# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Bluetooth Low Energy
It is also possible for a server to send to a client, unsolicited messages like notifications  
Figure 7: Bluetooth low energy protocol stack. that do not need any confirmation message to be sent by the client. A server is also required to send indication messages, which need confirmation messages to be sent by the client. The slave sends requests for responses and indications prior to transactions confirmation following a stop-and-wait scheme. Slaves can either write attributes values at the master. A framework defined by GATT performs the role of discovery services using the ATT attributes, and allows exchange of characteristics between devices interconnected. An attribute carries a set of characteristics that includes a value and properties of the parameter monitored by the device. For example, a humidity sensor needs humidity characteristics and attributes to describe this sensor, and to store its measurements. Thus, this sensor needs a further attribute to specify the measurement units. Creating specific profiles with the Low Energy Bluetooth standard takes place in the Generic Attribute Profile (GATT). GATT uses the Attribute Protocol (ATT) protocol in addition to the lower stack protocols, in order to introduce the subdivision of retained server attributes into services and features. Services can contain a set of features, which can include a single value (accessible from the client) and other numerical data that describe such features. Among the assignments of GAP profile specifications are: device role rights, discovery devices and services, as well as establishing connections and security. A new profile based on the existing profile requirements can be created following a profile hierarchy. The interoperability of different devices can be handled through application profiles. Bluetooth is designed to offer a low-cost alternative to Wi-Fi at the expense of the transmission range. Its transmission range is considerably shorter (up to 100 m LOS) and data rate does not exceed 721.2 Kbps in the classic Bluetooth Basic Rate version and can reach 3 Mbps with the Enhanced Data Rate feature. BLE operates at 1 Mbps rate on its physical layer, while its application layer can handle only 236.7 Kbps. In Bluetooth Low Energy, there are no subdivisions in power classes but only the maximum and minimum output power values of the transmitter are provided. Only an approximate value of the maximum reachable distance can be predicted. The low power required for transmission is the main feature of the Bluetooth Low Energy standard and this result is due to enhancements made on the classic version. These enhancements include reduced frequency band and shorter PDU packets [31]. An energy evaluation is offered at [32] using CC2640 radio chipset consumption reference measurements. The comparison is made when operating on 0 dBm transmission power by gathering the main characteristics of Bluetooth and BLE. Bluetooth v5.0 has no functional block included in its first and second layers when compared to versions v4.0, v4.1, and v4.2. A representation of the inter-layer communication structure and the relationship with Bluetooth layers of different Bluetooth versions can be seen in Figure 8. Device-to-device file transfers, wireless speakers, wireless headsets, and Body Sensor Networks are often enabled with Bluetooth versions. Finally, some characteristics of the Bluetooth BR and BLE technologies are summarized in Figure 9. This figure allows the comparison of their differences according to the PHY and MAC layer characteristics. Figure 8: Bluetooth power class classification.