The MAC layer characteristics include: unique network identifiers that allow up to 232 nodes to join one network, collision avoidance mechanisms, back-off time in case of collision, automatic retransmission to guarantee reliability, dedicated wakeup pattern that allows nodes to sleep when they are out of communication and hence save their power. G9959 MAC layer features include unique channel access, frame validation, ACK, and retransmission , .
LTE-A was started in 2009 with multiple releases that are continuously coming to support new technologies. It traditionally uses orthogonal frequency division multiple access (OFDMA) as a medium access technology, in which the frequency is divided into multiple subcarriers. The architecture of LTE-A consists of a core network (CN), a radio access network (RAN), and the mobile nodes. The CN is responsible for controlling mobile devices and to keep track of their IPs. RAN is responsible for establishing the control and data planes and handling the wireless connectivity and radio-access control. RAN and CN communicate using S1 link, as shown in Fig. 4 where RAN consists of the eNB’s to which other mobile nodes are connected wirelessly .
Moreover, the new releases of LTE-A (LTE Rel-13 and Rel-14) introduce new features that were designed to fit the upcoming 5G requirements . Rel-13 introduced three major features: FD-MIMO, enhanced spectrum and carrier aggregation, and new services for machine type commination. Full dimension multiple input multiple output (FD-MIMO) aims to increase spectrum efficiency using large number of antenna ports at the base station. The utilization of additional frequency resources is done by using unlicensed spectrum in addition to the already used licensed spectrum frequencies. In this way, more frequencies are used and it keeps backward compatibility with existing devices. Further, increasing the peak rate and efficient frequency resources distribution were used to enhance carrier aggregation in LTE-A. Moreover, LTE-A Rel-13 offered new services for machine type communication including cost reduction, support of extended coverage, indoor positioning, and broadcast and multicast support in a single cell.
LTE Rel-14 specification is expected to further enhance FD-MIMO with more antenna porta, robust transmission and reduced feedback. In addition, the release is expected to standardize latency reduction, vehicle to anything and downlink multi-user transmission which were discussed as feasibility studies in Rel-14 .
Fig. 4: LTE-A Architecture
It is a low-power consumption optimized protocol designed for scalable wireless networks with millions of devices. It supports redundant operation, location free, low cost, low power and energy harvesting technologies to support the future needs of IoT while enabling mobility and ease of use features .
Two standards can be used: Weightless-N and Weightless-W. Weightless-N was the first standard developed to support IoT requirements using TDMA with frequency hopping to minimize the interference. It uses ultra-narrow bands in the sub-1GHz ISM frequency band. On the other hand, Weightless-W provides the same features, but uses television band frequencies .
It has a dedicated channel assignment and, hence, has much more tolerance to interference and congestion problem. DECT/ULE supports FDMA, TDMA and time division multiplexing, which were not supported in the original DECT protocol .
The basic idea is to use efficient harvesting of motion, or any type of environmental energy, and convert it to usable energy using converters. This protocol has a relatively low packet size and is mostly used in heating, ventilation, and air conditioning IoT applications .
In addition to all the previously discussed data link protocols, near field communication (NFC), ANT, and International Society of Automation (ISA100.11a) can be used as well. These standards are rarely used in IoT as they are getting incomparable to the new upcoming ones, which we discussed in this section. NFC is used for short range communication in an ad-hoc manner. It operates at relatively low frequencies and uses radio frequency identifier to power on the receiver and start the peer-to-peer communication . ANT is a multi-cast wireless protocol that operates in a master-slave manner. It is mostly used for wireless sensor networks, operates at the 2.4 GHz frequencies and conceptually similar to Bluetooth low energy . ISA100.11a is the ISA standards developed for wireless networking in industrial automation control .
In this section, various data link protocols were briefly discussed with their main differences and usefulness in IoT medium access. Those protocols are mostly standardized by IEEE, ITU or other wireless standards organizations. Generally speaking, the most widely used standards in IoT are Bluetooth and ZigBee. IEEE 802.11ah, on the other hand, is the most compatible one with IEEE 802.11 which is the most used infrastructure in other wireless applications. Furthermore, some providers and IoT markets would seek for more reliable and secured technology and hence would use HomePlug for LAN connectivity. Newly arising LoRaWAN seems to be promising for outdoor applications.