# A Low-Cost IoT-Based System to Monitor the Location of a Whole Herd
## 4 Discussion
The major problem of RFID-based systems is that reading distance is less than a couple of meters, which limits its use to intensive environments. A solution based on active RFID has been tested to track the movements of wild animals (rats) [47], but its need for a network of data loggers distributed throughout the area to be monitored is a major drawback for commercial applications. Bluetooth has also been tested for cattle tracking [48]. Again, the solution described by these researchers is not practical for extensive production systems, since a large number of Bluetooth readers would be needed. The use of animals fitted with collars as network nodes to build mobile ad hoc networks is not new [16]. The ZebraNet project [49] was an early application of opportunistic sensor networks for wildlife monitoring. Wild zebras were fitted with special GPS collars, which sent all its data to encountered neighbors until their own data eventually reached the base station. In the case of the ZebraNet project, the base station consisted of a mobile vehicle for the researchers, which periodically moved around in the savanna and collected the data from the zebras encountered. This solution is quite sophisticated but has two major drawbacks for commercial farm applications. First, the cost is high, as each animal needs to be fitted with a GPS collar. Second, data update needs the vehicle to be moved around. The integration of LPWA networks (LoRa) and opportunistic networks (Bluetooth) has been recently proposed by [50] to overcome the second problem. The solution is similar to that described in the present paper, although these researchers did not test nor validated the system in real-world conditions. The performance of tag message reception is highly affected by the type of animal due to the differential behavior of sheep and cattle. The gregarious nature of sheep, exhibiting close spatial contact within the flock, has been extensively described in the scientific literature [51]. Beef cows are also gregarious animals, but the distance among animals on the pasture is normally larger than in the case of sheep. Using GPS collars, [35] found that sheep on the same pasture spent 75% of daytime within 51 m from each other, while cattle were within 111 m. These results are coherent with the findings of the present paper, as sheep would be placed inside the theoretical reading range most of the time, but not cows. Consequently, more tag messages would be captured per collar in a flock of sheep than in a cattle herd, as observed. However, the distance among animals on the grazing area does not only depend on the type of animal. Other factors, such as the habitat characteristics, the grass availability, the season or the time of the day, can affect animal behavior [52] and, subsequently, the performance of the proposed solution. Some authors even found behavioral differences in the average distance among sheep in a herd during the day and at night depending on the breed [53]. Thus, there are many factors to be considered in order to choose an optimal collar/tag ratio, which defines the cost per animal of the tracking solution. However, the most important criterion to make that decision is the use of location data. There are several utilities that would require only a location fix per animal per day, or even every few days, e.g., theft or death detection. This data can be obtained with a very low collar/tag ratio, especially in a sheep herd. Farmer management can also be useful to reduce collar/tag ratio. For instance, if animals are housed at night, a few collars could gather messages from lots of BLE tags. It is important to note that, according to the results of this work, the marginal cost of reading one more tag, in terms of additional collars, is high. This is probably because of the limited number of BLE tags used in the experiment.