# Performance Analysis of the IEEE 802.11p Multichannel MAC Protocol in Vehicular Ad Hoc Networks
## 1 Introduction  
Vehicular Ad Hoc Networks (VANETs) serve as one of the most important wireless communication technologies to implement all kinds of applications related to vehicles, road traffic, drivers, passengers and pedestrians [1]. It is considered that VANETs play an important part of the Intelligent Transportation System (ITS). VANETs consist of a set of vehicles equipped with communication devices, called On Board Units (OBUs), and a set of stationary units along the roads, called Road Side Units (RSUs). Through Vehicle To Vehicle (V2V) and Vehicle To Infrastructure (V2I) communications, vehicles can exchange information to support safety applications, such as emergency brake, cooperative collision avoidance, and automatic notification of crash on roads, and non-safety applications, such as infotainment, Internet access, video streaming, etc. [1; 2; 3]. IEEE 802.11p protocol [4] has been ratified as a standard to provide Wireless Access in Vehicular Environments (WAVE), which is based on the prioritized Enhanced Distributed Channel Access (EDCA) [5], and can support kinds of ITS applications by providing different level Quality of Service (QoS). The US Federal Communication Commission (FCC) has allocated 75 MHz in the 5.9 GHz band for Dedicated Short Range Communications (DSRC) to be used exclusively for V2V and V2I communications. The overall bandwidth is divided into seven 10-MHz channels, as shown in Figure 1. They include one Control Channel (CCH) for transmitting safety-related messages (e.g., emergencymessages and periodic beacon messages) and control messages such as WAVE Service Advertisement (WSA), and six Service Channels (SCHs) for non-safety applications (e.g., comfort and infotainment). IEEE 1609.4 protocol [6] extends Medium Access Control (MAC) layer operation of the IEEE 802.11p and defines a channel switching mechanism to enable to operate efficiently on seven DSRC channels. According to the IEEE 1609.4 protocol, nodes alternately perform the transmissions of safety-related messages on one CCH and the transmissions of non-safety messages on six SCHs during the fixed CCH Interval (CCHI) and SCH Interval (SCHI), respectively. Therefore, based on multichannel architecture, both safety-related and non-safety applications can be supported [7; 8]. On the CCH, each vehicle broadcasts two kinds of safety-related messages: periodic beacon and emergency (event-driven) message. Vehicles periodically broadcast beacons to perform cooperative vehicle collision avoidance such as cooperative collision warning and lane change warning [9], and under dangerous situations such as car accidents or emergency brake, the emergency messages will be triggered to disseminate to neighboring vehicles. In addition, the control messages such as WSA messages are also disseminated on the CCH, which are used to perform negotiations and reservations of the SCHs. On the other hand, the non-safety messages are transmitted on the SCHs. The traffic of safety messages has stringent requirements on highly reliable and real-time transmissions, while the non-safety applications require efficient and high throughput. Due to high vehicle mobility, ever-changing vehicle density and the requirements of different level QoS, the alternating feature of multichannel operation specified in the IEEE 1609.4 protocol leads to high packet losses on the CCH and low throughput on the SCHs [10]. Based on the EDCA mechanism, messages have different Access Categories (ACs) in a node with different priorities according to degree of emergency. In this paper, we only consider two types of ACs. Each AC on the CCH is as follows [7; 11; 12]: (1) AC\({}_{0}\) (with the higher priority) concerns the safety-related messages including periodic beacons and emergency (event-driven) messages; and (2) AC\({}_{1}\) (with the lower priority) concerns the control messages such as WSA messages. In VANETs, the direct broadcast on the CCH is an effective method to inform the neighborhood of safety-related messages.