cpriPcid ecpriPcid ecpriSeqid ecpriSeqid dataDirection payloadVersion filterIndex frameld subframeld slotld slotld symbolld sectionld sectionld symlno startPrbu startPrbu numPrbu udCompHdr (optional) Reserved (optional) udCompParam (optional) iSample (1st RE in PRB) qSample (1st RE in PRB) per RE iSample (12th RE in PRB) qSample (12th RE in PRB) udCompParam (optional) iSample (1st RE in PRB) qSample (1st RE in PRB) iSample (12th RE in PRB) qSample (12th RE in PRB) sectionld sectionld symlno startPrbu startPrbu numPrbu udCompHdr (optional) Reserved (optional) udCompParam (optional) iSample (1st RE in PRB) qSample (1st RE in PRB) iSample (12th RE in PRB) qSample (12th RE in PRB) udCompParam (optional) iSample (1st RE in PRB) qSample (1st RE in PRB) iSample (12th RE in PRB) qSample (12th RE in PRB) FIGURE 6-10 eCPRI U-plane message format. 226 Open Radio Access Network (O-RAN) Systems Architecture and Design IEEE1588. Operation and Maintenance (OAM) and control and management also rely on stan- dard IEEE/Ethernet mechanisms. Both RoE and eCPRI use a custom header including a Flow Identifier (RoE) or Physical Channel ID (eCPRI) to define the antenna/port/flow/link that bits are carried for. Assuming large payload/frame sizes to be supported (few KB/OFDM frame), the 10 S of Byte of Ethernet + protocol framing overhead constitute a very small percentage efficiency loss. The eCPRI standard 2 defines multiple eCPRI header types, only a subset of which is used by the O-RAN Control/User/Synchronization (CUS) fronthaul specification: 0000 0000b = IQ data message 0000 0010b = Real - time control data message 0000 0101b = transport network delay measurement message As an example, we show the CUS plane frame format for IQ data below (Fig. 6-10): Key header fields include: eCPRIVersion, part of the eCPRI common header. 0001b is interpreted as valid for all versions of eCPRI up to 2.0. Other values are initially reserved. eCPRIConcat, part of the eCPRI common header. Indicates concatenation of multiple eCPRI frames into a single