the A-MPDU sub-frame, and the other A-MPDU sub-frames can be recovered. All A-MPDU sub-frames must have the same destination on the radio link. On the other hand, the destination or the source address of the MPDU frame may be different. 4.1.3.2. A-MSDU frame The A-MSDU (Aggregate MAC Service Data Unit) frame is a sequence of A-MSDU sub-frames (Figure 4.3). Each A-MSDU sub-frame contains an A-MSDU header, an MSDU data unit and pad bytes. Header Header A-MSDU sub-frame A-MSDU sub-frame MAC header A-MSDU frame MPDU frame Figure 4.3. Structure of A-MSDU frame The A-MSDU header consists of three fields: the MAC addresses of the destination and source of the MAC frame and the length of the MSDU data unit. With the exception of the last A-MSDU sub-frame, padding bytes are added SO that the size of each A-MSDU sub-frame is a multiple of four bytes. Since the A-MSDU sub-frames of the same A-MSDU frame are contained in the same MPDU data unit, the same encryption applies to all the sub-frames. Both forms of aggregation may be combined: an A-MPDU frame may contain an A-MSDU frame. Wi-Fi Integration to the 4G Mobile Network 4.1.4. Control frames 4.1.4.1. Block acknowledgment The block acknowledgment mechanism improves the efficiency of the channel by grouping multiple acknowledgments in a single control frame. There are two types of mechanisms: immediate acknowledgment and delayed acknowledgment. The immediate acknowledgment mechanism is suitable for high- bandwidth and low-latency applications, whereas the delayed acknowledgment mechanism is suitable for applications that tolerate moderate latency. The original design of the acknowledgment mechanism requires that each transmitted frame be acknowledged separately by the Ack control frame (Figure 4.4). Standard acknowledgment Source Destination Immediate block acknowledgment Block Source Destination Block Delayed block acknowledgment Block Source Destination Block Figure 4.4. Block acknowledgment If the immediate acknowledgment mechanism is used, then the recipient must respo