# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Bluetooth IEEE 802.15.1  
WPAN IEEE 802.15.1 also called the Bluetooth Basic Rate (BR) is a global 2.4 GHz specification working with short-range wireless networking. It covers the versions v1.0, 1.0B with voice dialing, call mute, last number redial and a 10 m range as the main facilities and a v1.2 with adaptive frequency hopping added. Versions v2.0+Enhanced Data Rates (EDR) and v2.1+EDR added more capabilities such as improved resistance to radio frequency interference as well as improving indoor coverage and LOS range to 100 m. Fast transmission speeds and low power consumption mechanisms were also increased on the v2.0+EDR and v2.1+EDR versions. Version v2.1 counts on a sniff subtracting function that results in less transmissions do access the medium and so reducing interference. The evolution follows with v3.0 receiving enhanced power control to quickly adapt to the changing path loss of the new 5 GHz transmission frequency bands. Version 4.0 increased the modulation index, resulting in less energy used during transmission but still presented a certain medium consumption to complete the receiving process. Version 4.1 shows better alignment on pico-nets timing when the transmission suffers interference. In Version 4.2, the low energy is reinforced with the adoption of longer packet transmissions. This reduces the quantity of packets transmitted for the same information size, using a packet length extension technique. The present version of Bluetooth v5.0 has some improvements regarding transmission and receiving processes. The Bluetooth v5.0 improvements include slot availability mask, which allows the alignment of pico-nets timing with nearby LTE bands, an increased coding gain, increased symbol rate and a better channel selection algorithm. The IEEE 802.15.1 MAC layer is composed of Logical Link Control, the Adaptation Protocol (L2 CAP) layer, the Link Manager Protocol (LMP) layer, and the Base-band or simply the Physical layer. The Bluetooth MAC layer handles the communication types that can be asynchronous connectionless (ACL) or synchronous connection-oriented communication (SCO). Figure 4 shows the relationship between the Open Systems Interconnection model (OSI), the seven-layer model and the IEEE 802.15.1 standard. Figure 4: Mapping of OSI reference model to IEEE 802.15.1 Bluetooth stack. Figure 3: NFC active to passive device communication protocol stack. The Base-band Layer defined by the IEEE 802.15.1 standard operates in the 2.4-GHz Industrial Scientific and Medical (ISM) band using a short-range radio link and a fast frequency-hopping (FFH) transceiver. The Radio and the Base-band sub-layers define the Bluetooth physical layer. The Radio layer provides the physical links among Bluetooth devices with 79 different Radio Frequency (RF) channels spaced of 1 MHz, using a frequency hopping spread spectrum (FHSS) transmission technique. This FHSS technique increases the robustness of the link due to its capability to reduce the interference of nearby systems that may operate in the same frequency range. A Time Division Duplex (TDD) transmission scheme is specified to divide the channel into time slots of 625 us each, corresponding to a different hop frequency, simulating full-duplex communication in the same transmission channel. The radio link which reaches up to 100 m with LOS is obtained using the nominal power according to its power class and the irradiating system used. Three output power classes divide the Bluetooth Basis Rate protocol devices. Each class is characterized by a maximum power and a minimum output power, and, based on these values, the distance within which the device can communicate is defined according to the list in Figure 5.  
The main roles of the Bluetooth MAC layer are to set-up the physical connections between the master and slaves; send and receive packets along the physical channels; synchronize the network devices with the master clock and manage the different devices for power saving states [27].