The growing interest of the Internet of Things (IoT) in the recent years has revealed numerous wide-range, low- power, low bit-rate, wireless communication technologies. IoT refers to the interconnected of devices and sensors on a network. The devices are usually equipped with a telecommunication interface, and a processing and storage units. The network is normally referred as the Internet Protocol network. Unlike the traditional Internet, IoT's end- devices often have limited power, and send lower data rates to the network (Atzori, et al., 2010). Therefore, to do more with “less”, LoRa technology allows long-range connectivity for the IoT devices with low bit-rates without requiring construction and maintenance of complex multi-hop topologies (Raza, et al., 2017; Bor & Roedig, 2017).
LoRa is one of the spread-spectrum technologies that build up Low-Power Wide Area Networks (LPWAN), which enables to overcome the challenges of IoT applications, and promised to achieve its technology requirements. Among LPWAN technologies, LoRaWAN (LoRa Alliance, 2017) is the flagship and has gained significant attention from the public with its solution in the recent years. With the characteristic of LPWAN technologies to trade throughput for range, it is tested to have a coverage of more than 10 kilometres with ultra-low power usage, and the capability to support multiple applications with different transmission requirements while keeping network structures and management simple (Petajajarvi, et al., 2015; Adelantado, et al., 2017). However, the LoRa technology is still immature, its possibilities are yet unclear. Therefore, this paper aims to provide an overview of LoRa wireless technology and the LoRaWAN. The following sections will discuss on the related works, then investigate on the LoRa technology, and further discuss LoRaWAN performance and on its different classes of end- devices for different communication needs, its coverage capabilities, and its security.
Similar Implemented System
There are several communication technologies which aim at the low-power feature. The examples that will be discussed are the Bluetooth Low Energy (BLE) that works in short-range and Sigfox that aims to operate at a greater range.
Bluetooth Low Energy
Bluetooth was invented by a Swedish company, Ericsson, and was released in the late 1990s, it is aimed to replace cables to connect multiple devices wirelessly through a low power consumption short-range radio communication system. However, Bluetooth v1.0 did not begin well, manufacturers often adopt its alternative standard, such as ZigBee (Want, et al., 2013). A few revisions later, Bluetooth v4.0 was developed in 2010. It is fully compatible with classic Bluetooth and was extended with BLE capability, formerly known as Bluetooth Smart. This revision provides lower-power interface comparing to the previous revisions and has enhanced its pairing duration as BLE also provides rapid link enhancement. It also designs for lower power consumption by decreasing its throughput, with the main objective of running a wireless sensor for at least one year on a single coin cell (approximately 200MAHr) (Want, et al., 2013).
Sigfox (2009) is a French company that builds proprietary wireless networks, which allow remote devices to connect to access point at low power consumption with wide-reaching Ultra-Narrowband (UNB) signal that can penetrate through obstacles. Sigfox operates on the 868-MHz frequency band in Europe and 902-MHz in the United States, with the spectrum divided into 400 channels of 100 Hz (Margelis, et al., 2015). Sigfox claims that up to a million end-devices can be connected to one access point, its end-devices and can cover 3-10 km in the urban areas and 30-50 km of distance covered in the rural areas (Sigfox, n.d.; Augustin, et al., 2016). Its low-power consumption is achieved by sending only 12 bytes data packets at a throughput of 100 bps. Each end-devices can send up to 140 messages per day, each message takes approximately 6 seconds to transmit.
LoRa is a long-range wireless communication system and its physical layer is used in LoRaWAN. LoRa is renowned for its low power consumption even when the system is active and transmitting, it aims to have the system be usable in battery-operated devices for many months to years (Adelantado, et al., 2017) as the end-devices are not required to transmit more than a few bytes per transmission (LoRa Alliance, 2015). However, its connection between transmitter and receiver might be blocked by obstacles. In the case of lost connection, the receiver will generate noise rather than any usable signal. Therefore, the location of the sensors nodes must be considered, or additional equipment such as an amplifier or antenna gain would be necessary to establish a stable connection.
LoRa operates on free band spectrum, it is especially important to select hardware that suits frequency for your region (Battle & Gaster, 2017), 858Mhz for Europe, 915MHz for North America, and 433 MHz bands for Asia. However, transmission in any of those three bands do not have listen-before-talk and adaptive frequency agility mechanism implemented due to the regulation of European Telecommunications Standards Institute (ETSI, 2008), the channels shall follow with the 1% radio duty cycle, i.e. if radio is transmitted for a second, transmission for the next 99 seconds is not allowed (Haxhibeqiri, et al., 2017). LoRa is commonly referred to two distinct layers. Firstly, a physical layer that uses Chirp Spread Spectrum (CSS) (Springer, et al., 2000) modulation used to provide different data rates using different spreading factors depending on the communication distance (LoRa Alliance Technical Marketing Workgroup, 2015). Secondly, LoRaWAN is a MAC layer protocol which enables numerous end-devices to communicate with a gateway using LoRa modulation.
