Aerohive's New Scheduling Algorithm Could Boost 802.11n Performance 10-FoldAerohive's New Scheduling Algorithm Could Boost 802.11n Performance 10-Fold

The WLAN market is still pushing the limits of the technology in some very creative ways.

WLAN switch maker Aerohive has announced a major software upgrade for their operating software that can add a new angle to the WLAN infrastructure decision. Designated HiveOS 3.2, the software will be available immediately and will be a free upgrade for existing Aerohive users.The key element in the new release is a feature called Dynamic Airtime Scheduling, and it takes some interesting twists to improve WLAN throughput. WLANs are a peculiar type of shared media network, where the access point and all of the stations associated with it take turns sending on one shared radio channel. A station sniffs the air to determine if the channel is free and will normally send immediately after a pre-transmission waiting interval. If the channel is busy, the station invokes a random back-off counter that it uses to set a timer; when the time expires the station sends. The random nature of the back-off process helps stations avoid collisions.

The Wi-Fi Alliance has also added a quality of service (QoS) protocol designated Wi-Fi Multimedia (WMM) that provides higher priority access for time sensitive voice and video users; those higher priority transmitters are assigned shorter pre-transmission waiting intervals. Aerohive is taking that a step farther to address a problem we are seeing when higher speed stations share a channel with lower speed stations.

WLAN stations can operate at a range of transmission rates, and select the rate based on the signal-to-noise ratio; stations in closer proximity to the access point or with fewer obstructions in the path typically transmit at higher data rates. However, given that the channel is shared, lower speed stations impact the performance of higher speed stations, as those lower speed stations will take longer to send the same size frame.

Aerohive's Dynamic Airtime Scheduling technique involves a sophisticated weighted round robin queuing system that keeps track of how much time each station is occupying the channel. Wi-Fi certified devices are not allowed to deviate from the basic channel access mechanism, but Aerohive's scheduler works above that, taking advantage of the flow control mechanism that's built into TCP. The sequence and acknowledgement numbers in the TCP header in conjunction with the window size (i.e. the number of characters that can be sent without receiving an acknowledgement) dictate the rate that TCP will push data over the channel.

In a Wi-Fi environment where faster and slower stations are sharing a channel, TCP's flow control divides the available capacity roughly equally among all of the stations regardless of the data rate each is using. What Aerohive does, is push those higher speed users' (e.g. 802.11n users) traffic out first, allowing them to have far better throughput. The interesting thing is that the technique doesn't degrade the performance any further for the lower speed users. In effect, the scheduler makes the channel appear "bigger" to TCP's flow control mechanism in those higher speed stations.

The access point can directly control when frames are sent in the outbound direction, but it cannot directly inhibit inbound transmissions. However, the access point keeps track of the total amount of airtime a station is using both inbound and outbound to assign the weights in the round robin algorithm. Flow controlling the outbound traffic effectively delays the acknowledgements for inbound transmissions so indirectly, the process can controls inbound transmissions as well.

Most importantly, the technique does not impact the performance of time sensitive voice or video traffic if WMM QoS is being used. Voice frames will be assigned strict priority and will consistently be sent with the shorter pre-transmission waiting interval so they are still pushed to the front of the line. Once the voice requirements are served, the Dynamic Airtime Scheduling optimally assigns the remaining network capacity to the data users.

Aerohive has written a white paper describing the technique, and tested it with a Veriwave WLAN test tool. The results showed that the performance for those higher speed stations could increased ten-fold in some cases. As the vast majority of 802.11n stations will be operating on channels shared with legacy 802.11a, b, and g devices, this mechanism could allow users to see the full impact of the higher speed 802.11n radio.

On top of the scheduling, the system also has the ability to define traffic policies, so for example, a network administrator could define a policy that allocated more network capacity to internal users than to visitors, even though they are sharing the same channels.

The big question for buyers is: is this really going to work for me, and if so, to what extent? White papers and lab tests are all well and good, but buying a pig in a poke is not a good way to advance one's career. Fortunately Aerohive's WLAN cooperative control product architecture doesn't require a controller, so it could be possible to test the solution on a small-scale basis prior to making a full commitment. However, it is interesting to see that the WLAN market is still pushing the limits of the technology in some very creative ways.The WLAN market is still pushing the limits of the technology in some very creative ways.