IEEE 802.11 WLAN
The basic medium access mechanism of the IEEE 802.11 Distributed Coordination Function (DCF) is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). An 802.11 wireless station senses the wireless medium before transmitting any packet. If the medium is busy, it applies a random backoff mechanism to defer the wireless medium access. The backoff counter is a random value between 0 and the current Contention Window (CW) value, which is initialized to the minimum window size. When the 802.11 station senses that the medium is free for at least the time duration DIFS (DCF Interframe Space), it begins to decrease the backoff counter. When the backoff counter value is 0 and the medium is still idle, the station can begin transmitting packets. After the packet transmission is done, the station which successfully received the data should send an Acknowledgement (ACK) frame back to the sending station. A Short Interframe Space (SIFS) waiting time is applied before replying the ACK message. The whole transmission procedure ends when the sending station successfully receives the ACK frame. If a data transmission is detected failed (no ACK is received), the backoff counter CW is doubled, until it reaches the maximum contention window size (CWmax). The CW value is reset to CWmin after successful data transmission.
In basic IEEE 802.11 operation, no quality of service is supported. All traffic contends for channel access with the same DIFS, CW, CWmax, and CWmin. Therefore, there is no difference between each packet. IEEE 802.11e (IEEE, 2005) is developed for QoS support in wireless LAN. The Enhanced Distributed Channel Access (EDCA) provides differentiated QoS support (IEEE, 2005; Mangold, et al., 2003). In EDCA, there are four Access Categories (AC): AC_VO for voice transmission, AC_VI for video transmission, AC_BE for best effort traffic, and AC_BK for background traffic. Each AC has different CWmax, CWmin, Arbitration Interframe Space (AIFS), which is similar to DIFS, and duration of Transmission Opportunity (TxOP). For high priority AC, smaller contention window parameters and AIFS value are applied.
To date, IEEE 802.11e has become a fundamental part of 802.11 protocols. It has been merged with IEEE 802.11a,b,g as a new standard of 802.11 (IEEE, 2007). It is also widely supported by most commercial products. Most subsequent 802.11 family members, including the high-throughput 802.11n (IEEE, 2009), retain the support of EDCA for compatibility. Thus, a cross-layer design based on the popular EDCA mechanism can provide high compatibility to both existing and future 802.11 infrastructures and mobile devices.