## 2 Background  
### IEEE 802.11 DCF and EDCA  
As a fundamental MAC mechanism of the IEEE 802.11, the Distributed Coordination Function (DCF) is a simple and flexible scheme to share the medium among multiple stations. As shown in Figure 1, in DCF, stations contend for the chance of channel access by Carrier Sense Multiple Access mechanism with Collision Avoidance (CSMA/CA). When collisions happen, DCF adopts a Binary Exponential Backoff (BEB) algorithm to alleviate the congestion [12]. Two separate and distinct carrier-sensing functions are defined in IEEE 802.11 standard: Clear Channel Assessment (CCA) and the Network Allocation Vector (NAV). CCA is physical carrier sense, which determines whether the medium is idle or not, based on energy thresholds from the radio interface. NAV is virtual carrier sense, which is an indicator for the station to avoid potential conflicts by overhearing stations.  
Before starting a new transmission, each station must sense the status of the channel. The station is permitted to initiate its transmission only if it finds the channel is idle in an additional random backoff period plus a DCF Interframe Space (DIFS) duration. Otherwise, the transmission must be frozen until the medium is idle again. The backoff duration is composed of a multiple of time slots. Each active station generates a uniformly random backoff value from [0, \(CW-1\)], where CW is the contention window size. The backoff value is the number of time slots that a station has to wait before transmission. In the first transmission, CW is set to minimal value \(CW_{min}\), which is defined in the standard. When the transmission fails, the CW is doubled until it reaches maximum CW value \(CW_{max}\). Once the CW reaches \(CW_{max}\), the contention window is maintained at \(CW_{max}\) even if the next transmissions are still unsuccessful. The CW is set back to \(CW_{min}\) after a successful data transmission or when the retransmission counter exceeds the retry limit.  
To provide priority-based QoS for real-time applications, IEEE 802.11e task group enhances the DCF through a new channel access mechanism: Enhanced Distributed Channel Access (EDCA). As illustrated in Figure 2, four Access Categories (ACs) are defined in EDCA, namely voice (\(AC\_VO\)), video (\(AC\_VI\)), best-effort (\(AC\_BE\)), and background (\(AC\_BK\)) traffic. In EDCA, higher priority traffic uses shorter arbitration inter-frame space (AIFS). When an internal traffic collision happens, the higher priority access category obtains the data transmission chance, while the other ACs should restart the backoff procedures. With EDCA, high-priority traffic has a higher transmission chance than low-priority traffic by differentiating the backoff parameters for different ACs (Table 1).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline _AC_ & \(CW_{min}\) & \(CW_{max}\) & \(AIFS\) & \(T_{AIFS}\) \\ \hline _AC\_VO_ & 3 & 7 & 2 & 28 \(\upmu\)s \\ _AC\_VI_ & 7 & 15 & 2 & 28 \(\upmu\)s \\ _AC\_BE_ & 15 & 1023 & 3 & 37 \(\upmu\)s \\ _AC\_BK_ & 15 & 1023 & 7 & 73 \(\upmu\)s \\ \hline \hline \end{tabular}
\end{table}
Table 1: IEEE 802.11E EDCA parameter set.  
Figure 1: Example of the DCF mechanism.  
Figure 2: EDCA traffic priorities mapping.