e time that it takes for data to be success- fully transferred increases. This results in the system appearing to become slower for the users. In view of this, Wi-Fi may not provide a suitable QoS in its current form for systems where real-time data transfer is required. To introduce QoS, a new MAC layer was developed as part IEEE 802.11e. The user traffic is assigned a priority level prior to transmission. There are eight user priority levels. The transmitter prioritizes the data by assigning one of the four access categories. The QoS- enabled MAC layer has combined features from the DCF and PCF schemes into hybrid coordination function (HCF). In this approach, the modified elements of the DCF are termed the enhanced distributed channel access (EDCA), whereas the elements of the PCF are termed the HCF controlled channel access. A new class of interframe space called an arbitration interframe space (AIFS) has been introduced for EDCA. This is chosen such that the higher the priority the message, the shorter the AIFS and associated with this there is also a shorter CW. The transmitter then gains access to the channel in the normal manner, but in view of the shorter AIFS and shorter CW, the higher the chance of it gaining access CWmin(10) CWmin(7) CWmin(0) AIFS[i] Immediate access when medium is free > DIFS/AIFS[i] AIFS[i] Voice random backoff range Video random backoff range Best-effort random backoff range Contention window Contention window Slot time DIFS/AIFS Busy medium Backoff slots Busy medium Next frame Next frame Backoff window Defer access Decrement backoff as long as medium is idle Slot time Select slot and decrement Defer access backoff as long as medium is idle Figure 8.7 Medium access methods in IEEE 802.11 30]. 864 Chapter 8 to the channel. Although statistically a higher priority message will usually gain the chan- nel, this will not always be the case. IEEE 802.11 frame contains a MAC header, a variable length frame body (0-2304 bytes) and 32-bit frame check sequence. The MAC header contains inform