lsewhere in this document, whilst operating under the practical constraints imposed by HARQ turnaround times, bandwidth constraints, and SO on. R/LCID Fixed-size CE R/F/LCID/L Variable-size CE R/F/LCID/L MAC SDU R/F/LCID/L MAC SDU MAC subPDU CE1 MAC subPDU CE2 MAC subPDU W SDU MAC subPDU W SDU MAC subPDU W Pad (opt) FIGURE 6-7 MAC PDU example (downlink). Chapter 6 User-plane application components 223 The MAC PDU processing component is a traditional packet processing component. MAC PDUs are equivalent to Physical Layer Transport Blocks (TBs) and consist of a sequence of "subPDUs" which can be: MAC subheaders with padding or including padding. MAC subheaders with a Control Element (CE). The MAC layer defines dozens of Control Elements, used for buffer status reporting, DRX configuration and similar control components that can be communicated through small messages without going to the Radio Resource Control (RRC) layer control stack MAC subheaders with a MAC Service Data Unit (SDU). These MAC subPDUs carry RLC PDUs and therefore end-user payload. The MAC PDU is a concatenation of multiple MAC subPDUs, shown below in Fig. 6-7: Each subPDU has a header which includes the following fields: Logical Channel ID (LCID) field identifies the logical channel instance of the corresponding MAC SDU to enable transfer to the appropriate RLC entity. This can optionally be extended to extended Logical Channel ID (eLCID) Length (L) field that indicates the length of the corresponding MAC SDU or variable-sized MAC CE in bytes A Format (F) field that indicates the length of the Length field (8 bits of 16 bits) A Reserved (R) bit, set to 0 eCPRI protocol overview In LTE macro cell deployments, Common Public Radio Interface (CPRI) is the dominant form of sample transport between BBU and RRH. The CPRI standard defines a GSM/ WCDMA/LTE optimized synchronous framing mechanism that carries user data, manage- ment and control. There is extensively flexibility for supporting multiple carriers and different Packet Switched Circuit Switched 