# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Weightless
Among the operations carried out by BB are identification of structured allocations within a frame structure and the transmission of the frames in both uplink and downlink directions, using appropriate physical channels. The Contended Access (CA) procedure is also controlled by BB using the uplink contended access physical channel. The communication between LL and BB is done by a set of transport channels and its channels are defined according to the type of information of addressing that is used. Connecting logical channels to the BB transport channels is the LL. LL is also responsible for retransmission control, reliability of the logical channels and for data fragmentation and reassembly. These processes can be either acknowledged or not. The multiplexing and de-multiplexingprocesses of the transport channels into logical control or user data channels is done by the LL. Thus, the LL provides such logical channels to allow the data or control traffic between the end devices and the BS or a BSN through an acknowledged and reliable or an unacknowledged and unreliable packet stream. Radio Resource Manager (RRM) is present to control the traffic between an ED and its BS through Control Channels, using appropriate security provided by LL. The messages that control and maintain the connection between the ED and BS during the communication between them, is handled by RRM through the control channels. RRM also provides a downlink-only control message stream sent by the BSN, in order to maintain the link between the ED and BS. A Weightless protocol stack drawing can be seen in Figure 14.  
Payload data is conducted from an ED and its BS through user channels that provide specific channels for unicast data, multicast data, interrupt data, and acknowledgement data. While the unicast user data channel is a bidirectional channel between the ED and the BS, the multicast data channel is a downlink-only channel. The multicast user data channel is an uplink-only channel. Weightless-W is a bidirectional communication technology that works on a Television White Space (TVWS) spectrum of 470-790 MHz. Its multiple access mechanism uses frequency hopping (Frequency Hopping Multiple Access--FHMA) with Time Division Duplexing (TDD). This can separate and coordinate the uplink and downlink transmission intervals. The data rates range from 1 Kbps to 10 Mb/s and the battery life is from three up to five years depending on usage. This technology supports star topology and 128-bit AES encryption in its packets. The packets can carry up to 10 bytes of payload, but encryption can be implemented end-to-end depending on other mechanisms, such as the network core. The error correction system is based on a Forward Error Control (FEC) algorithm not specified by the manufacturer. Channeling is done with 16 to 24 channels of 5 MHz bandwidth, depending on the frequency of use. The modulation of the channels is adaptive and can reach high rates at short distances according to the need of the application. The modulation can start with the Differential Phase Shift Keying (DBPSK) and BPSK for greater distances. Lower rates from 1 Kbps use Quadrature Phase Shift Keying (QPSK) or 16-QAM, reaching peaks of 10 Mbps over short distances [68; 69; 70; 71]. Weightless-N is based on the Weightless-W standard and adapted for smaller distances and lower energy consumption (batteries last up to 10 years), sacrificing the transmission rate from 30 up to 100 Kbps. Weightless-N does not reach the data rate peaks that the Weightless-W can reach. Unlike Weightless-W, the Weightless-N is based on an Ultra Narrow Band (UNB) system and operates on the UHF frequency in the 800-900 MHz ISM band, providing only uplink communication. This system supports star topology and applies the UNB DBPSK (Differential Phase Shift Keying) modulation to  
Figure 14: Weightless protocol stack. ultra-narrow 200 Hz wide-band channels. With this simpler modulation, DBPSK, the device can save energy by making the battery last for up to 10 years [68].