## 5 Lightweight Clear Channel Assessment (LW-CCA)
### RSSI Check Time Models in LW-CCA  
This section examines the hardware and software RSSI status check time. Based on the phase 2 Section 5.1, LW-CCA divides the radio check time into \(T_{\Delta}\) and t_RSSI time periods. The following function is responsible for doing the RSSI validation under a period of time t_RSSI = 0.128 ms (8 symbols) and \(T_{\Delta}\) = 0.32 ms (20 symbols) in cc2420.c.  
static void wait_for_status(uint8_tstatus_bit) { trimer_clock_t t0; t0 = RTIMER_NOW(); while(!(get_status()& status_bit)& status_bit) && RTIMER_CLOCK_LT(RTIMER_NOW(), t0 + RTIMER_SECOND/10); } The LW-CCA, with respect to radio check time partition, considers two models for a dynamic check in the Rx mode.  
Model 1: This model considers RSSI check for measuring signal strength in Rx and uses default CCA check-in Tx mode. The RSSI status check time is by default method in CC2420 driver based on the sum of \(T_{\Delta}\) and t_RSSI.  
Model 2: In this model, considering \(T_{\Delta}=0\), the RSSI check time is considered to be equal with the eight-symbol period's case. In fact, the RSSI status check is performed over the intended time for the cc2420 radio chip. The rest of the conditions are the same as in Model 1. The following function is responsible for doing the RSSI validation under a period of time t_RSSI = 0.128 ms.  
static void wait_for_status(uint8_tstatus_bit) { trimer_clock_t t0; t0 = RTIMER_NOW(); while!((get_status()& status_bit)&& RTIMER_CLOCK_LT(RTIMER_NOW(), t0); } Table 7 shows a comparison between indicated models in terms of percentage of the listening duty cycle according to the scenario in Sections 3.3 and 3.4 in a 40 min (High Rate). It shows that ignoring \(T_{\Delta}\) in RSSI status check time for nodes based on model 2 results in about 11 percentage reduction in average of the listening duty cycle. The results in Table 7 are extracted based on the Collect-View output. The details of the average listen duty cycle calculation are available in Section 6.  
Therefore, each RSSI status check can be divided into two states: 8 internal symbols set for CC2420 radio hardware and 20 symbols for performing a reliable radio WakeUp on Rtimer equal to Equation (3). In fact, 8 and 20 symbols can be considered as the minimum and maximum periods required to perform a minimum radio check time, respectively. Figure 4 illustrates the performance of the RSSI status check. The file Msp802154 Radio.java in Cooja is responsible for simulating eight symbols for RSSI status check based on CC2420 radio datasheets.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **RxI Tx** & **Listen Duty-Cycle (\%)** \\ \hline Model 1 & RSSI | CCA & 1.451 \\ Model 2 & RSSI | CCA & 1.282 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Comparison of models of 1 and 2 in terms of percentage of listen duty-cycle.  
Figures 5 and 6 show changes in RSSI check time in nodes based on model 1 and 2 in Rx and Tx mode. In this simulated example, the power configuration and the network layers (except for the amount of RSSI check time in ContikiMAC) of the nodes are according to Tables 2 and 3, respectively. The difference between the radio checks is shown in Figure 6 with some zoom to make the difference clearer. In both models, the radio check is assumed to be based on default CCA in Tx mode.