6 Layer 2

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.0 Overview

Layer 2 is split into the following sublayers: Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Convergence Protocol (PDCP).

This clause gives a high level description of the Layer 2 sub-layers in terms of services and functions. The three figures below depict the PDCP/RLC/MAC architecture for downlink, uplink and Sidelink, where:

– Service Access Points (SAP) for peer-to-peer communication are marked with circles at the interface between sublayers. The SAP between the physical layer and the MAC sublayer provides the transport channels. The SAPs between the MAC sublayer and the RLC sublayer provide the logical channels.

– The multiplexing of several logical channels (i.e. radio bearers) on the same transport channel (i.e. transport block) is performed by the MAC sublayer;

– In both uplink and downlink, when neither CA nor DC are configured, only one transport block is generated per TTI in the absence of spatial multiplexing;

– In Sidelink, only one transport block is generated per TTI for each carrier.

Figure 6-1: Layer 2 Structure for DL

Figure 6-2: Layer 2 Structure for UL

NOTE 1: The eNB may not be able to guarantee that a L2 buffer overflow will never occur. If such overflow occurs, UE may discard packets in the L2 buffer.

NOTE 2: For a NB-IoT UE that only supports Control Plane CIoT EPS optimisations, as defined in TS 24.301 [20], PDCP is bypassed. For a NB-IoT UE that supports Control Plane CIoT EPS optimisation and S1-U data transfer or User Plane CIoT EPS optimisation, as defined in TS 24.301 [20], PDCP is also bypassed (i.e. not used) until AS security is activated.

Figure 6-3: Layer 2 Structure for Sidelink

6.1 MAC Sublayer

6.1.0 General

This clause provides an overview on services and functions provided by the MAC sublayer.

6.1.1 Services and Functions

The main services and functions of the MAC sublayer 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 logical channels of one UE;

– Priority handling between UEs by means of dynamic scheduling;

– MBMS service identification;

– Transport format selection;

– Padding.

The sidelink specific services and functions of the MAC sublayer include:

– Radio resource selection;

– Packet filtering for sidelink communication and V2X sidelik communication.

– Transmission carrier selection for V2X sidelink communication.

6.1.2 Logical Channels

6.1.2.0 General

Different kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred.

A general classification of logical channels is into two groups:

– Control Channels (for the transfer of control plane information);

– Traffic Channels (for the transfer of user plane information).

There is one MAC entity per CG. MAC generally consists of several function blocks (transmission scheduling functions, per UE functions, MBMS functions, MAC control functions, transport block generation…). Transparent Mode is only applied to BCCH, BR-BCCH, PCCH and SBCCH.

NOTE: For a NB-IoT UE that only uses Control Plane CIoT EPS optimisations, as defined in TS 24.301 [20], there is only one dedicated logical channel per UE.

6.1.2.1 Control Channels

Control channels are used for transfer of control plane information only. The control channels offered by MAC are:

– Broadcast Control Channel (BCCH)

A downlink channel for broadcasting system control information.

– Bandwidth Reduced Broadcast Control Channel (BR-BCCH)

A downlink channel for broadcasting system control information.

– Paging Control Channel (PCCH)

A downlink channel that transfers paging information and system information change notifications. This channel is used for paging when the network does not know the location cell of the UE.

– Common Control Channel (CCCH)

Channel for transmitting control information between UEs and network. This channel is used for UEs having no RRC connection with the network.

– Multicast Control Channel (MCCH)

A point-to-multipoint downlink channel used for transmitting MBMS control information from the network to the UE, for one or several MTCHs. This channel is only used by UEs that receive or are interested to receive MBMS.

– Single-Cell Multicast Control Channel (SC-MCCH)

A point-to-multipoint downlink channel used for transmismitting MBMS control information from the network to the UE, for one or several SC-MTCHs. This channel is only used by UEs that receive or are interested to receive MBMS using SC-PTM.

– Dedicated Control Channel (DCCH)

A point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. Used by UEs having an RRC connection.

– Sidelink Broadcast Control Channel (SBCCH)

A sidelink channel for broadcasting sidelink system information from one UE to other UE(s).

