# MAC Layer Protocols for Internet of Things: A Survey
## 3 Long Range MAC Layer Protocols
### SigFox
This medium access technique has no medium access collision control method, thus allowing the terminals to access the medium at any instant of time, at any frequency (within the operating band), without any prior perception of medium occupancy [113]. The RFTDMA technique favors the low power consumption because it does not use energy to sense the medium before transmitting. It does not use any type of synchronization package or beacon technique, which also favors the cost of the terminals due to no need for expensive oscillator circuits. Another feature is that in RFTDMA the transmission frequencies are chosen randomly but within a continuous interval. A disadvantage that can be cited is the probability of colliding terminal packets that are transmitted simultaneously. The Medium Access Control layer header adds field for device identification and error detecting codes. Sigfox devices are not synchronized with the network and can transmit anytime because Sigfox removed medium access control from its conception. The purpose of the frame layer is to receive the payload coming from the application layer; then, do the segmentation and deliver the fragments to the MAC layer. The MAC layer can identify and order the generation of the radio frames with a sequence number included. As usual on UNB systems, each transmission occupies a simple 100 Hz or 200 Hz bandwidth of available spectrum permitting high capacity for the SigFox network. High resilience is obtained as the SigFox signal power is squeezed into a narrow bandwidth supporting better behavior regarding interference. The energy concentration of the SigFox signal enables base  
Figure 23: Sigfox protocol stack. stations to demodulate it easily, even if more powerful and spread interference signals are received simultaneously [83]. Depending on global regions, the transmission power varies from 14 dBm (ETSI) up to 22 dBm (FCC). UNB brings long range features thanks to D-BPSK modulation plus the low bit rate transmission performed by SigFox. This results in a highly sensitive base station receiver. The sensitivity of the base station receiver changes depending on the transmission bit rates of the devices. Sensitivity operates around \(-142\) dBm for 100 bps and \(-134\) dBm for 600 bps transmission rates. Such sensitivity offers a very large link budget. The transmission range achieved is defined by the channel conditions, interference, and the received noise level. With a high coupled irradiating system, the device can transmit up to 16.15 dBm EIRP (Effective Isotropic Radiated Power) with a 5.15 dBi antenna gain at the base station. With this combination, the receiver sensitivity has a link budget from \(-142\) dBm up to 163.3 dBm. For transmission at 600 bps, the base station receiver sensitivity is 8 dB lower due the high transmission rate. However, it is compensated with 24.15 dBm EIRP devices transmission power. The typical sensitivity of the device considered as a receiver is \(-126\) dBm. Devices are free to transmit on any operational band carrier and the base stations are ready to receive from all carriers anytime, permitting devices to work with low frequency accuracy [120]. SigFox has tailored a lightweight protocol to handle small messages. When dealing with short message transmissions, conventional protocols are not suitable due the overhead imposed by their design. Sigfox works with 26-byte packets carrying 12-byte data payloads. This gives Sigfox high transmission efficiency when compared to its useful data transmitted, against 20 bytes of ethernet header to carry the same 12 bytes plus 52 complementing bits. The payload data that can be put in a SigFox message rages from zero bits to 12 bytes composing a maximum packet size of 26 bytes. With a light protocol frame size, each transmission has less data to send, less energy is used and so the battery life increases.