## 6 Communications
### Medium Access Control
Second, applications with high vehicle traffic imply the generation of numerous packets and a considerable amount of energy wasted in the transmission of their preambles. Thus B-MAC is more appropriate for applications involving low vehicle traffic densities such as those found in rural environments or some parking lots. In addition this protocol has demonstrated its feasibility to communicate with vehicles at the typical speeds of these scenarios [40] if this feature is needed. Regarding scheduled protocols, these are more complex protocols which usually need control messages in order to maintain synchronization and coordinate the radio transceivers active period according to different strategies. This is not an important problem if these protocols are used in road static deployments. However, it may become a serious issue if these protocols are considered for the vehicle to road infrastructure communications, given the limited communication time with travelling vehicles and possible packet losses. In contention based scheduled protocols nodes basically follow a common active/sleep schedule of their transceivers, using CSMA during their active period. Representative examples can be found in the reviewed literature. S-MAC [82], currently applied to ITS in [11], synchronizes local areas or clusters, implying that in applications with communication between adjacent clusters the data dissemination delay increases and that border nodes must maintain more than one schedule. In addition, the A-MAC protocol [83], used in [31] for a street parking application, outstands for its ability for adjusting the duty cycles of nodes according to their remaining energy in such a way that it can guarantee a predetermined network lifetime. Scheduled contention-less protocols comprise those based on TDMA and slot reservation. On the one hand, they offer the advantage of avoiding collisions, with the consequent improvement in performance. On the other hand, they also have some drawbacks such as their lack of scalability and adaptability due to the difficulty of introducing new nodes in the layout as well as the need for strict synchronization in order to align slot boundaries; the particular layout of road deployments though, alleviates some of these restrictions. This is the main reason why it is possible to find different protocols specifically designed for ITS systems [12, 76, 84]. These protocols exploit the layout of roads, characterized by the presence of strings of road nodes with few neighbors, to assign (and reutilize) time slots to non-conflicting nodes in the road (_i.e._ distant nodes). Similarly they assume a hierarchical architecture with more powerful master nodes which manage the rest of nodes. PEDAMACS [84, 85] in particular, used in [20, 26, 55], relies on a master node with a larger communication range which enables it to simultaneously transmit to all the managed nodes via one-hop communications with the purpose of scheduling communications and simultaneously achieving network synchronization; data transmissions from the managed nodes, conversely, occur in the opposite direction in a multi-hop manner in order to save energy. The protocol presented in [76], in turn, stands out for allocating some time slots exclusively for the communication between road nodes and vehicles. Another commonly used protocol is IEEE 802.15.4 [50], employed in different works such as [15, 35, 86], which allows low power communications and has a coverage range similar to other wireless technologies such as Wi-Fi and may be used both for contention based (CSMA) or contention-less (TDMA) communications. It should be remarked that this protocol was not designed with mobility in mind, therefore it is well suited for WSN road deployments but it has several problemswith mobile nodes related to association with 802.15.4 coordinators and synchronization [87], as a consequence its suitability for vehicle to roadside communication should be studied more in-depth.