 (optional) MAC-I (cont.) (optional) MAC-I (cont.) (optional) FIGURE 6-3 PDCP PDU example (data radio bearer, 12b SN). 218 Open Radio Access Network (O-RAN) Systems Architecture and Design An example of a PDCP header for a DRB with a 12-bit Sequence Number (SN) is shown below in Fig. 6-3. Remainder PDU formats including SRB and different SN lengths are shown in the 3GPP standard. The PDCP SN number range is configurable between 12 and 18 bits. The reasoning is explained in the RLC section below (Section 6.3). Receive side PDCP operation includes a state machine/process associated with each radio bearer that contains amongst others: a set of COUNT values that indicate next SDU to be transmitted, next SDU expected to be received and first SDU not delivered to upper layers. These values are used to support the in-sequence deliver and associated reordering process. The COUNT value is a concatenation of the hyper frame number and the PDCP SN. a set of timers for transmit (Discard Timer) and receive (Reordering Timer) operation supporting timeout operation when lower layers stop performing expected functions (i.e., packet transmission and reception) and associated buffers need to be emptied and reset. The PDCP protocol does not include any fragmentation and reassembly and thus oper- ates at the GTP/IP packet rate and not at the Transport Block cadence dictated by the Physical Layer and driving the RLC and MAC layers. Implementation of PDCP protocols in software is therefore typically done on IP packet buffers and associated buffer management algorithms as shown in Fig. 6-4. A typical memory management scheme is shown below. We show two memory domains in different shades here, one presecurity PDCP (IP/IPSec, GTP, PDCP) and one postsecurity PDCP (PDCP, RLC, MAC). The presecurity PDCP memory domain This memory domain is characterized by: Buffer size defined by transport packet size (IP centric buffer size optimization) Single-use buffers-depending on Dual Connectivity implementation at PDCP level Ethernet IPSec Payload Et