## 4 MAC Protocol Design
### Graded Tailoring Strategy  
In IEEE 802.15.4, two clear channel assessments (CCAs) will be performed before every data packet transmission. There are two situations, after the data packet or after the ACK packet, where the channel will be detected idle. In both cases, the CCA will be successfully performed. To distinguish the two cases and avoid collisions, the double-CCA mechanism is applied. Several improved methods were proposed to achieve better performance [41; 42; 43; 44]. In this article, we focus on frame tailoring and frame adapting, aiming to reduce the overhead caused by CCA [41; 42]. They are expected to reduce the power consumption and improve the network throughput. The heart of the strategies is to adjust the tail size of the data packet, and reserve enough time for the turnaround action, so that the ACK packet is transmitted at the next backoff boundary. This way, the existence of a complete unit before the ACK packet will be avoided, then only one CCA is required. In the frame tailoring strategy proposed by [PERSON] [41], by padding the valid data with zeros, the physics layer data unit (PPDU) occupies a fixed length in the last backoff unit and reserves enough time for node turn around (Tx-Rx or Rx-Tx). In contrast, in frame adapting proposed by [PERSON] [42], a threshold of the data packet tail length was introduced. Similarly, the threshold ensures that there is always enough time to perform the ACK packet in the coming backoff unit. When the packet tail length exceeds the given threshold, bytes will be chopped and transmitted in next packet. The packet adapting strategy was considered to have a better performance in packet delay and energy consumption saving, while their throughput improvement was nearly identical. However, in frame adapting, the improved packet delay performance obtained within the chopped bytes were not taken into account. Since the chopped bytes were expected to be transmitted in the next data  
Figure 6: Operating process of a ISWN node. packet of the same node, it may lead to extra packet transmission and intensify the channel contention. Thus, we propose an improved tailoring strategy named Graded Tailoring Strategy. The improved strategy is expected to reduce the adding of the valid bytes. We assume that a same backoff unit size is used, and each backoff unit includes 10 bytes or 20 symbols. Like tailoring strategy, zeros are padded to the data packet. However, the packet tail will be filled with multiple sizes instead of a uniform one. To simplify, we put forward a two grades of packet size tailoring, and the partition point is \(N\) (\(0<X\leq 8\)) and 8 (in symbols). Assuming that the size of a data packet tail is x, if \(N<X\leq 8\), then zeros will be padded to the packet and make the tail size to 8; if \(0<X\leq N\) or \(8<X\leq 20\), then zeros will be padded to make the tail size N. The strategy is demonstrated in Table 3.  
Note that when x is larger than 8, the new packet tail will extend to the next backoff unit. Also, assuming that the length of the valid data packet is randomly distributed, the ideal value of \(N\) is zero in order to reduce the invalid bytes. Besides, in CCA-ED mechanism, the channel energy checking last for eight symbols, here we assume that for energy detect, at least two symbol of data transmitting are necessary to ensure that the channel is assessed busy. Then the value of N is set to 2. The access processes of the standard strategy and the three improved strategies are shown in Figure 7.  
Figure 7: Graded Tailoring Strategies. \begin{table}
\begin{tabular}{l} \hline \hline
**Graded Tailoring Strategy** \\ \hline X: the original length of the data packet in the last unit \\ Y: the size of the packet tail after the graded tailoring \\ N: the partition point \\ Check the length of the physics layer data unit (PPDU) \\ If \(N<X\leq 8\) \\ \(Y\gets 8\) \\ else \\ \(Y\gets N\) \\ Stop \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Graded Tailoring Strategy.