## 3 O-MAC Protocol Design Goals and Architecture
### Interior Communication
#### 3.1.1 Optimized Data Frame Format Model  
The goal of the ODFF is to enable sensor nodes to maintain QoS provisioning. The ODFF is preferred over the IEEE 804.15.4 standard and modifies existing features and includes new features depicted in Figure 2. The architecture of the ODFF is compatible with a robust and reliable wireless link. The ODFF can individually detect every radio in a WSN as well as the process and layout of communications between the radios. It also works beyond the physical and MAC layers, providing the cross-layered support to connect with the routing protocol. The ODFF reduces the channel access time, data transfer time, preamble transfer time, acknowledgement time, and retry transmission time because the performance of small devices--especially sensor nodes--depends on the reduced amount of time. Hence, the timing overhead can affect the lifetime of the node. In addition, the ODFF can be used with low-duty-cycle MAC protocols due to the reduced preamble size, addressing fields, addition of automatic buffering and employment of the anycast message addressing methodology. These modifications also enable the ODFF to transfer more data compared with the IEEE 802.15.4 standard. Based on the modifications and inclusion of new features, we focus on the following parameters to determine the impact. Figure 2: Optimized data frame format. * Channel access time. * Data frame transfer time. * Transmission timing for acknowledgement. * Retry transmission. * Possible transfer data rate in the ideal and worst-case scenarios. In this section, the ODFF improves the CSMA portion of IEEE 802.15.4 and determines the time consumed in accessing the channel. As in IEEE 802.15.4, a sensor node in the ODFF must wait for a random back-off period within the range of {0, 2\({}^{\gamma}\)-1}. The node can wait for the channel up to 2\({}^{\gamma}\)-1 in the case of congestion, and then, it begins to access the channel again. Here, we set a value of 3 for \"\(\gamma\)\". Thus, the channel access time can be obtained as:  
\[\Delta\varphi+c=(2^{\gamma}-1)\times(U_{bt})^{2}+\omega\times\beta\]  
Hence:  
\[ \begin{split} C_{at}=\Delta\varphi+c\\ C_{at}=\Delta\varphi+c=(2^{\gamma}-1)\times(U_{bt})^{2}+\omega \times\beta\end{split}  \tag{1}\]  
We use a maximum wait time of \(\omega=8\) to send the data over the channel after a collision to determine whether there is any further possibility of a collision. Each symbol takes 16 \(\mu\)s.  
The ODFF reduced the preamble size from 4 bytes to 1 byte because the 4-byte preamble size creates overhead and reduces the throughput. Based on the standard transfer rate of the modem, we have calculated the data frame transfer time as:  
\[T_{f}=\frac{(P_{max}+P_{t}+F_{dil}+F_{l})\times\omega}{\Delta R} \tag{2}\]  
Transmission Timing for Acknowledgement  
We have already indicated that our objective is to reduce the preamble size. Therefore, the ODFF uses an 8-byte acknowledgement frame that can reduce the substantial amount of time in the acknowledgement frame, as shown in Figure 3. Based on the standard data rate of the modem, we can calculate the acknowledgement transmission time \"\(T_{ack}\)\" as:  
\[T_{ack}=\frac{\left(M_{pdu}+P_{l}+F_{dil}+F_{l}\right)\times\omega}{\Delta R} \tag{3}\]  
Retry Transmission  
The node can wait for a particular amount of time for the MAC acknowledgement prior to attempting a retry transmission. We have set a wait time of 48 symbols. Each symbol takes 16 \(\mu\)s. If the number of symbols increases, the transmitting node could stay for a longer time in an ideal mode. Based on our experiments, we have determined that the ideal wait time for MAC could be 30, 36, 42, 48 or 54 symbols. We chose 48 symbols, but a smaller number could be chosen instead. If the network consists of multi-hops, then a smaller number is not an ideal choice for a large-scale wireless sensor network. Therefore, a 48-symbol wait time works perfectly on single-hop as well as multi-hop WSNs.