IEEE 802.11e: QoS standard for Wireless Networks

Voice and multimedia applications on wireless networks make it necessary for IEEE 802.11 MAC layer standards to add QoS terms. After several years of work, the 802.11 standards committee has recently completed the preparation of the 802.11e standard. 802.11e is an extension of the original standard, adding Terms of Service Quality and a set of performance improvement standards.

The added terms enable network administrators to meet application requirements. Understanding the new access methods and the impact of these parameters on network performance is very important to ensure the optimal network performance.

Access Method in section 802.11

The original 802.11 standard identified two access modes for wireless network access: Distributed Coordination (DCF) and point coordination (PCF.

DCF uses the CSMA/CA (Multi-Point Access/conflict avoidance for carrier listening) protocol. A site starts sending signals only when it detects that the wireless media is idle. Different from Ethernet, websites cannot always detect conflicts. Therefore, this Protocol requires the receiving site to send a notification.

When the site detects that the wireless media is idle, it generates a random number within the range specified by the "competitive window" parameter. Then, the site starts counting down from this random number. When the counter reaches zero, if the media is still idle, the site starts to send signals.

PCF defines a polling protocol. It has never been widely used. DCF or PCF does not support any data priority.

New access methods in the 802.11e standard: EDCA and HCCA

The 802.11e standard defines the Hybrid Coordination function (HCF ). HCF replaces DCF and PCF with new access methods to provide improved access bandwidth and reduce latency for high-priority communication. The access method called "Enhanced Distributed coordinated access" (EDCA) extends the DCF function, and the access method called "hybrid control channel access" (HCCA) extends the PCF function.

EDCA specifies four access types, each of which corresponds to one type of data. Each access category is configured with four parameters: CWmin -- Minimum competitive window; CWmax -- maximum competitive window; TXOP -- sending opportunity limit; AIFS -- interval between arbitration frames.

Set these parameters for each type of data so that the network administrator can adjust the network according to the application combination and traffic.

In a DCF application, a website with data to be sent can only be sent when the media is idle. However, with the 802.11e standard, this site is waiting for an additional period of time. The length of the extra time period depends on the data type to be sent. Defines the additional waiting period for the AIFS value set for this data category.

The AIFS value should be smaller for speech classification data, and the AIFS value should be larger for email and FTP data. The speech must be delayed for a short time. A small AIFS value means that voice data can start the next stage of Network Competition faster than less sensitive communications.

After the AIFS period, the site generates a random number between CWmin and CWmax. The lower CWmin and lower CWmax values should be set for the category with a higher priority.

AIFS, CWmin, and CWmax should be set together. In this way, high-priority data can be accessed by the network in most cases. The sum of the AIFS and CWmax values set for the high-priority data must be greater than the sum of the AIFS and CWmin values set for the low-priority data. In this way, low-priority data will not be completely blocked.

TXOP is used for a sequence class to define the maximum length of one transmission. If the data to be sent is too large to be sent within the TXOP limit, the site will send the data multiple times.

The limit on the TXOP of voice data is small because the voice data packet is short. For FTP, email, and network data, a large TXOP limit should be set. In this way, when sending data, you do not need to send the data multiple times.

Access Points can control network workload by using transmission specifications (TSPECs. An access point can require each site to send a transport specification request for each transport class. This request specifies the amount of data requested by the site for each category and the latency of the site. If an access point calculates that the number of requests it receives from each site exceeds the network capacity, it rejects these requests. If an application is rejected, the requested site will no longer send the category data, and the category data must be combined into the data with a lower priority level.

Like PCF, HCCA is a polling protocol. When used, it is always able to obtain the permission to access the media, because it waits for a shorter time than any EDCA user's minimum AIFS time. HCCA can configure separate service quality settings for each application. The hybrid Coordinator (HC) located in the Access Point takes turns to query a single site and approves media access permissions based on the configured specific service quality settings. There is no competition here, so the delay of high-priority data will not be lost as network communication increases.

The IEEE 802.11e standard provides the service quality and enhanced network performance required by applications such as voice and multimedia. With the development of new applications and the use of the IEEE 802.11e standard, there is no doubt that further research and expansion of this Protocol will be required.

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