5G NR Protocol Stack (L2 & L3)

December 12, 2018 Leave a comment

3GPP has released TS 38.300 Rel 15 Ver 3.1.0 about NR and NG-RAN Overall Description: Stage 2, This standard comes with the detailed descriptions about 5G NR network and Protocol architecture.

5G NR User plane Protocol Stack is shown below :

5G NR Control plane Protocol Stack  is shown below :

When we closely see both LTE & 5G NR protocol stack, we could see many similarities between LTE protocol stack and 5G-NR protocol stack because LTE protocol stack is being taken as the base for the development 5G-NR.

5G-NR User plane contains PHY, MAC, RLC, and PDCP same as LTE and has introduced a new layer named as SDAP (Service Data Adaptation Protocol).

5G-NR Control plane contains layers same as LTE, here MME equivalent node named as AMF (Access and Management Mobility).

5G-NR Layer 3 (RRC) Functions:

The main services and functions of the RRC sub layer include:

  • Broadcast of System Information related to AS and NAS;
  • Paging initiated by 5GC or NG-RAN;
  • Establishment, maintenance, and release of an RRC connection between the UE and NG-RAN including: 1. Addition, modification, and release of carrier aggregation, 2. Addition, modification, and release of Dual Connectivity in NR or between E-UTRA and NR.
  • Security functions including key management;
  • Establishment, configuration, maintenance, and release of Signalling Radio Bearers (SRBs) and Data Radio Bearers (DRBs)
  • Mobility functions including Handover and context transfer; UE cell selection and re-selection and control of cell selection and re-selection; Inter-RAT mobility.
  • QoS management functions
  • UE measurement reporting and control of the reporting
  • Detection of and recovery from radio link failure
  • NAS message transfer to/from NAS from/to UE.

5G NR Layer 2 Functions:

The layer 2 of NR is split into the following sublayers:

  • Service Data Adaptation Protocol (SDAP)
  • Packet Data Convergence Protocol (PDCP)
  • Radio Link Control (RLC)
  • and Medium Access Control (MAC)

The two figures below depict the an overview of Layer 2 for downlink and uplink, where:

  • The physical layer offers to the MAC sublayer transport channels;
  • The MAC sublayer offers to the RLC sublayer logical channels;
  • The RLC sublayer offers to the PDCP sublayer RLC channels;
  • The PDCP sublayer offers to the SDAP sublayer radio bearers;
  • The SDAP sublayer offers to 5GC QoS flows;
  • Comp. refers to header compression and segm. to segmentation;
  • Control channels (BCCH, PCCH are not depicted for clarity).

<5GNRL2DownLinkFlowImage>

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SDAP Layer Functions :

The main services and functions of SDAP include:

  • Mapping between a QoS flow and a data radio bearer  (Due to new QoS framework)
  • Marking QoS flow ID (QFI) in both DL and UL packets ( DL: due to reflective QoS and UL: due to new QoS framework)

A single protocol entity of SDAP is configured for each individual PDU session, except for DC where two entities can be configured.

PDCP Layer Functions:

The main services and functions of the PDCP sublayer for the user plane include:

  • Sequence Numbering
  • Header compression and decompression: ROHC only
  • Transfer of user plane data
  • Reordering and Duplicate detection (if in order delivery to layers above PDCP is required)
  • PDCP PDU routing (in case of split bearers)
  • Re-transmission of PDCP SDUs
  • Ciphering and Deciphering
  • PDCP SDU discard
  • PDCP re-establishment and data recovery for RLC AM
  • Duplication of PDCP PDUs

The main services and functions of the PDCP sublayer for the control plane include:

  •  Sequence Numbering
  • Transfer of control plane data
  • Ciphering, deciphering
  • Integrity protection & Verification;Duplicate detection
  • Duplication of PDCP PDUs

RLC Layer Functions: 

The main functions and services of the RLC sub-layer depend on the transmission mode and include:

  • Transfer of upper layer PDUs
  • Sequence numbering independent of the one in PDCP
  • Error Correction through ARQ
  • Segmentation and re-segmentation
  • Reassembly of SDU
  • RLC SDU discard
  • RLC re-establishment

Note:  no concatenation and no reordering

MAC Layer Functions

The main services and functions of the MAC sub layer include:

  • Mapping between logical channels and transport channels
  • Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels
  • Scheduling Information Reporting
  • Error correction through HARQ
  • Priority handling between UEs by means of dynamic scheduling
  • Priority handling between logical channels of one UE by means of logical channel prioritization
  • Padding

A single MAC entity can support one or multiple numerologies and/or TTI duration’s and mapping restrictions in logical channel prioritization controls which numerology and/or TTI duration a logical channel can use.

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Reference :

3GPP TS 38.300 Rel 15 Ver 3.1.0

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