nd to a BlockAckReq frame with a BlockAck frame (Figure 4.4). When the recipient sends the BlockAck frame, the initiator retransmits all frames that are not acknowledged in the BlockAck frame, either in another block or individually. 802. 11n Interface If the delayed acknowledgment mechanism is used, then the recipient must respond to a BlockAckReq control frame with an Ack control frame. The recipient must then send his response in a BlockAck control frame, which the initiator acknowledges by an Ack control frame (Figure 4.4). The block acknowledgment mechanism has been introduced with the QoS mechanism. The block acknowledgment mechanism was initially optional, but the efficiency gains coupled with the aggregate frame transmission resulted in BlockAck control frame support being required for the 802.11n interface. The initial definition of the block acknowledgment mechanism took into account the processing of frame-related sequence numbers and fragment numbers. For the 802.11n interface, the block acknowledgment mechanism can be compressed, thus only processing the sequence number. 4.1.4.2. Control frame structure The BlockAckReq control frame is transmitted by the source of several MAC frames for block acknowledgment (Figure 4.5). Fragment number Initial segment (value to ZERO) number Initial segment TID value number Frame Duration BlockAckReq Control Control Information Multi Compressed Reserved TID_INFO Policy Bitmap Frame Duration BlockAck Control Control Information TID value Initial segment Bitmap number Initial segment number Bitmap Figure 4.5. Control frame structure Wi-Fi Integration to the 4G Mobile Network The BAR or BA Control field contains the following information: - Ack Policy: this bit indicates whether the block acknowledgment is immediate (bit to ZERO) or not (bit to ONE); - Multi-TID: this bit indicates whether the block acknowledgment applies to several priority levels (bit to ONE) or not (bit to ZERO); - Compressed Bitmap: this bit indicates whether the block acknowledgment is compressed (