## 4 Data Transfer Solutions  
Transferring the sensed raw or preprocessed data has key importance on the system performance. Each kind of data transfer method has different limitations and gives constraints in terms of communication speed, power consumption, range, cost, and complexity, just to name a few. Existing systems and experiments may use two different approaches. Data can either be recorded inside the bolus or forwarded to some kind of receiver using a wireless channel. The former method can be used universally, while the latter one can be applied only in the case of fistulated cattle. The advantage of storing recorded data inside the bolus is the high available capacity, which allows continuous measurement with high sample rates even with multiple sensors. The most ubiquitous storage media are different SD cards [53]; however, the storage medium was not mentioned explicitly. Regarding the application of an intra-ruminal telemetric sensing device, there may occur some discontinuity in data transmission caused by the distance between the animal and the receiver or the increased signal degradation due to the lying position [32]. In adult cows, an increased packet loss for weighted ruminal boluses was also found [54]. Year by year, new radio communication solutions are developed and this progress can be also observable in bolus research. The first experiments date back to 1974, when [PERSON] and [PERSON] [55] developed the first RFID system. In the field of dairy cattle farming, automatic milking systems were the first adapters of this new technique [56], followed by other precision systems, where RFID was used to identify each animal for the robotic arms in [49], where ear tags were used to monitor the feeding behavior of the animals. The main advantages of passive RFID tags are the low price and the low energy consumption. Sensing is only possible with active RFID technology [52], but its communication range is very limited [55]. For digital radio communication, two possible directions are available. Either an existing standard can be used, or a different solution can be developed based on the existing radio chips. The two most well-known radio communication standards are 802.11 (WiFi) and 802.15.1 (Bluetooth). Both use the 2.4 GHz industrial, scientific, and medical ISM band; therefore, they are prone to the absorbing effect of water. These solutions are easy to interface to a laptop or a handheld computer [55]; however, the communication is limited to the vicinity of the examined animal. Another important drawback of using WiFi or any Bluetooth standard besides bluetooth low energy (BLE) is the relatively high power consumption. There are several different digital radio communication modules operating in various frequencies with different modulations. Transceivers operating in the UHF (ultra-high-frequency) ISM bands (315, 434, and 818 MHz) require larger antennas than their 2.4 GHz counterparts, but the absorption is much higher in the latter case. [PERSON] et al. [9] used the 433.92 MHz frequency band with a moderate 4 mW power to provide a bidirectional data link between the bolus sensor and the base station. The antenna of the bolus sensor was an inefficient helical one; the handheld unit was equipped with a quarter-wave whip antenna. In Ref. [57], a special workaround was proposed to compensate for high attenuation of the body of the animals. They used neck collar transponders to extend the communication range of the bolus sensors. In Ref. [32], a similar method was described, where the 433 MHz band was used and an additional transponder was placed to one of the legs of the animals. Many of the aforementioned solutions used one of the built-in wireless communication protocols of the computers of that time. Although the application of those solutions seems to be obvious, their parameters are not optimal to use in a bolus. The bandwidth of WiFi and even Bluetooth is unnecessarily high, while they are quite limited in terms of communication range or the number of simultaneously handled devices.