# Adaptive Polling Medium Access Control Protocol for Optic Wireless Networks
## 2 Related Studies  
Existing MAC protocols fall into three major categories: (i) non-contention protocols such as polling and token passing, (ii) contention-based protocols such as Aloha and carrier sense multiple access (CSMA), and (iii) channelization-based protocols like frequency division multiple access (FDMA) and time division multiple access (TDMA) [28]. In non-contention protocols, one device is controlling other devices and each device is assigned a specific time for using shared resource; therefore collision cannot occur. Contention-based protocols do not require control device that controls other devices. Usually contention-based protocols involve asynchronous competition in order to get access to sharedresources. However, contention-based protocols suffer from hidden nodes interference, low system utilization, and throughput degradation at high network loads. The IEEE 802.15.7 standard defines the physical layer and the MAC layer for short range wireless optical communication. The specifications of MAC layer allow four ways of accessing a shared channel such as (i) beacon enabled slotted random access (ii) beacon enabled unslotted random access (iii) non-beacon enabled slotted random access and (iv) non-beacon enabled unslotted random access [12]. Moreover, the IEEE 802.15.7 standard allows three types of network topologies namely peer-to-peer, star, and broadcast network topologies. Note that the beacon enabled modes divide time into superframes that are bounded by beacons transmitted via a coordinator. The authors in [29; 30; 31], present MAC protocols for the convergence of fiber and short radio wave based wireless communication. The medium-transparent MAC protocol proposed in [29; 31] provides seamless and dynamic bandwidth allocation in radio over fiber networks via the contention-based mechanism. In [32] the authors present the broadcasting based MAC protocol for VLC. This protocol exploits the channelization techniques such as TDMA to enhance the quality of service in VLC. Non-contention MAC protocol for ethernet passive optical network (EPON) is presented in [33]. This protocol eradicates collision and increases system utilization because communication operates in non-contention mode. Cooperative MAC protocol for VLC is proposed in [34], wherein the authors aim at enhancing reliability and increasing the coverage area. Moreover, this protocol can be exploited for VLC based vehicular communication. The contention-based MAC protocol for VLC that minimizes dual transmission over several access-points is presented in [35]. The novel energy efficient medium access scheme for VLC is studied in [36] to improve the performance of the CSMA/CA considering unsaturated conditions. [PERSON] et al., in [37] investigate the contention-based full-duplex MAC protocol for VLC. [PERSON] et al., in [38], present the novel VLC MAC protocol that combines time division multiple access (TDMA) and code division multiple access (CDMA). The authors in [38] consider delay minimization in vehicle to vehicle communication. Moreover, in [39] the polling-based MAC protocol for wireless networks is presented; wherein the focus is reducing polling overhead, dropping probability, and delay. Furthermore, the resource allocation scheme that exploits channelization technique such as TDMA for indoor VLC is presented in [40]. The energy consumption minimization problem for VLC is studied in [40], wherein, energy minimization subject to throughput maximization is the key points of consideration. [PERSON] et al., in [41] present the visual recognition based MAC protocol that aims at minimizing collision by using both radio and visible light spectrum. The proposed protocol in [41] increases throughput by 40% comparing to CSMA/CA. In order to mitigate the problem of signal interference caused by intersecting angle of irradiance, the authors in [42] propose the hidden avoidance enabled CSMA/CA protocol for VLC. The dynamic contention window-based MAC protocol that increases channel utilization for VLC is presented in [43].