Controller Based WLANs

Will the throughput be affected for the 802.11g client in the presence of a 802.11b client in the same RF environment?

by on ‎07-09-2014 02:14 PM

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.3.2.0 and later. 

The throughput for the 802.11g client would definitely drop in the presence of 802.11b client in the same RF environment. The drop in throughput is solely because of the overhead in additional RTS/CTS sent out for the 802.11b client to be aware of the 802.11g transmissions (RTS protection mechanism).   

However with recent changes to ARM (ARM 2.0), the AP would be able to deliver ‘Fairness’ across mixed-mode clients to improve overall available RF capacity. This adjustment would prevent bottlenecks in an environment with mixed clients and ensure that the legacy 802.11b client does not throttle down the performance of 802.11g client (or N clients). 

ARM 2.0 is available from ArubaOS 3.3.2.0 and above. 

Traffic Shaping 
In a mixed-client network, it is possible for slower clients to bring down the performance of the whole 
network. To solve this problem and ensure fair access to all clients independent of their WLAN or IP 
stack capabilities, an AP can implement the traffic shaping feature. This feature has the following three 
options: 

default-access: Traffic shaping is disabled, and client performance is dependent on MAC 
contention resolution. This is the default traffic shaping setting. 
fair-access: Each client gets the same airtime, regardless of client capability and capacity. This 
option is useful in environments like a training facility or exam hall, where a mix of 802.11a/g, 
802.11g, and 802.11n clients need equal to network resources, regardless of their capabilities. 
preferred-access: High-throughput (802.11n) clients do not get penalized because of slower 
802.11a/g or 802.11b transmissions that take more air time due to lower rates. Similarly, faster 
802.11a/g clients get more access than 802.11b clients. 

With the traffic shaping feature, an AP keeps track of all BSSIDs active on a radio, all clients connected to the BSSID, 
and 802.11a/g, 802.11b, or 802.11n capabilities of each client. Every sampling period, airtime is allocated 
to each client, giving it opportunity to get and receive traffic. 

The specific amount of airtime given to an individual client is determined by: 
client capabilities (802.11a/g, 802.11b or 802.11n) 
amount of time the client spent receiving data during the last sampling period 
number of active clients in the last sampling period 
activity of the current client in the last sampling period 

The bw-alloc parameter of a traffic management profile allows you to set a minimum bandwidth to be allocated to a virtual AP profile when there is congestion on the wireless network. You must set traffic shaping to fair-access to use this bandwidth allocation value for an individual virtual AP. 

Configuring Traffic Shaping Using the WebUI 
Traffic shaping is configured in an 802.11a or 802.11b traffic management profile: 
1. Choose Configuration > All Profiles. The All Profile Management window opens. 
2. Choose QoS to expand the QoS section. 
3. Choose 802.11a Traffic management profile or 802.11g Traffic management profile section. 
4. In the Profiles list, choose the name of the traffic management profile for which you want to 
configure traffic shaping. 
(If you do not have any traffic management profiles configured, click the Traffic Management profile 
drop-down list and select New. Enter a name for a new profile in the Profile Details pane.) 
5. In the Profile Details pane, click the Station Shaping Policy drop-down list and select either 
default-access, fair-access, or preferred-access. 
6. Click Apply to save your changes. 

Configuring Traffic Shaping Using the CLI 

You must be in config mode to configure traffic shaping in a traffic management profile. To enable traffic shaping, issue the 
following command: 
wlan traffic-management-profile <profile> fair-access|preferred-access 

To disable band steering, use the default-profile parameter: 
wlan traffic-management-profile <profile> default-access

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