# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Bluetooth IEEE 802.15.1
In summary, the base-band functionalities are clock synchronism, management timers, addressing and assessment devices, physical channel handling, channel hop selection, physical link supervision, logical transport management, logical link management, formatting and ordering bits, bit-stream processing, error checking, error correction, definition of ARQ (Automatic Repeat Request) schemes, managing link controller operations, support for general audio recommendations, audio level control, audio paths, frequency mask, and others. A Link Management Protocol is a control protocol responsible to establish base-band and physical layer links. Functions such as connection establishment and release, among others, are Link Management (LM) features acquired by LMP utilization that handles a Synchronous Connection-Oriented (SCO) link, and an Asynchronous Connectionless Link (ACL). A SCO physical link establishment is a symmetric point-to-point connection between the master and a specific slave. It is used to deliver delay-sensitive traffic, such as voice service, and works as a circuit-switched connection between the master and the slave. The ACL link is a point (master) to multi-point (slaves) in a pico-net domain and works as a packet-switched connection, which considers the Bluetooth devices that support point-to-multi-point connections. To ensure the integrity of the data and to guarantee a reliable delivery of the data, ACL uses a fast Automatic Repeat Request scheme. The Logical Link Control and Adaptation Protocol (L2 CAP) layer is a channel-based abstraction between the base-band and service application layer. The L2 CAP layer handles segmentation and reassembly of application data, and multiplexing and de-multiplexing of multiple channels over a shared logical link [28]. The L2 CAP layer is responsible for handling the size adjustment of the maximum transmission unit (MTU) when the application layer data is larger than the MTU of the base-band layer. The L2 CAP layer can segment the application data in order to seize the maximum MTU transmitted based on the size of the MTUs received by the application This feature reduces the overhead on the information sent thus, improving the efficiency. Endpoint peer devices receive a Channel Identifier (CID) used for signaling purposes associated to its connections, for ACL and SCO  
Figure 5: Bluetooth power class classification. data communication between L2 CAP devices. Some CIDs are reserved for the L2 CAP layer as a logical channel required to meet Bluetooth standards and is reserved for signaling purposes. Connectionless channels support 'group' or multi-point communication, while connection-oriented channels are dedicated to peer-to-peer connections only [29]. Figure 6 elucidates the data channelization, links and transport structure. Bluetooth relies on the contention-free token-based multi-access networks as logical topology. End point devices are mentioned as slaves. A cluster of up to seven slave devices can be attached to a master device, in order to have access to the channel, composing a pico-net cellular topology. The master is responsible to manage the polling process messages to authorize a slave device to access the channel and transmit its data. These features increase the range to more than 100 m with a LOS path in an outdoor environment also increasing its indoor environment range to about 40 m. The utilization of beacons as sense mechanisms and the larger message capacity of 255 bytes improves the communication performance. Data rates have a theoretical value of 2 Mbps but with a practical recommendation of 1.6 Mbps considering overheads. A pico-net data rate can reach up to 1 Mbps, which represents the channel capacity not considering the overhead introduced by the adopted access control techniques and polling scheme. The coverage areas of the pico-nets can be overlapped forming a scatter-net topology when at least one unit exchanges data with more than one master.