# Performance Comparison of NB-Fi, Sigfox, and LoRaWAN
## 3 An Overview of NB-Fi, Sigfox, and LoRaWAN
### Channel Access and Operation Modes
#### 3.3.3 LoRaWAN  
In LoRaWAN networks, the operating channels can be 125 kHz, 250 kHz, or 500 kHz wide. A LoRaWAN operator divides the available spectrum into several channels. The number of channels depends on regional restrictions and network options. Some channels are  
Figure 8: Multiple frames U-procedure in Sigfox.  
Figure 7: DRX transmission procedure in NB-Fi (here, the first transmission attempt is unsuccessful).  
used for data transmission (hereafter called the main channels), some channels are reserved for join requests sent by sensors, wishing to connect to the server, and one channel is used for downlink responses by the BSs (hereafter called downlink channel).  
LoRaWAN sensors can operate in three modes called Classes A, B, and C. Class A is the default operation mode supported by all LoRaWAN devices. Class A sensors are capable of bi-directional communications. They transmit data if needed and can receive data only shortly after transmitting. Downlink communications from the server at any other time will have to wait until the next uplink transmission.  
Let us consider a Class A sensor. Figure 9 illustrates its transmission procedure. If the sensor has some data to transmit, it randomly selects one of the main channels and transmits the frame. Note that, during the transmission, the frame occupies the whole channel. \(T_{1}\) seconds after the transmission, the sensor listens to the same uplink channel where the data were transmitted for a short time. By default, \(T_{1}=1\) s. If the sensor receives a downlink frame intended for it and the MIC field checks out, then the sensor does not open the second receive window. Otherwise, the second receive window is opened \(T_{2}=T_{1}+1\) s after the end of uplink transmission. During this receive window, the sensor listens to the downlink channel. If a preamble is detected during one of the receive windows, the sensor does not close the receive window until the downlink frame is demodulated. If the sensor does not receive any of the acknowledgments, it retransmits the frame after a random delay within the retry window [1;3] s. The sensor retransmits the frame until it receives an acknowledgment or exceeds a configurable retry limit. The receive window duration is configurable and should be long enough to receive a downlink frame.  
The second mode is optional and is called Class B [33]. While operating in this mode in addition to Class A features, sensors are capable of scheduled downlink communications. BSs occasionally transmit a time-synchronized beacon that specifies the schedule. Sensors receive these beacons and listen to the channel at the scheduled time.  
The third mode is also optional and is called Class C. A Class C sensor can receive downlink frames continuously, except for times when the sensor is transmitting. Similarly to NB-Fi, Class C mode is not energy efficient, and, thus, is not often used by battery-powered devices. Therefore, we do not consider Class C sensors in this paper.  
It is worth noting that Class A LoRaWAN sensors work similarly to NB-Fi end devices, operating in the DRX mode, while Class C sensors operate similarly to end devices in the CRX mode.