The main purpose of the LTE system PDCP protocol layer data packet is sent or received the peer PDCP entity. The completion sub layer mainly the following aspects: IP header compression and decompression, encryption of data and signaling, and the signaling integrity protection. The following figure shows the main functional model of the PDCP layer of the user plane and the control plane.
In the control plane, ciphering and integrity protection is mandatory function; and in the user plane, header compression reliable (the ROHC) is mandatory functions, data encryption is an optional function, where the data may be user data, may be applied layer signaling, such as SIP, RTCP and the like.
PDCP is located in the upper side of the UE RRC and user plane, or provide a relay service to the eNodeB side, comprising transmitting user plane data, transmission of the control plane data, header compression, ciphering, integrity protection.
PDCP layer to a lower layer service can be provided comprising: transparent data transfer service, acknowledged data transmission services (including indication of successfully transmitted PDCP PDU), unacknowledged data transmission services (sequential transmission, or discarding the packet replication) and the like.
Specifically, the user-plane PDCP layer include the following functions.
● header compression and decompression, supports only one compression algorithm that ROHC algorithm.
● Data transfer user plane, i.e. from the NAS receives PDCP SDU data forwarding sublayer to the RLC layer, and vice versa.
● RLC AM submit to the order of the upper PDCP PDU when reestablishing process.
● When the RLC AM PDCP SDU reestablishing process of duplicate detection of lower layer.
● RLC AM retransmission of switching the PDCP SDU.
● data encryption.
● timer-based SDU discard uplink.
DETAILED PDCP layer function of the control plane comprises the following.
● encryption and integrity protection.
● control plane data transmission, i.e. data received PDCP SDU from the RRC layer, and forwarded to the RLC layer, and vice versa.
Compared to PDCP layer of the UMTS system, the LTE system, the PDCP layer exhibits the following features.
● compression algorithm is simple, supports only one compression algorithm.
● does not support lossless relocation.
● must support encryption.
PDCP entity
PDCP entity located in the PDCP layer. For a UE, a plurality of PDCP entities can be defined. Each PDCP entity for carrying user plane data may be configured to use a header compression technique, data for each PDCP entity carries one radio bearer. The current protocol version, only supports ROHC protocol, each PDCP entity can only be used up to a ROHC instance.
A PDCP entity is associated with a control plane or a user plane, depending on what data it carries radio bearer. The following figure shows a schematic view of the PDCP layer function.
PDCP PDU structure
The following figure shows the schematic structure of the PDCP PDU.
Wherein PDCP PDU length is an integral multiple of a byte, the PDCP header may be one or two bytes long.
