5G-NR: Protocol Stack

                                

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.

User Plane Protocol Stack

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.

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;

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 numerology and/or TTI duration and mapping restrictions in logical channel prioritization controls which numerology and/or TTI duration a logical channel can use.

5G Layer 1 | 5G PHYSICAL Layer

Following are the functions of 5G layer 1 i.e. PHYSICAL (PHY) Layer. 

• Error detection on the transport channel and indication to higher layers
• FEC encoding/decoding of the transport channel
• Hybrid ARQ soft-combining
• Rate matching of the coded transport channel to physical channels
• Mapping of the coded transport channel onto physical channels
• Power weighting of physical channels
• Modulation and demodulation of physical channels
• Frequency and time synchronisation
• Radio characteristics measurements and indication to higher layers
• Multiple Input Multiple Output (MIMO) antenna processing
• Transmit Diversity (TX diversity)
• Digital and Analog Beamforming
• RF processing