## 4 Reference Scenarios
### Traffic Patterns  
In WHAN scenarios, the amount of user data transmitted is usually small and, most of the time, only consist of a few bytes [1,14]. The actual amount of traffic depends on the number and the type of WHAN devices and how frequently they are used. To evaluate the performances of the network settings investigated, the degrees of device activities are varied in the network, as well as the roles of the devices. As a result, three different traffic scales are defined for the evaluations: low, medium and high traffic. This allows one to observe how the WHAN performs under a variety of situations and reveals how the performance of a certain communication protocol stack configuration scales with increasing traffic load. Two possible causes for the generation of data are considered:  
1. Aperiodic events: An aperiodic event can be, for example, an alarm generated by a motion sensor or by the command to turn on/off a device caused by toggling a switch. The amount of bytes that it takes to encode the information that needs to be transmitted is assumed to be three bytes. The total amount of bytes for the payload and the headers of such a message fits in the payload of an IEEE 802.15.4 packet, only requiring transmission of a single frame.
2. Periodic events: For example, a periodic event can be a heartbeat signal from the nodes of a network that needs to be sent to a security center regularly or temperature information that is periodically collected and reported to an HVAC controller. The messages include aggregated information, such as sensor data and/or plain text, as part of status reports or log files, to name some examples. As a consequence, the total amount of bytes of a periodical event is assumed to surpass the maximum payload available in one MAC frame. Thus, three packets are assumed to be generated for a periodic event.  
Depending on the roles of the nodes described in Section 4.1, the type of traffic they generate is defined as shown in Table 3. For example, a light switch (Role A) may only generate aperiodic events, whereas a temperature sensor (Role B) would generate periodic events for temperature statistics, but it also would generate an aperiodic event if the temperature exceeds a certain threshold. Aperiodic events always generate data for a random destination within the network.  
Periodic events are generated with a fixed interarrival time of 60 s that does not vary with the traffic scale. Instead, the number of nodes producing periodic traffic is varied to achieve different traffic loads in the network. The interarrival time values for aperiodic events, \(t_{s}\), are defined to follow an exponential probability distribution function, the individual values of which can be generated through the use of a uniformly random variable, R, with the formula:\[t_{s}=\frac{-ln(1-R)}{\lambda}, \tag{6}\]  
where \(R\) is uniformly distributed between zero and one and \(\lambda\) is the average number of events per minute. Different traffic scales are achieved by varying \(\lambda\) and the number of nodes generating the aperiodic event data. For low, medium and high traffic loads, \(\lambda\) is defined to be 0.75, 1.25, and 1.75, respectively.