## 6 Performance Evaluations  
### Simulation Setup and Performance Metrics  
The performance of ATLAS is evaluated through extensive simulations using the ns-2 [20] simulation tool. A network of 30 \(\times\) 30 \(m^{2}\) area is used, where 20 sensor nodes are deployed in a uniform random distribution. A gateway-rooted 2-hop routing tree is pre-established using the deployed nodes where the first-level nodes serve as the cluster-heads. The transmission and interference ranges of the nodes are set at 15 m and 33 m, respectively. In WBAN, the sensors are mostly located on the human body and we have chosen a large transmission range and network size to simulate the increasing demands for the both in-body and on-body nodes deployment and network scalability. Such design choice can be validated by the CodeBlue prototyped medical sensor network platform [29]. Furthermore, the radio capacity is set at 250 kbps. Table 2 summarizes the simulation parameters, most of which are extracted from the implementation of LDTA-MAC [3] in conjunction with the IEEE 802.15.4 standard. Finally, to yield more stable results, an average of fifty simulation executions is performed, each of that is 100 s long.  
In this study, we compare the performance of ATLAS with LDTA-MAC and IEEE 802.15.4, since both of these protocols include the superframe oriented CAP, CFP and IP modes. Both the LDTA-MAC and IEEE 802.15.4 protocols are implemented for a single-hop star topology and sensor nodes make direct transmissions to the gateway. In ATLAS, the MAC operation modes are conveyed through the initial beacon, including the time-slot assignment of the current superframe. Moreover, at each  
\begin{table}
\begin{tabular}{l c c c} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Channel data rate & 250 kbps & Buffer size & 40 \\ Payload size & 32 Bytes & Super-frame period & 1 s \\ Beacon size & 11 Bytes & SIFS & 192 \(\mu\)s \\ PHY header & 6 Bytes & CCA check delay & 128 \(\mu\)s \\ MAC header & 8 Bytes & Time-slot length in CFP & 1,920 Symbols \\ Back-off window & 16 & Symbol time & 16 \(\mu\)s \\ Retry limit & 4 & Simulation time & 100 s \\ \hline \end{tabular}
\end{table}
Table 2: Simulation Parameters.  
data-Ack beacon we make provisions for assigning time slots and announcing other network parameters. Note that for LDTA-MAC, in addition to the beacon frame, an extra notification frame is used for time-slot assignment.  
To analyze the performance of the protocols, we have used the following metrics: (i) _Average duty cycle_--the ratio of the per cycle average active time to the entire cycle time, expressed as a percentage; (ii) _Energy consumption_--the average energy consumption of all the nodes of the network, expressed in Joules; (iii) _Aggregate throughput_--the sum of the sizes of the total packets received by the gateway per unit simulation time; (iv) _Delivery ratio_--the ratio of the total number of packets received by the gateway to the number of packets generated at the sensor nodes; and (v) _End-to-end delay_--the average time interval between a packet generated at a sensor and received at the gateway. Note that both the aggregate throughput and packet delivery ratio reflect the data fidelity effectiveness of the protocols. Here, the data fidelity is defined as how well the demands (_i.e._, aggregate throughput and delivery ratio) of application data are fulfilled by the MAC protocols. The data fidelity may be hampered if significant packet drops occur in the network, thereby resulting in low aggregate throughput and delivery ratio.