NR & LTE implements PDCP in both the User Plane and Control Plane. This is unlike UMTS, where PDCP was only found in the User Plane. The main reason for the difference is that PDCP in NR & LTE takes on the role of security, i.e. encryption and integrity.
Note: The PDCP sublayer is configured by upper layers RRC. The PDCP sublayer is used for RBs mapped on DCCH and DTCH type of logical channels. The PDCP sublayer is not used for any other type of logical channel.
Each RB (except for SRB0) is associated with one PDCP entity. Each PDCP entity is associated with one, two, or four RLC entities depending on the RB characteristic (e.g uni-directional/bi-directional or split/non-split) or RLC mode:
- For split bearers or for RBs configured with PDCP duplication, each PDCP entity is associated with two UM RLC entities (for same direction), four UM RLC entities (two for each direction), or two AM RLC entities (for same direction).
- Otherwise, each PDCP entity is associated with one UM RLC entity, two UM RLC entities (one for each direction), or one AM RLC entity
- The figure Above shows the functional overview of the PDCP layer.
- In transmitting the operation for each PDCP SDU, the PDCP entity first starts the discard timer and assigns COUNT associated with the PDCP SDU, performs header compression and
then integrity protection and ciphering which is configurable at a DRB level.
- If split bearer is configured, the PDCP PDU can be delivered to either the RLC primary path or the secondary path based on condition whether the total amount of data
volume is equal or above the configured threshold.
- In receiving operation, the PDCP entity determines the COUNT value of the received PDCP PDU and performs deciphering and integrity verification for the packet.
- Duplication detection and removal is then performed.
- If for a DRB, out-of order-delivery is not configured, reordering is also performed. Then after header decompression, the data packet is ready to be delivered to upper layers.
- The PDCP entities are located in the PDCP sublayer.
- Several PDCP entities may be defined for a UE.
- Each PDCP entity is carrying the data of one radio bearer.
- A PDCP entity is associated either to the control plane or the user plane depending on which radio bearer it is carrying data for.
- Figure above represents the functional view of the PDCP entity for the PDCP sublayer.
Services provided to upper layers
- transfer of user plane data;
- transfer of control plane data;
- header compression;
- integrity protection.
Services expected from lower layers
A PDCP entity expects the following services from lower layers per RLC entity.
- acknowledged data transfer service, including indication of successful delivery of PDCP PDUs;
- unacknowledged data transfer service.
- Transfer of data (user plane or control plane).
- Maintenance of PDCP SNs
- header compression and decompression using the ROHC protocol
- ciphering and deciphering
- integrity protection and integrity verification
- timer based SDU discard
- for split bearers, routing
- reordering and in-order delivery
- out-of-order delivery
- duplicate discarding.
- PDCP entity handling
- Data transfer
- SDU discard
- Status reporting
- Data recovery
- Data volume calculation
- Header compression and decompression
- Ciphering and deciphering
- Integrity protection and verification
- Handling of unknown, unforeseen, and erroneous protocol data
- PDCP duplication
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