5G NR Radio Protocol Stack (Layer 2 and Layer 3)

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3GPP has released specification 38.300 V1 this month about NR and NG-RAN Overall Description: Stage 2, This standard comes with the detailed descriptions about 5G NR network and Protocol architecture.

NR Radio User plane and Control Protocol Stack  is shown in below figures:

   

Figure #1 User Plane Protocol Stack                                             Figure #2 Control Plane Protocol Stack

When we closely see both the 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 line 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).

On another side, the control plane of 5G-NR is identical to LTE, here MME equivalent node named as AMF (Access and Management Mobility Function).

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     Addition, modification, and release of carrier aggregation, 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 reselection and control of cell selection and reselection; 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 sub layers:

  • 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 Layer 2 architecture for downlink and uplink, where:

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

Figure #3 Downlink Layer 2 Structure                                                    Figure #4 Uplink Layer 2 Structure

SDAP (Service Data Adaptation Protocol) Protocol 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 (Packet Data Convergence Protocol) 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 data
  • Reordering and Duplicate detection (if in order delivery to layers above PDCP is required)
  • PDCP PDU routing (in case of split bearers)
  • Retransmission 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;
  • Ciphering, deciphering and integrity protection;
  • Transfer of control plane data;
  • Duplicate detection;
  • Duplication of PDCP PDUs.

RLC (Radio Link Control ) Layer Functions: 

The main services and functions of the RLC sublayer 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 (Media Access Control) 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 durations and mapping restrictions in logical channel prioritization controls which numerology and/or TTI duration a logical channel can use.

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