LoRa stands for “Long Range” and is a low-power, wide area physical-layer networking protocol for IoT. It operates under the unlicensed band below 1 GHz. LoRaWAN, based on LoRa, provides higher-layer mechanisms for bi-directional communication, end-to-end security, mobility and localization services for realizing large-scale IoT applications.
You may often hear LoRa and LoRaWAN terms being used interchangeably, which is okay, as they are part of the same concept, but it is good to know about the distinction. LoRa stands for the physical layer protocol for connecting IoT devices with gateways and is a proprietary technology (owned by the company Semtech). LoRaWAN is an open-source standard for establishing end-to-end connectivity based on LoRa and providing development support for IoT applications. This is depicted in Figure 3.1.
Table 3.1 provides the typical specifications of the LoRaWAN protocol about the spectrum, data rate, transmission range, etc. We will not go into the details here, but it is important to note that the data rates are much less (in the order of kilobits per second) when compared to typical Internet connections (megabits per second). In return, we need very little transmission power (less than 25 milliwatts) while we can achieve impressive transmission ranges. This transmission power of 25mW is 40 times smaller than the maximum transmission power of mobile phones for typical cellular connections.
Table 3.1: Typical LoRaWAN specifications
|Spectrum||Unlicensed frequency bands within 433-915 MHz range
125/250 KHz channels
|Modulation and Access||LoRa modulation (Chirp Spread Spectrum (CSS))
LORAWAN MAC (Aloha)
|Data Rate||~ 250 bps – 50 kbps|
|UE Max Tx Power||14 dBm (25 mW)|
|Range||~ 5 km (urban), 20 km (rural)|
|Energy Efficiency||Device Classes & Adaptive Data Rate (ADR) mechanism|
LoRaWAN has a simple architecture with end devices connecting to gateways, which simply serve as bridges to the Internet and to the applications. One interesting characteristic of LoRaWAN architecture is that each packet from an end device can be received and relayed by any gateway in the range. In other words, the end devices do not associate with a specific gateway, unlike in WiFi and mobile networks. The network server then handles duplications among packets coming from multiple gateways, among the security and other network functions. This architecture is depicted in Figure3.2.
LoRaWAN architecture components are further shown in Figure 3.3, followed by a list of the functionalities and responsibilities of each component.
The main mode of communication in IoT or sensor networks is UPLINK (from end devices towards the network/application). On the other hand, it is well known in telecommunications that keeping the radio on for listening the channel (for downlink traffic) is a major source of energy consumption. Therefore, when it comes to downlink, LoRaWAN standard defines three different classes of end-point devices to address the different needs of a wide range of applications:
Figure 3.4 picturizes the device classes in LoRaWAN protocol and summarizes the key facts for IoT communications using LoRa.
For class A devices, any communication with the network is initiated by the end device (uplink). As mentioned earlier, continuously listening the channel for any incoming data (downlink) for IoT devices consumes significant energy. Therefore a “receive window” is only opened for downlink communication (from gateway to the end node) after an uplink transmission.
After the device sends an uplink packet, there is a guaranteed opportunity for the network to send packets back to the device. In between the uplink (UL) and downlink (DL) windows, the devices is in a sleep mode. This cycle of switching between sleep and transmission/reception modes is usually called the duty cycle. These concepts are illustrated in Figure 3.5.