# A Reference Model for Monitoring IoT WSN-Based Applications
## 4 Model Description
### Monitoring Layer
#### 4.1.2 Components  
At this level, the main source of data is the \"Monitoring Data Point\" (MDP). It is not a physical point but rather a logical one, corresponding to the information to be obtained. For instance, a MDP may be a communication routine which generates a software trap in a \"_successful_transmission_\" event. Other example could be a MDP to measure the remaining battery in a mode.  
In order to obtain the MDPs' data, it is necessary to use so-called \"probes\", which acquire these data from the observed system. For instance, a probe may be a trap collector module (software probe) waiting for a \"_successful_transmission_\" event. Or it may be the circuit (hardware probe) that measures the remaining battery life in a mode.  
It is necessary to define at least one probe for each MDP. Probes (Figure 2a) may consist in a single capture point, as the located in a sniffer [32; 34] or a test bed which physically measures the signals on a PCB. There also exist composite probes [36] (Figure 2b), being composed by two or more elements. In this case, at least two elements must be present to acquire the desired data: an element in the monitored mode and other correspondent element located in the monitor mode.  
Figure 2: Example of (**a**) Single probe and (**b**) Composite probe.  
The MDP is the main concept of the Monitoring Platform. Data acquired by probes usually consists of a trace of events, which reflects the behavior of a single MDP in the WSN. In this line, the WSN-MP main function is to collect all the relevant MDPs' traces in an efficient way, joining and synchronizing them to study the behavior of the WSN.  
This methodology abstracts the monitoring requirements from implementation issues, hiding part of the technical complexity of a monitoring platform design. This way, the designer can focus mainly on what it is intended to be captured and how to process and display that information, following the desired specifications.  
The interchange of information inside this layer must be performed by means of the Monitoring Layer Protocol (MLP), which must ensure the coordinated subsystems operation. This protocol is a crucial point of the standardization and reutilization of elements of the monitoring tool.  
Most of existing proposals do not implement analysis or visualization subsystems. This way, there is not a protocol to communicate data from capture subsystem. In other cases, the storage format is the only interchange route. Other tools, such as [34; 39; 40], make a huge effort to process obtained data, using their own protocol. Unfortunately, all these works were made without following any standards, and then is not possible to reuse them directly.  
The WSN-MP designer is responsible from the definition of each MDP to study, as well as the different probes which must be associated to them. The combination of an MDP and its associated probes must define which data monitors (the MDP), and how these data is monitored (its associated probes), by defining hardware and software tools, elements and procedures.  
Acquisition system inside the probes will produce a stream of raw, unformatted data. The lower layer (Information Layer) is responsible from applying the appropriate actions to express this information in a common standard way.