 of SDUi. RLC SDU RLC header RLC header RLC PDU Figure 3.18 RLC PDU structure. Reproduced with permission from © 2009 3GPP [14] 3GPP Evolved Packet System (EPS) UE/E-UTRAN Radio Bearers entity PDCP-SAP PDCP-SAP C-SAP sublayer PDCP entity PDCP entity PDCP PDU RLC SDU RLC UM-SAP RLC AM-SAP sublayer Figure 3.19 A structural view PDCP sublayer. Reproduced with permission from © 2009 3GPP [18] 3.6.3 PDCP Sublayer The PDCP is the point of interaction of the Layer 2 with RRC (layer 3). Figure 3.19 shows a possible architecture for the PDCP sublayer which is based on radio interface protocol architecture defined in [14]. Each Radio Bearer (i.e., DRB (data RB) and SRB (signaling RB), except for SRBO) is associated with one PDCP entity. Each PDCP entity is associated with one or two (one for each direction) RLC entities depending on the RB characteristic (i.e., uni-directional or bi-directional) and RLC mode. The PDCP entities are located in the PDCP sublayer. The PDCP sublayer is configured by upper layers [14,18]. 3.6.3.1 PDCP Entities The PDCP entities are located in the PDCP sublayer and several PDCP entities may be defined for a UE. Each PDCP entity is carrying the data of one radio bearer. Each PDCP entity carrying user plane data may be configured to use header compression. 3GPP is currently supporting ROHC (Robust Header Compression Protocol) and every PDCP entity uses at most one ROHC instance. 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 3.20 represents a functional view of the PDCP entity for the PDCP sublayer which is based on the radio interface protocol architecture defined in [14]. 3.6.3.2 Services and Functions PDCP provides its services to the RRC and user plane upper layers at the UE or to the relay at the evolved Node B (eNB). The maximum supported size of a PDCP SDU is 8188 octets. PDCP is used for SRBs and DRBs mapped on DCCH and DTCH type of logical channels. PDCP is not used for any other type of logical chan