Overview of IEEE 802.11E, the WLAN Standard
IEEE 802.11e is an addendum to the IEEE 802.11 Wireless Local Area Network (WLAN) standards.
The IEEE 802 LAN standards recognize that video streaming, voice over IP (VOIP), cell phone users and other wireless network users have different bandwidth needs and varying quality of service (QoS) requirements. The original IEEE 802.11 standard did not take into consideration the different requirements of different types of wireless traffic when determining quality of service of wireless users.
Access Categories in IEEE 802.11
The IEEE 802.11e standard's QoS MAC allows up to four different transmit queues to be run. Each transmit queue has its own access priority, service level and bandwidth allowances. For example, video and voice traffic over Wireless Local Area Network (WLAN) may be given higher priority access to bandwidth over internet browsing. Or video and voice traffic may be configured to packet sizes and packet burst frequency to minimize bandwidth usage without interrupting the user's experience.
The four IEEE 802.11e standard's transmit queues or access categories (AC) are: VO or voice, VI for video, BE for best effort and BK or background. Each access category has its own EDCA parameters. When there are competing signals, the IEEE 802 WLAN standard states that the frame or packet from the highest priority access channel is sent first.
Defining Transmission Opportunity
Transmission opportunity (TXOP) is the time interval in which an access channel has the right to initiate transmissions without other access channels competing for that time interval.
During a TXOP, the access channel can send multiple data packets if it can do so in the allowed time frame. The length of the transmission opportunity (TXOP) is set in the TXPO limit parameter between 0 and 255. The maximum TXOP limit is 8,160 microseconds.
If the channel becomes idle during the TXOP limit, the hybrid controller may reclaim the channel and use it to send a different set of data. The Arbitrary Distributed Interframe Space (AIFS) is a time frame set by the controller or in system parameters. If the channel is idle for a time period equal to the Arbitrary Distributed Interframe Space (AIFS), the access channel can be assigned to a new, incoming data stream.
IEEE 802.11e includes an enhanced MAC-level QoS protocol. The Enhanced Distributed Channel Access (EDCA) runs the IEEE 802.11e MAC protocol.
IEEE 802.11e Medium Access Control or MAC uses a Hybrid Coordination Function (HCF) for channel access. In comparison, the DCF is used to operate the IEEE 802.11 or original MAC protocol.
The Hybrid Control Channel Access (HCCA) is part of the HCF but is not mandatory. The HCCA determines the transmission opportunity time interval given to 802.11e wireless network points.
Equations Specified in IEEE 802.11e
IEEE 802.11e calculates the resource need by data stream based on the mean data rate and average packet size. Average data rate is measured in bps. The service interval is measured in microseconds.
Data packets are stored during the service interval (SI) before being transmitted. The TXOP is supposed to be long enough to send all stored data packets saved during the service interval at the next data transmission interval.
The application mean data rate times the service interval divided by the nominal MAC Service Data Unit (MSDU) size yields the average number of data packets stored in the buffer during the service interval.