# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Weightless  
Weightless is the name of a set of LP-WAN protocols for wireless communication networks with low transmission rates. In this set, Weightless have the variations Weightless-P, Weightless-N and Weightless-W. These technologies are standardized by the Weightless Special Interest Group (Weightless SIG) [67]. The Weightless network is a typical star topology system composed of the end devices (ED) and the base stations (BS). EDs are the sensor nodes or are also called leaf nodes and the base stations (BS) concentrate the communication with EDs. The interconnection with the base stations composes the base station networks (BSN) that, among other things, manages the system facilities such as authentication, roaming and radio resource allocation and scheduling. The physical layer of the Weightless protocol has one variant for high data rates and another for low data rates. In both cases, the functional blocks that compose the physical layer for downlink are Forward Error Correction (FEC) encoding, interleaving, whitening, Phase Shift Keying (PSK)/Quadrature Amplitude Modulation (QAM) modulation types control, spreading factor used, cyclic prefix insertion, sync insertion and Root Raised Cosine (RRC) pulse shaping. The combination of the modulation type, FEC rates and spreading factor parameters used impacts the final transmission rate. This transmission rate can vary from 125 Kbps to 16 Mbps. The data rate of 125 Kbps is through a modulation of (\(\pi/2\)) BPSK with an FEC rate and spectral scattering. In addition, 16 Mbps is achieved when the modulation used is 16-QAM without the use of the FEC mechanism and the scattering factor spectral is reduced. Depending on the availability of FEC encoder module, the interleaving module may be present or not. When present, the interleaving block provides time diversity and increases the robustness of the process adding a processing gain. The whitening module uses a known random sequence to scrambler the bit stream turning it into a pseudo white noise, and increasing the receiver synchronization performance. A spreading module is necessary to spread the modulated data that receives a cyclic prefix insertion, in order to reduce the multi-path transmission effects. This characteristic adjusts the frame conversion from the time domain to frequency domain. The synchronization pattern necessary to receive processes is then inserted by the sync insertion module. The RRC pulse shaping acts as a digital filter to reduce the radiation that surpasses the transmission radio frequency band. On the receiving process, appended modules are necessary to coarsen the time offset estimation and correction. It is necessary to find out the start of the burst, the fine frequency offset estimation and correction, channel estimation and equalization and timing detection to determine payload start position. Like many other systems, Weightless uses channels to exchange data between the protocol layers. They are classified according to their role as control channels, logical channels transport channels, and physical channels. To allow base-band data exchange, the Physical layer (PHY) has three physical channels. They are named the downlink channel, uplink channel and uplink contended access channel. To transmit data from the base station to one or multiple EDs, the downlink channel is used. To uplink communication, from an ED to the access point, the uplink channel is used. The uplink contended access physical channel is also used to transmit data from the end devices to the base station. This channel is contended by the end devices and several EDs are allowed to transmit at the same time using this channel. A Base-Band (BB) sublayer is responsible for providing the transport channels to transport the data to or from the Link Layer (LL). This is done by connecting the transport channels to the physical channels of PHY.