n Fig. 6-6. Remainder PDU formats are shown in the 3GPP standard. The RLC SN number range is configurable as we said above. The appropriate width to be used depends on the target Round Trip Time of the RLC protocol and the target bit rate of the air interface, which together define the maximum amount of outstanding/open RLC sequence numbers for either acknowledgment (AM mode) or receive side re-ordering (UM mode). For example, assuming a maximum air interface latency associated with HARQ of 10Ms (multiple retransmissions in a worst-case scenario) and a 1 Gbps connection with 390B packet size, we can estimate the re-ordering window to be 1 Gbps/[390B X (8 bit/Byte)] X 20Ms ~ = 3200 packets. A 12b SN would be appropriate for this use-case. Note: The AM mode 18b sequence number option can complicate "straightforward" imple- mentations where the retransmission algorithm is implemented using software array implemen- tations that rely on SN indexing to find a PDU context. 18b corresponds to 256 K entries into this array which (in a DU) can lead to unrealistic memory dimensioning requirements. Receive side RLC operation includes a state machine/process associated with each logical channel that contains amongst others: a window of SN that are not or partially received. On arrival of a PDU that is out-of- sequence, this will either be ignored (if this SN has already been received and is thus interpreted as a duplicate) or the window is advanced to include the latest SN (if the received PDU is ahead of the window). a reassembly timer for each RLC (channel) context. Reception of a PDU that contains a partial SDU, this reassembly timer is started. The associated state machines support fragmentation/reassembly and reliability through retrans- mission of lost SDU (fragments). The segmentation process involves unpacking an RLC PDU into RLC SDUs, or portions of SDUs. The RLC PDU size is based on transport block size and thus dictated 222 Open Radio Access Network (O-RAN) Systems Architecture and Design by the MAC layer (schedule