# Performance Analysis of the IEEE 802.11p Multichannel MAC Protocol in Vehicular Ad Hoc Networks
## 3 Background
### MAC Sublayer in IEEE 802.11p  
The IEEE 802.11p protocol employ EDCA mechanism in IEEE 802.11e [5] for contention-based prioritized QoS support. According to EDCA mechanism, a station (node) can implement up four ACs with different priorities corresponding to voice, video, best effort and background traffic. Each AC has an independent MAC queue entity, which can be identified by a set of distinct channel access parameters, including CW, AIFS and Arbitrary Inter-Frame Space Number (AIFSN[AC]). The AIFSN[AC] is used to determine the duration of AIFS[AC] according to  
\[\text{AIFS}[\text{AC}]=SIFS+\text{AIFSN}[\text{AC}]\times\sigma \tag{1}\]  
where AIFSN[AC] \(\geq\) 2 corresponds to Distributed Inter-Frame Space (DIFS) of IEEE 802.11, and \(SIFS\) and \(\sigma\) represent the duration of a Short Inter-Frame Space (SIFS) and a slot time, respectively. The default EDCA parameters setting for ACs in IEEE 802.11p is shown in Table 2 where AC\({}_{0}\) corresponds to the highest priority and AC\({}_{3}\) corresponds to the lowest priority.  
The operation of IEEE 802.11e EDCA backoff procedure described in [5] is illustrated in Figure 2. Each station has four AC queues acting as four independent stations. Note that, in this paper, we only consider two types of ACs. Each frame from the higher layer arrives at the MAC layer with a specific priority value, and enters a specific queue. The EDCA mechanism relies on the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) technique to contend and access channel which a station must probe the channel before transmission to determine whether it is busy or idle. On one hand, if only one AC queue has backlogged data at a time in a station, and the station will sense the channel idle for the duration of AIFS[AC] as in Equation (1) before attempting to transmit it. If the channel is sensed as busy, then the station defer its transmission of an additional backoff interval. The backoff interval is calculated as a random number of slot times uniformly selected from [0, CW[AC]]. At the first transmission attempt, the backoff interval for an AC in EDCA is randomly selected from [0,  
* Note: H/E: Hidden terminal/Erroneous channel, BBP: Backoff Blocking Probability, BSIP: Backoff Stage Increase Probability; AD: AIFS Differentiation, VC: Virtual Collision, CS: Channel Switch, D: Delay, T: Throughput, PRR: Packet Reception Rate, NS/S: Non-saturation/Saturation, B/U: Broadcast/Unicast, FCP: Frame Collision Probability, FBP: Frame Blocking Probability, FVCP: Frame Virtual Collision Probability, FFP: Frame Failure Probability, CBP: Channel Busy Probability.  
\end{table}
Table 1: Comparison of different models.  
CWmin[AC]], and it is doubled at every retransmission with an upper limit equal to CWmax[AC]. The smaller is AIFS[AC] or CWmin[AC], the higher is the priority in channel access. On the other hand, since each station has four different AC queues, there is a probability that more than one AC queue initiates a transmission at the same time. Therefore, an internal collision occurs inside a station, also called virtual collision. We will discuss this issue in detail in Section 4.2.