Draytek VigorAP 903, User Manual

Page: 60 / 153

54
you have to specify the rate manually.
Download Limit Define the maximum speed of the data downloading which will be used for
the wireless station connecting to Vigor device with the same SSID.
Use the drop down list to choose the rate. If you choose User defined,
you have to specify the rate manually.
Auto Adjustment Check this box to have the bandwidth limit determined by the system
automatically.
Total Upload Limit When Auto Adjustment is checked, the value defined here will be treated
as the total bandwidth shared by all of the wireless stations with the same
SSID for data uploading.
Total Download Limit When Auto Adjustment is checked, the value defined here will be treated
as the total bandwidth shared by all of the wireless stations with the same
SSID for data downloading.
After finishing this web page configuration, please click OK to save the settings.
II-3-9 Airtime Fairness
Airtime fairness is essential in wireless networks that must support critical enterprise applications.
Most of the applications are either symmetric or require more downlink than uplink capacity; telephony
and email send the same amount of data in each direction, while video streaming and web surfing involve
more traffic sent from access points to clients than the other way around. This is essential for ensuring
predictable performance and quality-of-service, as well as allowing 802.11n and legacy clients to coexist
on the same network. Without airtime fairness, offices using mixed mode networks risk having legacy
clients slow down the entire network or letting the fastest client(s) crowd out other users.
With airtime fairness, every client at a given quality-of-service level has equal access to the network's
airtime.
The wireless channel can be accessed by only one wireless station at the same time.
The principle behind the IEEE802.11 channel access mechanisms is that each station has equal
probability
to access the channel. When wireless stations have similar data rate, this principle leads to a
fair result. In this case, stations get similar channel access time which is called airtime.
However, when stations have various data rate (e.g., 11g, 11n), the result is not fair. The slow stations
(11g) work in their slow data rate and occupy too much airtime, whereas the fast stations (11n) become
much slower.
Take the following figure as an example, both Station A(11g) and Station B(11n) transmit data packets
through VigorAP 903. Although they have equal probability to access the wireless channel, Station B(11n)
gets only a little airtime and waits too much because Station A(11g) spends longer time to send one packet.
In other words, Station B(fast rate) is obstructed by Station A(slow rate).
To improve this problem, Airtime Fairness is added for VigorAP 903. Airtime Fairness function tries to
assign
similar airtime
to each station (A/B) by controlling TX traffic. In the following figure, Station B(11n)
has higher probability to send data packets than Station A(11g). By this way, Station B(fast rate) gets fair
airtime and it's speed is not limited by Station A(slow rate).