## 1 Introduction  
Wireless Sensor Networks (WSN) are one of the industrial applications that benefit the most from the license-free nature of the Industrial, Scientific and Medical (ISM) band. Nevertheless, the ISM band has to be shared with other devices and systems using standard communication protocols such as Wireless Local Area Network (WLAN) or Bluetooth [1]. This situation leads to interferences in the communication channel and, as a result, produces (pseudo-) random transmission errors. Re-transmitting interfered packets might eventually succeed, but at the expense of increased latency and energy consumption of the devices. Lost packets and increased latency directly affect the QoS of the network, first by the direct loss of arbitrary packets and second by the side effects of the missed packets such as breaking a multi message or state-full process that has to be started from the beginning (i.e., pairing, network establishment, discovery, etc.) [2]. Additionally, not all interferences are unintentionally produced by coexisting networks. External attackers may try to block the communication channel in order to perform a Denial of Service (DoS) [3] attack. DoS security attacks are specifically designed for interfering with the communication link, either hindering or totally blocking the communication channel. Thus, communication protocols that do not appropriately manage the communication channel may not be able to provide a satisfactory security level. All in all, interference avoidance mechanisms that reduce the need for retransmissions are highly desired in WSN domains in order to minimize energy consumption, reduce unnecessary degradation of QoS and reliability as well as to raise security. In this paper we present a method for supporting the design of communication solutions for saturated and dynamic environments from the point of view of channel interference. This method characterizes frequency hopping techniques based on the quality of the available frequency channels. Frequency hopping techniques relay on the use of multiple frequency channels over time as opposed to statically allocating and using a single frequency channel. The main idea is that, in case of channel interference, frequency hopping helps in minimizing the side effects by avoiding (at least temporally) the interfered frequency. Figure 1 shows a frequency hopping schedule example in which multiple frequency channels are selected at different time instants. The core of the method is based on several standard frequency hopping techniques as well as quality metrics for the characterization of available and valid frequency channels. The proposed method homogenizes the notation used to describe the different techniques and normalizes the different quality metrics based on the statistical properties of the RSSI values. Thanks to the homogenization and normalization, the different techniques can be fairly compared based on quality and performance indicators. Additionally, the method is flexible enough to accommodate new frequency hopping techniques, quality metrics and performance indicator approaches. Thus, it is scalable and extensible with new additions. All the techniques and metrics are combined into a two-dimensional matrix. Each technique and metric pair represents a possible solution which provides a subset of selected or preferred channels and the sequence in which the channels are used. For some techniques, this sequence is purely random while for others the sequence will change based on specific parameters. For instance, in techniques based on probabilistic channel usage, there are channels within the subset which are used more often because they are considered to be of \"better\" quality. On the other hand, and regarding quality metrics,  
Figure 1: Example of a Frequency Hopping schedule. some metrics are better suited for characterizing static interferences while others are better used for dynamic interferences that hop between channels.