NSA-ENDC Interfaces

It ‘s possible to transmit the user plane between both nodes to the EPC but control plane is only possible on the ENDC side.

GTP (GPRS Tunneling Protocol) . S1 user plane is tunneled between the LTE node and SGW using GTP-U. 

UDP (User Datagram Protocol)

S1AP (S1 Application Protocol)

SCTP (Stream Control Transmission Protocol)

5G Architecture

In option 3 there are 3 deployment scenarios for NSA NR they consist of at least two nodes, one for LTE and one for NR.

LTE-NR Dual Connectivity feature 

-The LTE-NR Dual Connectivity feature introduces the support for EN-DC in the NR Node used in an NSA deployment.

-The feature covers the fundamental interaction between LTE and NR in the EN-DC context.

-The feature makes it possible to configure split DRBs with one leg in LTE and one leg in NR.

 

The benefits of this setup are the following:

  Higher peak rate of network data traffic.

  Sustainable capacity and performance growth.

-CONTROL PLANE

  Master Cell Group (MCG) SRB (SRB1, SRB2): SRB (Signal Radio Bearer)

  Direct SRB between the master node and the mobile device that can be used for conveying master node RRC messages which can also embed secondary node RRC configurations.

  Split SRB (SRB1+SRB1S, SRB2+SRB2S): 

  SRB that is split between the master node and the secondary node (at a higher layer, PDCP) towards the mobile device, allowing a master node RRC message to be sent via the lower layers (RLC, MAC and PHY) of either the   master node or secondary node; or to be sent via the lower layers of both the master and secondary nodes. Here, the master node RRC message can also embed secondary node RRC configurations.

  Secondary Cell Group (SCG) SRB (SRB3): 

  Direct SRB between the secondary node and the mobile device by which secondary node RRC messages are sent.

-USER PLANE

  MCG DRBs: DRB (Data Radio Bearer)

  Bearers terminated at the master node and using only the master node lower layers.

  MCG split DRBs:

  Bearers terminated at the master node but that can use the lower layers of either the master node or secondary node; or can use the lower  layers of both the master and secondary nodes.

  SCG DRBs:

  Bearers terminated at the SN and using only the secondary node lower layers.

An additional data radio bearer has been introduced in EN-DC:

  SCG split DRBs:

  Bearers terminated at the secondary node but that can use the lower layers of either the master node or secondary node, or can use the lower   layers of both the master and secondary nodes.

Adopting current LTE DC procedures for the introduction of the SCG split DRB entails that every bearer type change will require a re-establishment in the PDCP layer, as well as in some cases signaling towards the core network to switch the path from the core network to the radio access network. To minimize the impact of such bearer type changes, and minimize implementation and testing efforts on mobile devices, 3GPP has agreed to harmonize the bearer definitions. With the harmonized bearer concept, there will be only two kinds of bearers from the mobile device perspective:

  Direct DRBs: 

  DRBs with only one lower layer configuration, either corresponding to LTE or NR lower layers. If the LTE lower layers are configured, either the   NR or the LTE version of the higher layer can be used by the bearer. If the NR lower layers are configured, the NR version of the higher layer will   be used.

  Split DRBs: 

  DRBs with two lower layer configurations, corresponding to LTE and NR lower layers. It will always use the NR version of the higher layer. In case   of split DRBs, packet duplication, when it is finalized as a new functionality in Release 15, could be used for additional reliability.

Limitations.

It is not recommended for EN-DC UEs to use VoLTE due to the lack of support for important VoLTE related features.

The VoLTE performance is expected to be poor due to the following:

  TTI (Transmission Time Interval) Bundling cannot be used.

  Handover support is limited.

  RRC re-establishment for EN-DC UEs is not supported.

LTE Channel

The PBCH (Physical Broadcast Channel) carries the periodic downlink broadcast of the RRC MasterInformationBlock message. Note that system information from BCCH (Broadcast Control Channel) is scheduled for transmission in the PDSCH (Physical Downlink Shared Channel).

The PDCCH (Physical Downlink Control Channel) carries no higher layer information and is used for scheduling uplink and downlink resources. Scheduling decisions, however, are the responsibility of the MAC layer, therefore the scheduling information carried in the PDCCH is provided by MAC. Similarly the PUCCH (Physical Uplink Control Channel) is used to carry resource requests from UEs that will need to be processed by MAC.

The PHICH (Physical Hybrid ARQ Indicator Channel) is used for downlink ACK/NACK of uplink transmissions from UEs in the PUSCH (Physical Uplink Shared Channel). It is a shared channel and uses a form of code multiplexing to provide multiple ACK/NACK responses.

The PCFICH (Physical Control Format Indicator Channel) is used to indicate how much resource in a subframe is reserved for the downlink control channels. It may be either one, two or three of the first symbols in the first slot in the subframe.

The PRACH (Physical Random Access Channel) is used for the uplink transmission of preambles as part of the random access procedure.

The PDSCH and the PUSCH are the main scheduled resource on the cell. They are used for the transport of all higher-layer information including RRC signalling, service-related signalling and user traffic. The only exception is the system information in PBCH.

X2 Interface in LTE

With the removal of the RNC from the access network architecture, inter-eNB handover is negotiated and managed directly between eNBs using the X2-C interface. In LTE implementations that need to support macro diversity, the X2-U interface will carry handover traffic PDUs (Protocol Data Units) between eNBs. X2-C (control plane) signalling is carried by the X2AP (X2 Application Protocol), which travels over an SCTP association established between neighboring eNBs.



X2AP performs duties similar to those performed by RNSAP (Radio Network Subsystem Application Protocol), which operates between neighboring RNCs over the Iur interface in UMTS R99 networks. X2-U (user plane) traffic is carried by the existing GTP-U (GPRS Tunnelling Protocol – User plane), as employed in UMTS R99 networks. The facilities provided by the X2-U interface are only expected to be required if macro-diversity handover is supported.  Both sub-types of the X2 interface travel over IP: SCTP/IP for the X2-C and UDP/IP for the X2-U.





X2 Interface Architecture : -



The X2 interface is designed to provide a logical signalling and traffic path between neighbouring eNBs.  The term ‘neighbouring’ in this sense refers to eNBs that generate adjacent cells between which UEs would be expected to request handovers. The X2 interface is the functional successor to the UMTS Iur interface, which interconnects neighbouring RNCs.



An eNB is only expected to support X2 interfaces to neighbouring sites with which there is a realistic possibility of handover events occurring; an individual eNB would not be required to support X2 interfaces to all eNBs in the network. Indeed, the X2 is an optional interface and all of its functions can be performed indirectly via the S1 and the MME/S-GW if direct connections are not supported.