Content for  TS 38.300  Word version:  18.3.0

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 (one HARQ entity per cell in case of CA);
• Priority handling between UEs by means of dynamic scheduling;
• Priority handling between logical channels of one UE by means of logical channel prioritisation;
• Priority handling between overlapping resources of one UE;
• Padding.

A single MAC entity can support multiple numerologies, transmission timings and cells. Mapping restrictions in logical channel prioritisation control which numerology(ies), cell(s), and transmission timing(s) a logical channel can use (see clause 16.1.2).

Different kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred. Logical channels are classified into two groups: Control Channels and Traffic Channels. Control channels are used for the transfer of control plane information only:

• Broadcast Control Channel (BCCH): a downlink channel for broadcasting system control information.
• Paging Control Channel (PCCH): a downlink channel that carries paging messages.
• 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.
• 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.

Traffic channels are used for the transfer of user plane information only:

• Dedicated Traffic Channel (DTCH): point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.

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;
• 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.

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.

The HARQ functionality ensures delivery between peer entities at Layer 1. A single HARQ process supports one TB when the physical layer is not configured for downlink/uplink spatial multiplexing, and when the physical layer is configured for downlink/uplink spatial multiplexing, a single HARQ process supports one or multiple TBs.

The RLC sublayer supports three transmission modes:

• Transparent Mode (TM);
• Unacknowledged Mode (UM);
• Acknowledged Mode (AM).

The RLC configuration is per logical channel with no dependency on numerologies and/or transmission durations, and ARQ can operate on any of the numerologies and/or transmission durations the logical channel is configured with. For SRB0, paging and broadcast system information, TM mode is used. For other SRBs AM mode used. For DRBs, either UM or AM mode are used.

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 (UM and AM);
• Error Correction through ARQ (AM only);
• Segmentation (AM and UM) and re-segmentation (AM only) of RLC SDUs;
• Reassembly of SDU (AM and UM);
• Duplicate Detection (AM only);
• RLC SDU discard (AM and UM);
• RLC re-establishment;
• Protocol error detection (AM only).

The ARQ within the RLC sublayer has the following characteristics:

• ARQ retransmits RLC SDUs or RLC SDU segments based on RLC status reports;
• Polling for RLC status report is used when needed by RLC;
• RLC receiver can also trigger RLC status report after detecting a missing RLC SDU or RLC SDU segment.

The main services and functions of the PDCP sublayer include:

• Transfer of data (user plane or control plane);
• Maintenance of PDCP SNs;
• Header compression and decompression using the ROHC protocol;
• Header compression and decompression using EHC protocol;
• Compression and decompression of uplink PDCP SDUs: DEFLATE based UDC only;
• Ciphering and deciphering;
• Integrity protection and integrity verification;
• Timer based SDU discard;
• For split bearers, routing;
• Duplication;
• Reordering and in-order delivery;
• Out-of-order delivery;
• Duplicate discarding.

Since PDCP does not allow COUNT to wrap around in DL and UL, it is up to the network to prevent it from happening (e.g. by using a release and add of the corresponding radio bearer or a full configuration).