802.11 - Why lower rate communication affect other higher rate devices

(take from CWNA-PW0-105)

Airtime Fairness

One of the important features of 802.11 is its ability to support many different data rates. This allows older technologies to still communicate alongside newer devices, along with enabling devices to maintain communications by shifting to slower data rates as they move away from an access point. The ability to use these slower data rates is paramount to 802.11 communications; however, it can also be a huge hindrance to the overall perfor- mance of the network, and to individual devices operating at faster data rates.

Since 802.11 is contention based, each radio must contend for its turn to communicate, then transmit, and then go back to the contention process. As each radio takes its turn transmitting, the other 802.11 radios must wait. If the transmitting radio is using a fast data rate, the other radios do not have to wait long. If the transmitting radio is using a slow data rate, the other radios will have to wait a much longer period of time. When 802.11 radios transmit at very low data rates such as 1 Mbps and 2 Mbps, effectively they cause medium-contention overhead for higher data rate transmitters due to the long wait time while the slower devices are transmitting.

To try to understand this, look at Figure 8.9. The top portion of the figure illustrates the normal operation of two stations each sending eight frames. One station is sending eight frames at a higher data rate and the other station is sending eight frames at a lower data rate. If a high-speed and a low-speed device coexist in the same WLAN, they have to share or contend for the time to transmit. In other words, both stations get an equal number of times to access the RF medium even though one of the stations is capable of transmitting at a higher rate. Because there is no priority given to the station with the higher data rate, both stations finish transmitting their eight frames over the same period of time.

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Instead of allocating equal access to the network between devices, the goal of airtime fairness is to allocate equal time, as opposed to equal opportunity. Airtime fairness can provide better time management of the RF medium. In the bottom half of Figure 8.9, airtime fairness is enabled, you can see the station with the higher data rate transmission is given priority before the station with the lower data rates. Effectively this is a much better use of transmission time because the higher data rate station does not have to remain idle waiting during the lower data rate transmission. Notice that the faster station transmitted all eight frames in a much shorter time period, and the slower rate station still sent all eight frames in about the same period as before. Airtime fairness effectively achieves better time management of the medium by cutting down on wait times.

There currently are no 802.11 standards or amendments that define airtime fairness or how to implement it. Nor is there any requirement for a vendor to implement it. Most ven- dors use airtime fairness mechanisms only for downstream transmissions from an AP to an associated client. Airtime fairness mechanisms are normally used for prioritizing the higher data rate downstream transmissions from an AP over the lower data rate downstream trans- missions from an AP. At least one vendor also makes claims of upstream airtime fairness capability. Any implementation of airtime fairness is a proprietary solution developed by each WLAN vendor. No matter how each vendor implements their solution, the underlying goal is essentially the same: to prevent slower devices from bogging down the rest of the network.

Although each WLAN vendor takes their own approach to implementing airtime fair- ness, it is typical for them to analyze the downstream client traffic and assign different weighting based on such characteristics as current throughput, client data rates, SSID, and distance from the AP. Algorithms are used to process this information and determine the number of opportunities for each client’s downstream transmissions. If implemented prop- erly, airtime fairness makes better use of the medium by providing preferential access for higher data rate transmissions.