## 4 Performance
### Network Efficiency  
The network efficiency measures the efficiency of packet transmission in a sensor node. If a packet is successfully delivered from a source node to a sink node, the efficiency is perfect (100%). However, if a packet is dropped somewhere in a network during transmission, the transmissions from the source node to the relaying node that dropped the packet become unnecessary. If many packets are dropped due to congestion, the network efficiency decreases. The network efficiency is related directly to energy because unnecessary packet transmissions unnecessarily dissipate a large amount of energy, which is a very critical resource.  
Figure 14 shows the network efficiency of the CODA, CODA-Comp, CRRT and ACT. The total amount of sensed data per second slightly exceeded the available bandwidth at the beginning of the simulation, and hence, the efficiency of the CODA slightly decreased. During congestion, the CODA process drops overflowed packets and controls one sensor node among child nodes and sets it to the sleep state for some time. Therefore, the network efficiency decreased due to the overflowed packets in  
\begin{table}
\begin{tabular}{|l|l|} \hline
**Parameter** & **Value** \\ \hline Radio Range & 100 m \\ Number of sensor nodes & 100 \\ Simulation Time & 5,000 sec \\ Max Network Bandwidth & 40 packets/sec \\ Sampling Interval & 10\(\sim\)1,000 ms \\ Queue size on a sensor node & 64 \\ Max transmission interval & 255 ms \\ Event Change & 2,000 sec \\ \hline \end{tabular}
\end{table}
Table 1: Specifications of simulation environment.  
\begin{table}
\begin{tabular}{|l|l|} \hline MCU & ATMEGA 128 L 8 MHz \\ \hline Memory & 4 K RAM/128 K FLASH \\ \hline  & Chipcon CC2420 \\  & IEEE 802.15.4/ZigBee compliant \\ RF & 2.4 GHz Frequency band \\ Transceiver & 250 Kbps Transmission data rate \\  & -24 dBm to 0 dBm RF power \\  & 20 m to 30 m Indoor range \\ \hline \end{tabular}
\end{table}
Table 2: Specifications of simulated mode.  
the congested nodes. As the back pressure (making one child node sleep) propagated slowly, the packet overflow decreased, and as a result, the network efficiency increased. However, as sensor nodes generated more packets and thus the network congestion became serious, more packets were dropped. As a consequence, the network efficiency drastically decreased. In the case of CODA-Comp, the packet is compressed before sending at the source node and thus less congestion is occurred. Therefore, CODA-Comp shows higher network efficiency than CODA. CRRT shows higher network efficiency than CODA-Comp. CRRT optimized the retransmission at the hop-by-hop communication and thus less packets were lost and thus the network efficiency was higher than CODA.  
On the other hand, the ACT exhibits high efficiency at the beginning of simulation, because it reduces the generated packets with the APC. Although the total number of sensed data packets is increased and network congestion becomes serious, the ACT exhibits high network efficiency. Moreover, the network efficiency of CODA decreases continuously, while that of the ACT increases with time. With CODA, the back pressure is propagated slowly, and the packet flows greatly fluctuate in case of congestion. Therefore, the packet overflows increase, which leads to a decrease in the network efficiency. On the other hand, the ACT controls the quantization step size in proportion to the congestion state, and thus, the total number of packets in the network is decreased. Moreover, it propagates the back pressure fast with adaptive queue operation. As a consequence, its network efficiency is increases slowly. ACT also shows highest value at the average network efficiency [Figure 14-(B)]. The confidence interval was 95%.