ta PDUs and RLC control PDUs. RLC data PDUs are used by TM, UM, and AM RLC entities to transfer upper layer PDUs. RLC control PDUs are used by AM RLC entity to perform ARQ procedures. The STATUS PDU is used by the receiving side of an AM RLC entity to inform the peer AM RLC entity about correctly received as well as lost RLC data PDUs. As shown in Figure 7-25, the TMD PDU consists of only a data field, and does not include any RLC headers. The UMD PDU consists of a data field and a UMD PDU header. The UMD PDU header consists of a fixed part (i.e., fields that are present for every UMD PDU) and an extension part (i.e., fields that are present for a UMD PDU when necessary). The fixed part of the UMD PDU header itself is octet-aligned and consists of Framing Information, Extension, and Sequence Number (SN) fields. The extension part of the UMD PDU header itself is octet-aligned and includes Extension bits and Length Indicators. The AMD PDU consists of a data field and an AMD PDU header. The AMD PDU header comprises a fixed part (i.e., fields that are present for every AMD PDU) and an extension part (i.e., fields that are present for an AMD PDU when necessary). The fixed part of the AMD PDU header itself is octet-aligned and consists of Data/Control (D/C), Re-segmentation Flag (RF), Polling bit (P), Framing Indicator (FI), Extension bit (E), and Sequence Number (SN). The extension part of the AMD PDU header itself is octet-aligned and consists of Extension and Length Indicator fields. The STATUS PDU consists of a payload and an RLC control PDU header [11]. The packet processing stages in 3GPP LTE in various sub-layers is illustrated in Figure 7-26. 7.4.4 ARQ and HARQ in LTE The E-UTRA supports ARQ and HARQ functionalities. The ARQ functionality provides error correction by re-transmissions in the acknowledged mode at Layer 2. The HARQ functionality improves packet transmission and detection between peer entities at Layer 1. The HARQ within the MAC sub-layer is characterized by an N-process Stop-and-Wait protocol and 