geted example (and as such, RTP0UDPoIP). RoHC is a complex protocol (two state machines, various field encoding methods, Cyclic Redundancy Check (CRC), etc.) and as such gives a high loading on cores when implemented in pure software. The initial version is called ROHC vl and based on RFC 3095 and RFC 4815 (with clarifications in RFC 3759). It defines UDP/IP, RTP/UDP/IP, ESP/IP and Uncompressed Profiles but is a highly complex standard with many challenges on interpretation and by implication on interoperability. Typical VolP codec packet (eg G.723.1) 40 Byte 24 Byte IP Header Payload Token Payload 1-4 Byte FIGURE 6-5 Robust header compression (RoHC) for PDCP PDU payload size reduction. 220 Open Radio Access Network (O-RAN) Systems Architecture and Design Because of this, a simplified version was defined in 2007 in RFC 4995. This version clari- fies many of the concepts used but doesn't change the RoHC framework originally defined in RFC 3095. Building on RFC 4995, RFC 4996 defines a TCP profile. RoHC has been incorporated in 3GPP standards since release 8 (first LTE release) as part of the PDCP standard. RLC Protocol As we explained before, the main role of the Radio Link Control (RLC) layer is transferring of upper lay SDUs to the MAC layer for transmission over the Physical Layer. This includes three modes of operation: Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). UM and AM modes both add an RLC header to the packet that contains the following fields: SN-this allows the receiver to support in-sequence delivery of the packet to the upper layer. In AM mode, the SN is also used to convey acknowledgment information back to the transmitter for the purpose of triggering retransmission. RLC sequence numbers can be 6 or 12 bits wide for UM mode, or 12 or 18 bits for AM mode operation. Segmentation Offset (SO) field which allows the receiver side to know which piece of a segmented RLC SDU packet buffer the received PDU belongs to. This field is combined with the SI field. Segmentation Ind