## 6 Communications
### Medium Access Control  
The Medium Access Control (MAC) layer provides link-level data addressing and access control mechanisms to the physical medium (radio) shared by nodes in a multi-point network. The design of MAC protocols for WSN is different from that for other wireless networks since it is mainly focused on power conservation, which implies finding a trade-off between different metrics such as latency and throughput in order to extend the network lifetime. In the scientific literature, many MAC protocols suited for WSNs can be found as well as survey works according to different purposes, including [77], where real-time MAC protocols are considered, and [75], which deals with energy-aware protocols. Conversely, MAC protocols for VANETs must provide high reliability and low delay without having to deal with the limitations inherent to WSN. To this aim, several works study how the selection of the MAC concerns the communications among vehicles, including [78], which describes the main features of IEEE 802.11 standards applied to the VANET environment, and [79, 80], which provide two alternative classifications for MAC protocols. Among the existing protocols and standards, it should be emphasized the recently release of the IEEE 802.11p [52] standard which is intended to offer an efficient communication in the V2V (vehicle-to-vehicle) as well as in the V2I (vehicle-to-infrastructure) scenarios. The focus of this work is on the WSN tier of the system. The approach which has been taken to review how both existing (general purpose) and specifically designed MAC protocols apply to ITS scenarios follows the classification of protocols proposed in [75] because the different categories considered have an important impact on the performance of the system. According to the classification, on a top level we can differentiate between (i) unscheduled or random protocols, where nodes operate independently; (ii) scheduled protocols, which organize communications in an ordered way, with radio transceivers following a coordinated scheduling of their duty cycles; and (iii) hybrid protocols, which combine different scheduled and unscheduled techniques. In addition, sub-classifications for each one are also presented, where it is interesting to remark that scheduled protocols may be divided into contention and contention-less based protocols. Unscheduled protocols are popular in the WSN domain because they do not require neither clock synchronization among nodes nor global topology information. Nevertheless, these protocols may suffer higher rates of packet collisions since they usually do not provide any means for avoiding them, apart from carrier sensing. In an ITS scenario they provide two important advantages: (a) dynamic node joining, which is very useful in ITS since it facilitates communication with moving vehicles, and (b) adaptability to changes in topology. The latter have proven to be very important in some specific applications such as those requiring WSN deployments in parking lots. In them, the arrival of vehicles and its associated distortion on communications (see Section 5.1) may provoke the loss of existing links as well as worsening synchronization as shown in [28], favoring the adoption of unscheduled protocols. Within these protocols, preamble based ones are the most commonly found. In them, a sender probes the receiver with repetitive sequences of bits until the latter awakes. These sequences are denoted as preambles in the B-MAC protocol [53], which is currently selected for many ITS works [40, 67, 81]. In addition the B-MAC protocol is sometimes inadvertently used by developers because many commercial sensor nodes advertising 802.15.4 compliant transceivers actually use B-MAC by default. The use of this protocol in ITS scenarios has two additional consequences. First,end-to-end latencies are high for data dissemination even with short preambles since prior to each forwarding a preamble must be transmitted.