sfi

NDP

ENTERPRISE IRELAND

NCNRC

Communications Network Research Institute

WLAN Resource Controller (WRC)

Mark Davis

The WRC application has been developed as a WLAN bandwidth provisioning application that greatly simplifies the use of the IEEE 802.11e/WMM EDCA functionality in implementing radio resource management in WLANs. The WRC automatically tunes the EDCA parameters of a QoS-enabled access point (QAP). The only requirement by the user is to specify the minimum capacity Cmin for each Access Category (AC).

Under the IEEE 802.11e/WMM MAC enhancement standard two new modes of MAC operation are supported: Enhanced Distribution Coordinate Access (EDCA) and Hybrid Controlled Channel Access (HCCA). The WRC application described here pertains to the EDCA mode of operation only. Essentially, the EDCA mode differentiates the channel access probability among the different traffic categories. Under the EDCA mode of operation packets are classified according to different traffic categories at the network layer, and these are mapped to four prioritized transmit queues at the MAC layer, called access categories (ACs). Associated with each AC is a set of parameters that controls the access probability to the wireless medium. Configuring the EDCA parameters separately for each AC is intended to allow users to introduce access probability differentiation between the traffic categories. However, in this approach the EDCA parameters are used to tune the access efficiency values.

The basic strategy here is to specify a minimum capacity (Cmin) for each AC and then to tune the EDCA parameters in such a way as to realize these capacities.

tune EDCA parameters

The goal of the WRC algorithm is to ensure that each AC can be accommodated on the network by ensuring that it is provided with a sufficient capacity to ensure an acceptable QoS. The capacity allocation across the ACs may be controlled through the access efficiencies associated with each AC which in turn are determined by the EDCA parameter settings. Essentially the WRC adaptively tunes the EDCA parameters in such a way that the capacity is shared out among all the competing ACs according to their minimum capacity requirements.

Radio Resource Conroller

The WRM application is used to provide real-time measurements of the access efficiency factors for all the AC streams. These AEF measurements, i.e. {AEF}AC, provide one of the inputs to the RRC, the other input being the minimum capacity requirements of each AC, i.e. {Cmin}AC. The RRC then determines the updated EDCA settings required for each AC, i.e. {AIFSN, ECWmin, ECWmax}AC. These new EDCA settings are then broadcast in the beacon management frames transmitted by the QAP.

There may also be other traffic streams where it may not be possible to control the access efficiency factor, e.g. traffic from non-QoS enabled nodes and nodes not associated with the QAP in question. The RRC includes this traffic in its calculations of the new EDCA parameters.

There are two main parts to the RRC: The first part calculates the target values for the access efficiency factors {AEF}TAC required to satisfy the minimum capacity requirements for the ACs, i.e. {Cmin}AC. The second part uses the measurements of {AEF}AC produced by the WRM application to calculate the new values for the EDCA parameters for each AC, i.e. {AIFSN, ECWmin, ECWmax}AC.

Radio Resource Conroller


Example using the WRC Application

Consider the following scenario where we wish to transmit separate traffic streams to two wireless clients. One client receives a video stream, while the other client receives a data stream. The video and data traffic are generated by a video server and traffic generator located on the wired network.

A demo of this test scenario may be downloaded from our software download page.

The QAP used here is a Cisco Aironet 1200 which has been flashed with the firmware version IOS 12.3(8) JA which allows us to access the IEEE 802.11e/WMM capability of the device. The QAP has been configured with a QoS policy where the Differentiated Services Code Point (DSCP) values in the IP header are used to apply a particular Class of Service (CoS). Each CoS is then mapped to a particular AC. So, by appropriately setting the DCSP field, we can determine which AC queue a packet is sent to for transmission.

In the case of the video stream its packets are tagged with a DSCP value so that its packets are sent to the AC_VI queue of the QAP. Similarly, the data stream packets are tagged so that they are sent to the AC_BE queue.

We have determined that the minimum bandwidth requirement for the video stream is 0.25, while for the data stream the minimum bandwidth requirement is 0.05. So, we have

The particular scenario used is this example involves two cycles of ramping the load of the data stream from zero to saturation. In the first cycle, the RRC function is switched off and one can observe how the rising load reduces the capacity of the video stream. In the second cycle the RRC function is switched on and the EDCA parameter settings are adaptively tuned to protect the capacity of the video stream.

Radio Resource Controller Testbed

The first step is to configure the WRC application which involves specifying the WLAN channel, in this case channel 1. Other settings include specifying the WLAN adapter used (in this case the AirPcap USB adapter from CACE Technologies, see www.cacetech.com) and the measurement interval (i.e. every second the EDCA settings will be updated).

RRC config

The second step involves specifying the minimum capacity settings for each of the ACs. In the top panel we first specify the MAC address and SSID of the QAP. This is facilitated by the Search function which allows the user to select the QAP from the list in the bottom panel.

