# Design and Implementation of a Mixed IoT LPWAN Network Architecture
## 2 Related Technologies
### Sigfox  
Sigfox [17] is not only the name of an IoT technology but also a network operator that provides a complete solution, which covers since the collection of data from objects in any area of the world under coverage up to the transfer of these data towards the information system of any possible customer. The business model is based on invoicing for the connectivity provided to the devices. Since 2009, Sigfox has had a rapid growth, and in November 2018 is available in 53 countries, with an estimated coverage area of 5 million square kilometres [18]. Currently, each base station reaches an area from 3 to 10 km in urban environments and from 30 to 50 km in rural areas, offering different services for up to 1 million devices [19]. Before a Sigfox device can be registered in the network, it must pass a certification process defined by Sigfox's official specifications, thus serving to ensure compatibility and quality of service. In these certification tests, both the link layer protocol and the RF radiation performance are evaluated, giving rise to a classification based on the transmission power [20]. When the certification process is passed, an official certificate is provided, which will later be necessary for the registration of any other device of the same model. In addition, each device is associated with a pair of ID/PAC identifiers that are necessary for the registration in the Sigfox network. The 'Device-ID' is a globally unique 32-bit identifier that is recorded in the non-volatile memory of the device and it does not change during the lifetime of the device. The PAC (Porting Authorization Code) proves the ownership of the device and changes each time the device is registered or transferred. For transmission of information, in the physical layer of the protocol is used a UNB (Ultra Narrow Band) radio technology, with a channel width of 100 Hz, DBPSK (Differential Binary Phase Shift Keying) modulation and a bit rate of 100 bps [21]. The modulation is the simplest one of the phase shift type, offering the highest immunity to noise due to the fact that the maximum difference of symbols is 180\({}^{\circ}\). By concentrating the radiation in a very small bandwidth, the energy density is increased and a link more immune to signal interference with higher bandwidth and power is achieved [22]. It is possible to achieve long-range wireless links of up to 163.3 dB of total loss with 14 dBm of transmit power, using an antenna with 2.15 dBm gain, a base station receiver with a sensitivity of \(-142\) dBm and a reception antenna of 5.15 dBm [23]. In Figure 1, it can be seen that each transmission period of the device lasts around 2 s and is repeated 3 times at different frequencies in order to increase the possibilities of reception by the network. The base stations maintain reception in a 192 KHz segment around the central frequency (868,130 MHz in Europe) and the devices can transmit on any frequency within that segment [24]. If a response is requested, a reception window of 25 s is opened and a confirmation of reception is subsequently generated. For the transmission from the network to the device, GFSK (Gaussian Frequency Shift) modulation is used at a speed of 600 bps and a channel is chosen according to the one used for uploading within the 200 kHz downlinksegment and the European central frequency of 869,525 MHz [25]. Therefore, it is a bidirectional but asymmetric communication, which is always initiated by the device and only is produced a downloading communication if it is requested by this device. The number of uploading messages per device to the network is limited to 140 per day and the number of downloading messages is limited to 4. In the data link level (MAC, Medium Access Control) associated with the OSI model (Open System Interconnection), each uplink frame has a total length of 26 bytes (208 bits). Up to 12 bytes (96 bits) of this data link frame can be a payload, that is, a message defined by the user.