LoRa network is a “star-of-stars” topology, its end-devices are designed to deliver status indicators directly to a gateway. The low-power end-devices are programmed to only transmit data to remote locations by using LoRaWAN bidirectional long-range communication. LoRa gateways in its coverage will pick up the data packet to validates its authenticity. Upon successful validation, data packets will be pushed to the network server to verify its timestamps and pushed to the application server for notification, e.g. when a cone placed at the side of the highway is knocked over, message will be transmitted from the end-device (cone), so that road services team will be notified.
LoRaWAN is a MAC protocol that enables the communication between multiple devices and their gateways. It is developed to use LoRa physical layer for sensor networks. LoRaWAN can be used without LoRa radio, but it will not be practical. There are several components required to form a LoRaWAN network, which is the end-devices (sensors) that is used to communicate with gateways using LoRa, gateways are mainly used for forwarding the received data packets to a network server which decodes data packets it received and prepare data packets to be returned to the end-devices. However, peer-to-peer (P2P) communication is not supported by LoRaWAN protocol due to its cost and complexity. Its data packets can only be transmitted from an end-device to the network server, or vice-versa.
LoRaWAN has three classes of end-point devices for different communication needs. Class A end-devices are mostly battery powered, it is most widely adopted due to its power efficiency, often used as a tracking device, smoke alarm, water meter sensor, and more. This class of device allows bi-directional communication following the Aloha protocol, which only checks for incoming messages immediately when the other end sent a message. However, data packets cannot be sent at the same time, it must wait for a duration until the next transmission. Class B end-devices have low latency, it allows bi-directional communication with scheduled receive slots. It only listens to incoming messages at fixed intervals (every 128 seconds) (Battle & Gaster, 2017). Class C end-devices have no latency, due to that, it is the most power consumption among all classes, it allows bi-directional communication with continuously listening for incoming messages (Battle & Gaster, 2017), the server can initiate transmission at any time.
LoRaWAN wireless network is used as Wide Area Network (WAN), due to its wide coverage capabilities, it benefits a huge group of IoT applications that are currently using other networks. In a recent field test, LoRaWAN can achieve long communication range of 2-3 km in the average urban environment, 5-10 km in the rural areas, 30 km communication range was reached on the water (Petajajarvi, et al., 2015; Adelantado, et al., 2017). Recently, LoRaWAN has achieved a world record of long-range data packet transmitting at distance of 702km (The Things Network, 2017), which is achieved by sending the sensor to the sky.
LoRaWAN features low-power consumption capabilities, its battery could last up to 10 years of a lifetime (Adelantado, et al., 2017). LoRaWAN is programmed to optimize its battery life during the inactive period by switching into the deep-sleep mode. To further achieve low-power consumption, its transmitting speed is extremely low. Although sending media over this type of network is not expected, it is more suitable for extremely small sensor data packets for triggering operations or monitoring purposes.
Security has been a vital aspect of all wireless technology, end-devices must be activated to participate in a LoRaWAN network. There are two ways to activate an end-device, which are Over-The-Air Activation (OTAA) and Activation-By-Personalization (ABP) (LoRa Alliance, 2015). LoRaWAN utilizes two layers of security, which are at the network layer with network session key (NwkSkey) and the application layer with application session key (AppSKey). LoRaWAN uses separate keys for both layers to reduce the risk of compromised keys (Dongare, et al., 2017). These mechanisms eliminate mischievous activities, such as malicious end-nodes injection or take advantage of the network without knowing the end-nodes keys (Oniga, et al., 2017). OTAA provides 128 bits key length AES encryption signing, which its encrypted packets are sent over the network. It uses join procedure with a join-request and join-accept message exchange is used for each new session, in the case of a reset or power-outage, end-devices can re-join network automatically (Augustin, et al., 2016; Battle & Gaster, 2017). For the ABP, both session keys are directly stored in the end-devices (Augustin, et al., 2016).
LoRa is a long-range and low-power telecommunication system for the IoT. LoRa physical layer used in LoRaWAN, which is an open source technology with the specification available at no charge. LoRaWAN is an LPWAN protocol which is very similar to Aloha. However, its performance may be affected by the load on the link increases. LoRaWAN has already proven its ability to deliver low power, long distance data transmission in rural, low-dense building environments, and on the sea. Its network coverage and throughput can be affected drastically due to the difference of location and range. Although, it has limited signal penetration capability, but is acceptable to most IoT-devices with its CSS spectrum which is robust against interference.
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