-establishment procedure for RLC AM; Re-transmission of PDCP SDUs at handover for RLC AM; Ciphering and deciphering; Timer-based SDU discarding in the uplink. The main services and functions of the PDCP in the control-plane include Ciphering and Integrity Protection, as well as Transfer of control-plane data. Figure 5-9 illustrates one possible structure for the PDCP sub-layer (without restricting other implementations), and is based on the radio interface protocol architecture defined in reference [21]. A PDCP PDU is a bit string that is byte aligned. As shown in Figure 5-9, each Radio Bearer (RB) 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 [22]. The PDCP entities are located in the PDCP sub-layer and are configured by upper layers [21]. The Control SAP (C-SAP) is the PDCP interface with the RRC layer. Several PDCP entities may be defined for a UE. Each PDCP entity carrying user-plane data may be configured to use header compression. Each PDCP entity is carrying the data of one radio bearer. The 3GPP LTE supports the ROHC protocol as specified by reference [9]. Each PDCP entity uses one ROHC instance. A PDCP entity is associated with either the control-plane or the user-plane, depending on which radio bearer is carrying data. Figure 5-10 shows the functional decomposition of the PDCP entity in PDCP sub-layer. The figure is based on the radio interface protocol architecture defined in reference [20]. The PDCP provides services to the RRC sub-layer and upper layers in the user-plane at Radio Bearers C-SAP (Interface with RRC) PDCP-SAP PDCP-SAP PDCP entity PDCP entity PDCP Sublayer PDCP -PDU RLC-SDU RLC UM-SAP RLC AM-SAP RLCSublayer PDCP Header PDCP SDU PDCP PDU FIGURE 5-9 PDCP sub-layer and structure of PDCP PDU [21] CHAPTER 5 The IEEE 802.16m Convergence Sub-Layer In order Delivery and Duplicate Sequence Numbering Detection (U-plane) Header Compress