6.1.2.2 Traffic Channels

Traffic channels are used for the transfer of user plane information only. The traffic channels offered by MAC are:

– Dedicated Traffic Channel (DTCH)

A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink. DTCH is not supported for a NB-IoT UE that only uses Control Plane CIoT EPS optimisations, as defined in TS 24.301 [20].

– Multicast Traffic Channel (MTCH)

A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE. This channel is only used by UEs that receive MBMS.

– Single-Cell Multicast Traffic Channel (SC-MTCH)

A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE using SC-PTM transmission. This channel is only used by UEs that receive MBMS using SC-PTM.

– Sidelink Traffic Channel (STCH)

A Sidelink Traffic Channel (STCH) is a point-to-multipoint channel, for transfer of user information from one UE to other UE(s). This channel is used only by sidelink communication capable UEs and V2X sidelink communication capable UEs. Point-to-point communication between two sidelink communication capable UEs is also realized with an STCH.

6.1.3 Mapping between logical channels and transport channels

6.1.3.1 Mapping in Uplink

The figure below depicts the mapping between uplink logical channels and uplink transport channels:

Figure 6.1.3.1-1: Mapping between uplink logical channels and uplink transport channels

In Uplink, the following connections between logical channels and transport channels exist:

– CCCH can be mapped to UL-SCH;

– DCCH can be mapped to UL- SCH;

– DTCH can be mapped to UL-SCH.

6.1.3.2 Mapping in Downlink

The figure below depicts the mapping between downlink logical channels and downlink transport channels:

Figure 6.1.3.2-1: Mapping between downlink logical channels and downlink transport channels

In Downlink, the following connections between logical channels and transport channels exist:

– BCCH can be mapped to BCH;

– BCCH can be mapped to DL-SCH;

– BR-BCCH can be mapped to DL-SCH;

– PCCH can be mapped to PCH;

– CCCH can be mapped to DL-SCH;

– DCCH can be mapped to DL-SCH;

– DTCH can be mapped to DL-SCH;

– MTCH can be mapped to MCH;

– MCCH can be mapped to MCH;

– SC-MTCH can be mapped to DL-SCH;

– SC-MCCH can be mapped to DL-SCH.

6.1.3.3 Mapping in Sidelink

Figure 6.1.3.3-1: Mapping between Sidelink logical channels and Sidelink transport channels

In Sidelink, the following connections between logical channels and transport channels exist:

– STCH can be mapped to SL-SCH;

– SBCCH can be mapped to SL-BCH.

6.2 RLC Sublayer

6.2.0 General

This clause provides an overview on services, functions and PDU structure provided by the RLC sublayer. Note that:

– The reliability of RLC is configurable: some radio bearers may tolerate rare losses (e.g. TCP traffic);

– Radio Bearers are not characterized by a fixed sized data unit (e.g. a fixed sized RLC PDU).

6.2.1 Services and Functions

The main services and functions of the RLC sublayer include:

– Transfer of upper layer PDUs;

– Error Correction through ARQ (only for AM data transfer);

– Concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer);

– Re-segmentation of RLC data PDUs (only for AM data transfer);

– Reordering of RLC data PDUs (only for UM and AM data transfer);

– Duplicate detection (only for UM and AM data transfer);

– Protocol error detection (only for AM data transfer);

– RLC SDU discard (only for UM and AM data transfer);

– RLC re-establishment, as defined in TS 24.301 [20];

6.2.2 PDU Structure

Figure 6.2.2-1 below depicts the RLC PDU structure where:

– The PDU sequence number carried by the RLC header is independent of the SDU sequence number (i.e. PDCP sequence number);

– A red dotted line indicates the occurrence of segmentation;

– Because segmentation only occurs when needed and concatenation is done in sequence, the content of an RLC PDU can generally be described by the following relations:

– {0; 1} last segment of SDU_i + [0; n] complete SDUs + {0; 1} first segment of SDU_i+n+1 ; or

– 1 segment of SDU_i.

Figure 6.2.2-1: RLC PDU Structure

6.3 PDCP Sublayer

6.3.0 General

This clause provides an overview on services, functions and PDU structure provided by the PDCP sublayer.

