## 3 IoT Traffic Models  
Among IoT applications, there are some popular real implements, features and traffic models that are discussed in this section [35]. There are two main identified traffic model trends that are considered for IoT applications: (i) periodical and (ii) event trigger traffic as explained below [36]. * **Periodical:** This is a typical IoT traffic model applied to passive monitoring applications. A monitor generates stable traffic every predetermined period to update the state of object. This kind of traffic is always predictable between an IoT end-user device and an IoT data platform collector (IoT-DPC). For example, smart city applications, IoT in agriculture, smart grids, healthcare and farming devices send periodic messages along with statistics to the IoT-DPC for monitoring. * **Event Trigger:** This is a typical IoT traffic model applied to dynamic monitoring applications. Event trigger happens occasionally to declare that an operation may be required. Although these events add uncertainty to the traffic flow, it is still under control due to its probability of presence. Devices in smart homes, wearables, connected car, industrial IoT and smart retail might randomly generate information to the IoT-DPC. These events only arise in a low frequency; otherwise, real-time monitoring might not be necessary. Based on periodical, event trigger or both, the IoT traffic model can be characterised for specific applications. In the following, the most popular IoT applications are elaborated. 1. _Smart Home (event trigger)_ includes diverse real-time monitor and event triggered actions. Typical scenarios include home condition retrieve, preconfigured intelligent services and remote control of home appliances, achievable through sensors and Internet-connected devices at home [37]. For people who would like to manage from outside home, collected data are sent to the distant application IoT-DPC via a home gateway, such as a modem, and then the IoT-DPC tries to reach the user via mobile or wireless networks. 2. _Wearables (event trigger)_ are portable devices that have the capability to save and transmit data. Similar to mobile phones, these wearables may upload or download files and communicate over the Internet. As they are always carried by users and activated erratically, it is an arduous task to predict the access point and traffic utilisation. 3. _Connected Car (event trigger)_ is capable of communicating with other internal and external IoT devices, as well as accessing the Internet. Sensors embedded in the vehicle ensure the performance of driving, while modules exchanging data with peer vehicles avert the chance of a crash. As a connected car is able to gather information from devices nearby, it acts as a mobile collector in the IoT [38]. 4. _Industrial IoT (event trigger)_ is a future project that pushes remarkable changes in both application and technology in the manufacturing system, the objective is to achieve service-oriented industry [39]. The key point is that users hand over their behaviour and feedback to the factory in real time, and the factory reacts to these statistics by reconfiguring manufacturing process and regulating orders from suppliers [40]. As there are too many possibilities and combinations of customer feedback in this area, IoT traffic is unpredictable in these applications unless manufacturers predefine a duration for the comments. 5. _Smart City (periodical/event trigger)_ employs sensors all over the city to monitor the real-time state from environment to parking space. The architecture consists of three tiers: IoT sensor, IoT gateway and server. In the IoT sensor tier, devices are deployed in the facilities for dedicated tasks, which means the traffic is stable for a kind of service under a given gateway [41]. 6. _IoT in Agriculture (periodical/event trigger)_ manages to read the status of farmland, such as soil moisture and temperature, to optimise the growing conditions without manual intervention [42].