Hybrid Access Techniques for Densely Populated Wireless Local Area Networks

Abstract

The IEEE 802.11p Task Group has recently released a new standard for wireless access in vehicular environments (WAVE). It constitutes an amendment to the 802.11 for Wireless Local Area Networks (WLANs) to meet the requirements of applications related to roadsafety involving interand intra-vehicle communications as well as communications from vehicle to the roadside infrastructure. Indeed, the importance of the targeted applications has forced authorities to allocate some dedicated bandwidth (nearby the 5.9GHz) to ensure the security of the communications. However, despite the suitability of this standard for use in high-speed vehicular communications, it is not possible to pass over the unprecedented market penetration of the popular 802.11 networks, the so-called WiFi networks. Before we can see a world where all the cars are equipped with 802.11p devices, current and nearfuture applications might probably run on the original 802.11. Moreover, interaction between humans and vehicles will probably be carried out by means of the 802.11, which is the standard that is flooding most of personal tech devices, such as laptops, mobile phones, gaming consoles, etc. Therefore, it is important to keep on working in the improvement of the 802.11 Standard for its use in, at least, some vehicular applications. This is the main motivation for this chapter, where we focus on the Medium Access Control (MAC) protocol of the 802.11 Standard, and we propose a simple mechanism to improve its performance in densely populated applications where it falls short to provide users with good service. Envisioned applications include those were a high number of vehicles and pedestrians coexist in a given area, such as for example, a crossing in a city where all the cars share information to coordinate the drive along the crossing and prevent accidents. Into more detail, the Distributed Coordination Function (DCF) is the mandatory access method defined in the widely spread IEEE 802.11 Standard for WLANs [1]. This access method is based on Carrier Sensing Multiple Access (CSMA), i.e., listen before transmit, in combination with a Binary Exponential Backoff (BEB) mechanism. An optional Collision Avoidance (CA) mechanism is also defined by which a handshake Request to Send (RTS) – Clear to Send (CTS) can be established between source and destination before the actual transmission of data. This CA mechanism aims at reducing the impact of the collisions of data packets and to combat the hidden terminal problem. The DCF can be executed in either ad hoc or infrastructure-based networks and is the only access method implemented in most commercial hardware. Despite the doubtless commercial success of the DCF, the simplicity