6.3.1 Services and Functions

Except for NB-IoT, the main services and functions of the PDCP sublayer for the user plane include:

– Header compression and decompression using ROHC and/or EHC ;

– Compression and decompression of uplink PDCP SDUs: DEFLATE based UDC only;

– Transfer of user data;

– In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM;

– For split bearers in DC (only support for RLC AM) and LWA bearers (only support for RLC AM and RLC UM): PDCP PDU routing for transmission and PDCP PDU reordering for reception;

– Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM;

– Retransmission of PDCP SDUs at handover and, for split bearers in DC and LWA, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM;

– Ciphering and deciphering;

– Timer-based SDU discard in uplink;

– Duplication of PDCP PDUs;

– For PDCP duplication, reordering and duplicate detection at the receiver.

For NB-IoT UE when AS security is activated, the main services and functions of the PDCP sublayer for the user plane include:

– Header compression and decompression: ROHC only;

– Transfer of user data;

– In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM;

– Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM;

– Ciphering and deciphering;

– Timer-based SDU discard in uplink.

NOTE 1: When compared to UTRAN, the lossless DL RLC PDU size change is not required.

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

– Ciphering and Integrity Protection;

– Transfer of control plane data.

Except for NB-IoT, the main services and functions of the PDCP sublayer for the control plane also include:

– Duplication of PDCP PDUs;

– For PDCP duplication, reordering and duplicate detection at the receiver.

NOTE 2: For a NB-IoT UE that only supports Control Plane CIoT EPS optimisation, as defined in TS 24.301 [20], PDCP is bypassed. For a NB-IoT UE that supports Control Plane CIoT EPS optimisation and S1-U data transfer or User Plane CIoT EPS optimisation, as defined in TS 24.301 [20], PDCP is not used until AS security is activated.

6.3.2 PDU Structure

Figure 6.3.2-1 below depicts the PDCP PDU structure for user plane data, where:

– PDCP PDU and PDCP header are octet-aligned;

– PDCP header can be either 1 , 2 or 5 bytes long.

Figure 6.3.2-1: PDCP PDU Structure

The structures for control PDCP PDUs and for control plane PDCP data PDUs are specified in TS 36.323 [15].

6.4 Carrier Aggregation

In case of CA, the multi-carrier nature of the physical layer is only exposed to the MAC layer for which one HARQ entity is required per serving cell;

– In both uplink and downlink, there is one independent hybrid-ARQ entity per serving cell and one transport block is generated per TTI per serving cell in the absence of spatial multiplexing. Each transport block and its potential HARQ retransmissions are mapped to a single serving cell;

– HARQ operation is asynchronous for Licensed-Assisted Access (LAA) SCells.

Figure 6.4-1: Layer 2 Structure for DL with CA configured

Figure 6.4-2: Layer 2 Structure for UL with CA configured

In case of CA in sidelink, which applies to V2X sidelink communication, there is one independent HARQ entity per carrier used for V2X sidelink communication and one transport block is generated per TTI per carrier. Each transport block and its potential HARQ retransmissions are mapped to a single carrier.

Figure 6.4-3: Layer 2 Structure for Sidelink with CA configured

6.5 Dual Connectivity

In case of DC, the UE is configured with two MAC entities: one MAC entity for MeNB and one MAC entity for SeNB. Figure 6.5-1 below describes the layer 2 structure for the downlink when both CA and DC are configured. In order to simplify the figure, the BCH, PCH, MCH and corresponding logical channels are not included. Also, only UE_n is shown as having DC configured.

Figure 6.5-1: Layer 2 Structure for DL with CA and DC configured

Figure 6.5-2 below describes the layer 2 structure for the uplink when both CA and DC are configured. As explained in clause 4.9.2, SRBs are always handled by the MeNB and as a result, CCCH is only shown for the MeNB. For a split bearer, UE is configured over which link (or both) the UE transmits UL PDCP PDUs by the MeNB. On the link which is not responsible for UL PDCP PDUs transmission, the RLC layer only transmits corresponding ARQ feedback for the downlink data.

Figure 6.5-2: Layer 2 Structure for UL with CA and DC configured