RRC Capacities Setup Dialog

In the second panel, the Minimum Capacities are entered for each of the ACs. The Total Capacity is also calculated. If the user attempts to exceed a Total Capacity of 1.0, an error message pops up and advises the user to re-enter the capacities. This panel also allows the user to specify what set of EDCA settings to use when the WRC application is first launched. The user may select between the default 802.11b and 802.11e settings that are specified in the standard.

The Filtering panel allows a user to apply smoothing to the displayed data.

The third step involves running the WRC application. There are two main modes of operation - Monitor and Control. In the Monitor mode, the application simply monitors the capacity of the network and does not make any adjustments to the EDCA parameters, i.e. the RRC function is switched off. In the Control mode, the RRC function is switched on and the EDCA parameters are adaptively tuned in response to changing network load conditions (in order to satisfy the minimum capacity requirements specified for each of the ACs).

RRC Capacities Screen

The Capacity panel shows the variation in the minimum capacities experienced for each of the ACs. The non RRC is the minimum capacity available to all the other stations that are not associated with the QAP or that cannot be controlled (i.e. legacy non-QSTAs).

Selecting the AC_BE panel shows the performance of the AC_BE queue of the QAP. The upper graph shows the variation in Cmin(AC_BE) with time (i.e. the green trace). The two cycles of the ramped data load can be clearly seen here (i.e. the red trace). The dashed horizontal line corresponds to the Cmin(AC_BE) = 0.05 setting specified for this AC.

It can be seen that Cmin(AC_BE) < 0.05 over the first cycle of ramping the load (where the RRC function is switched off). However over the second cycle, where the RRC function is switched on, we have Cmin(AC_BE) > 0.05.

RRC Best Effort Capacity Screen

The second graph shows the tuning action of the RRC function in tuning the Access Efficiency Factor (AEF) via adjustments to the EDCA parameters. The purple trace shows the target AEF value calculated for the AC_BE class while the blue trace shows its measured or actual value. Over the first cycle of the ramped load, the target AEF value remains fixed as the RRC function has been switched off. However, in the second cycle with the RRC function switched on, the EDCA parameters are adaptively tuned to ensure that the actual AEF value closely tracks the calculated target AEF value.

By selecting the AC_VI panel, the effects of the tuning actions can be seen on the AC_VI class. In the upper graph, the variation in the minimum capacity can be seen. Clearly for the first cycle of the ramped AC_BE load the minimum capacity Cmin(AC_VI) < 0.25 which fails to meet the specified requirement. However, when the RRC function is switched on, the minimum capacity Cmin(AC_VI) > 0.25. Also, since Cmin(AC_VI) > Load the stream will always have a sufficient availability of bandwidth to satisfy its requirements.

RRC Video Capacity Screen

The tuning action of the RRC function can be in the lower graph where the EDCA parameters are adaptively tuned to ensure that the actual AEF values now track the calculated target AEF values.

Finally, the adjustments made to the EDCA parameters can be observed by clicking on the EDCA panel. The upper graph shows the variation in the value of the AIFSN parameter for each of the ACs, while the lower graph shows the variation in the ECWmin parameter.

RRC EDCA Screen

Over the first cycle of the ramped AC_BE load, the EDCA parameters remain unchanged as the RRC function is switched off (the default 802.11e settings are used here). However for the second cycle, with the RRC function switched on, the adjustments to the EDCA parameters can be clearly seen.

This graph clearly shows the need to adaptively tune the EDCA parameters in response to a changing network load. Furthermore, all the EDCA parameters need to be tuned together, it is not possible to tune the EDCA parameters for any one AC in isolation from the others.

The most significant benefit of this application is that the user does not have to worry about how to set up the EDCA parameters - the RRC algorithm takes care of this. The only input required from the user is to specify the minimum capacity requirements for each of the AC classes.

Specify the minimum working bandwidth requirement and walk away

You can download a demo version of the WRC application that will allow you to playback a number of trace files including the example shown here. The download also includes a guide to using the WRC application.

Download the WRC_demo

Benefits of the WRC Appplication

The main benefit of the WRC is that it greatly simplifies the task of implementing radio resource management on IEEE 802.11e/WMM WLANs. One the major obstacles hindering the widespread use of the IEEE 802.11e QoS facility is a poor understanding of how to configure the EDCA parameter settings. Firstly, there are too many of them, three for each of the four access categories which makes a total of 12 parameters that need to be set. The other major problem associated with setting the EDCA parameter values is that it is not possible to tune them individually for each access category in isolation from the other access categories. Also, the default values specified in the standard are of limited use since their effectiveness or otherwise is to a large extent determined by the nature of the network load. All of which makes it extremely difficult to implement effective radio resource management on IEEE 802.11e/WMM networks.

The WRC removes all these obstacles as the only input required from the user is to specify a set of minimum capacities (i.e. bandwidths) for each of the access categories {Cmin}AC. The user avoids having to deal with the EDCA parameter settings. Moreover, the user never has to worry about the changing nature of the network load, since the WLAN RRC automatically makes the necessary adjustments to the EDCA parameter settings in order to preserve the minimum capacities allocated to each of the access categories.

In summary the main advantages of the WLAN RRC are:

Or in other words:

Download the WRC_demo