Implementing QoS in a WLAN

Q: Considering that VoIP requires low bit rate, low latency, and QoS-preferred connections, how is it possible for a shared medium, CSMA/CD, OFDM RF technology implementations of 802.11x capable of providing true QoS? Would you agree that frame/cell tagging “over the air” be better referred to as class of service instead of QoS? – Ian, Dallas

The Wizards reply:

Dan Simone, Trapeze Networks

You draw a good distinction between class and quality of service. QoS typically refers to bandwidth guarantees, and wireless technology cannot provide such guarantees. At the core, however, what’s needed for good service to VoIP devices on WLANs is some form of prioritization.

Look for a system that classifies the traffic according to VoIP device or application (typically performed by the WLAN switch in a second-generation enterprise design) and then prioritizes delivery of the traffic in the access point. To achieve good performance, the system will need to support multiple queues per user, always sending all users’ voice packets before any data packets.

Dr. Vaduvur Bharghavan, Meru Networks

Yes, it is possible for a shared medium such as 802.11 to provide QoS connections utilizing mechanisms within the 802.11 standard without requiring proprietary changes to the wireless stations.

I would also agree that frame/cell tagging “over the air” is closer to the class of service concept in which traffic is prioritized based on class but not reserved. Having frame/cell tagging allows the endpoints on each end of the wireless link to prioritize the higher class service above the lower class service. This can assist the traffic classification so that when transmitting packets, the higher priority packets are sent before the lower priority ones.

However in wireless networks, such as 802.11, this is not sufficient to provide QoS since there are many contenders attempting to transmit packets at the same time. Due to this, the frame/cell tagging only allows prioritization of traffic between each endpoint and the access point. This lets the access point schedule downstream traffic across all stations based on the traffic class.

Because there are many senders in the upstream direction, there is no central policy enforcement to ensure that higher traffic class packets are sent first, and scheduling is done across all stations. For upstream traffic, each station can (if it has traffic classification/scheduling implemented) schedule its own upstream traffic to ensure higher priority packets are transmitted first. However, it has no control/knowledge to be able to coordinate between all stations in order to ensure that global traffic scheduling is provided. Therefore, in a wireless medium such as 802.11 it becomes even more important to have reserved QoS due to the low bandwidth and shared medium.

In order to provide reserved QoS there are several components that the system requires. It requires traffic classification in order to identify which packet flows (identified by source/destination IP, port, protocol) are to be treated as QoS as well as to determine the amount of resources that will be used. The resource usage depends not only on the bandwidth requirements of the application but in an 802.11 system also depends on the link speed between the access point and each wireless station, as well as interference from neighboring access points and clients. In addition to the classification, a policy manager is required to account for the resources available and in use by the system, which must adjust as the network changes dynamically in 802.11.

Finally, the traffic must be scheduled, which requires access points to enforce the QoS policy and adjust its behaviour as the network is operating. This requires real-time access to the channel in order to respond to events on the air in sufficient time to adapt to the changes observed so that QoS can be maintained.

Michael Montemurro, Chantry Networks

There is standards work being done in the IEEE to extend LAN-based QoS mechanism to WLAN. The 802.11e Task Group is looking to address this problem with two solutions: Enhanced Distribution Channel Access (EDCA) and Hybrid Coordination Function (HCF). EDCA is an extension of the traditional LAN-based queuing mechanisms, where packets are tagged and transmitted using prioritized channel access functions. HCF is a polled access mechanism where different traffic streams are given a dedicated time slot for communications. Both mechanisms take advantage of admission control signaling which preserve QoS streams between the network and the wireless user.