In this section, the conventional GS scheme and its underlying problem will first be described.
Furthermore, the proposed GFS mechanism for block acknowledgement will be further ex-plained as follows.
5.2.1 Conventional Greedy Scheme
The conventional block acknowledgement mechanism denoted as the GS scheme is described in this subsection. For the generalization of data aggregation mechanisms, a system parameter called window size W is introduced to denote the maximum number of data packets that are contained within a single frame. The characteristics of the conventional GS scheme is explained with the example as shown in Fig. 5.1(a) under W = 4 as follows.
In the conventional GS scheme, the transmitter assigns each data packet with an SN for packet identification, e.g., SN = 1 for D1 and SN = 2 for D2. As illustrated in Fig. 5.1(a), the first aggregate frame consisting of W data packets from D1 to D4 is delivered by the transmitter; while both D2 and D4 packets denoted with the ∗ sign are considered corrupted.
The block ACK packet consists of an additional SSN field in front of its bitmap array to record the sequence number of the starting packet within the aggregate frame, e.g., SSN = 1 denotes that D1 is the starting packet in the first frame; and SSN = 2 represents D2 as the starting packet for the second frame. Moreover, the bitmap array of size W within the block ACK packet indicates the packet correctness based on the consecutive SNs.
As shown in Fig. 5.1(a), the bitmap from the first block ACK packet b = [1010] with SSN
= 1 contains the correctness information of the aggregated data packets from SN = 1 to SN
= 1 + (W − 1) = 4, i.e., for the consecutive D1 to D4 data packets. It is noticed that the one value within the bitmap denotes that the corresponding packet is correctly received; while the zero value implies that the correctness of the corresponding packet should be determined by the transmitter since the packet can be either corrupted or previously acknowledged.
After the reception of the first block ACK packet, the transmitter redelivers the previously unacknowledged D2 and D4 packets together with the newly scheduled D5 and D6 packets in the second aggregate frame. Associated with SSN = 2 and bitmap b = [1011] in the second block ACK packet, the receiver notifies the transmitter that D2, D4, and D5 packets are correctly received while the correctness of D3 packet is undetermined. Based on the previous reception within the transmitter, it can be observed that packet D3 was successfully received such that it is not required to be retransmitted. Furthermore, it is worthwhile to notice that the correctness of packet D6 becomes undetermined even though it is correctly received by the receiver. The performance degradation due to the insufficient bitmap for recording the correctness information can therefore be perceived.
5.2.2 Proposed Greedy Fast-Shift (GFS) Scheme
The greedy fast-shift (GFS) block acknowledgement mechanism is proposed in this subsection as an enhanced version of the conventional GS scheme. The main objective of the proposed
GFS scheme is to mitigate the performance degradation due to insufficient bitmap of the conventional GS scheme. The procedures of proposed GFS scheme with W = 4 is illustrated in the exemplified schematic diagram of Fig. 5.1(b). Inherited from the conventional GS scheme, the characteristics of both the SN and SSN fields in the transmitter side are still preserved.
For efficiency consideration, the mechanism of retransmitting both the corrupted packets and the new-coming packets are also employed. The major differences between the proposed GFS mechanism and the conventional GS scheme are on the methodologies to determine the SSN value at the receiver side. In the GS scheme, the SSN of the feedback block ACK packet is determined by the SN of first packet in the aggregate frame. In other words, the receiver does not change the value of SSN, which is actually assigned by the transmitter. On the other hand, in the proposed GFS scheme, the SSN value is given by the receiver as the SN of the first unreceived data packet. Consequently, the SSN value of the block ACK packet may not necessarily be equal to the SN of first packet in the aggregate frame. It can be a larger value or even beyond the largest SN in the aggregate frame.
The bitmap array, i.e., the acknowledgement window, specified in the block ACK packet of the proposed GFS scheme still represents the packet correctness based on the consecutive SNs starting with the SSN value. Considering the same case as in Fig. 5.1(a), the packets D2 and D4 in the first aggregate frame of Fig. 5.1(b) are corrupted. Instead of receiving the block ACK with SSN = 1, the transmitter receives a block ACK packet with SSN = 2 in the proposed GFS scheme since the SSN is determined by the receiver as the SN of the first unreceived data packet, i.e., the packet D2. Likewise, after the reception of the first block ACK packet, the transmitter redelivers the previously unacknowledged D2 and D4 packets together with the newly scheduled D5 and D6 packets in the second aggregate frame. After receiving the second aggregate frame with correct D2, D4, D5, and D6 packets, the receiver knows that the data packets from D1 to D6 are correctly received. Therefore, the receiver assigns the SSN to be 7 which is the SN of the first unreceived packets. It can be observed that the SSN value will be larger than the SN’s range of the aggregate frame. Afterwards, the receiver sends the block ACK packet with SSN = 7 and bitmap b = [0000], which indicates that the receiver does not receive packets from D7 to D10 and all the packets before D7, i.e.,
D1 to D6, are correctly received.
Instead of providing explicit acknowledgement to each packet in the aggregate frame, the proposed GFS scheme adopts a hybrid method to both implicitly acknowledge the correctly received packets before the SSN and also explicitly provide the correctness information of those packets after the SSN. Contributing to this hybrid method, the SSN value will grow faster than that in the conventional GS scheme, resulting in the phenomenon of fast-shift acknowledgement window specified in the block ACK packet. The performance degradation due to the insufficient bitmap can therefore be mitigated. The detailed processes and the analytical model of the proposed GFS scheme are further explained in Subsection 5.3.2.