (eNB) would be connected to the EPC via the S1 interface and to the secondary-node (en-gNB) via the X2 interface.

Note: en-gNB could also be connected to the EPC via the S1-U interface and other en-gNBs via the X2-U interface.(Please read my 5G deployment article for more understanding)



MR-DC with the 5GC (5G core):


This dual connectivity can be supported only when operator already has 5G core.
The possible MR-DC options are mentioned below:

1) NGEN-DC


In this MR-DC, NG-RAN supports NG-RAN E-UTRA-NR Dual Connectivity.
In this dual connectivity, device (DUT) is connected to one ng-eNB which acts as a master-node and one gNB which acts as a secondary-node.

Here master-node (ng-eNB) shall connect with the 5GC and secondary-node (gNB) shall connect with ng-eNB via Xn interface.

2) NE-DC


In this MR-DC, NG-RAN supports NR-E-UTRA Dual Connectivity.
In this dual connectivity, device (DUT) is connected to one gNB which acts as a master-node and one ng-eNB which acts as a secondary-node.

Here master-node (gNB) shall connect with the 5GC and secondary-node (ng-eNB) shall connect with gNB via Xn interface.

3) NR-DC


In this MR-DC, NG-RAN supports NR-NR Dual Connectivity.
In this dual connectivity, device (DUT) is connected to one gNB which acts as a master-node and another gNB which acts as a secondary-node.

Here master-node (gNB) shall connect with the 5GC via NG interface and Secondary-node (gNB) shall connect with master gNB via Xn interface.

Note: The second gNB could also be connected to the 5GC via the NG-U interface.

Reference: 3gpp 37.340

Dual Connectivity:


Dual Connectivity feature implies that a device is simultaneously connected to two cells.
The E-UTRA-NR Dual Connectivity (EN-DC) feature provides support for 5G New Radio (NR) with EPC.
A DUT (device) which is connected to an eNodeB acts as a Master Node (MN) and an en-gNB acts as a Secondary Node (SN).

The eNodeB is connected the S1 interface to the EPC through and through the X2 interface to the en-gNB.

The en-gNB can be connected through the S1-U interface to the EPC and through the X2-U interface to other en-gNBs.

If the Device supports dual connectivity with NR, then it must set the DCNR bit to "dual connectivity with NR supported" in the UE network capability IE of the Attach Request/Tracking Area Update Request message.

If the Device indicates that it supports dual connectivity with NR in the Attach Request/Tracking Area Update Request message, and the MME decides to restrict the use of dual connectivity with NR for the UE, then it sets the RestrictDCNR bit to "Use of dual connectivity with NR is restricted" in the EPS network feature support IE of the Attach Accept/Tracking Area Update Accept message.

In the EPS network feature support IE of the Attach Accept/Tracking Area Update Accept message, ff the RestrictDCNR bit is set to "Use of dual connectivity with NR is restricted", the UE gives the indication that dual connectivity with NR is restricted to the upper layers.

If the DUT supports DCNR and it is also configured on MME, and if HSS sends ULA/IDR with "Access-Restriction" carrying "NR as Secondary RAT Not Allowed",  "NR Restriction" bit is sent by MME, set in "Handover Restriction List" IE during Attach/TAU/Handover procedures.

Similarly, the RestrictDCNR bit is set to "Use of dual connectivity with NR is restricted" by MME in the EPS network feature support IE of the Attach Accept/Tracking Area Update Accept message.

Accordingly, UE provides indication to the upper layers that dual connectivity with NR is restricted.

In the "Initial Context Setup Request" message for TAU and Attach procedure, the "Handover Restriction List" IE is present with data forwarding procedure, in the "Handover Required" message for S1 handover procedure and in the "Downlink NAS Transport" message for TAU without active flag procedure.


Comments

  1. map90003 July 2019 at 13:26

    Is this only for NR devices connecting to both EPC and NGC? i.e. not for an LTE device to enable connectivity to a NGC?

    ReplyDelete
    Replies
    1. Blacktechoguys22 September 2019 at 02:09

      gNB serving NR devices using the NR user-plane and control-plane
      protocols and ng-eNB serving LTE devices using the LTE user-plane and control-plane protocols

      Delete

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