Quality of Service in Wireless Networks – WMM and WMM-SA
This article introduces the QoS mechanisms followed by a wireless network like WMM(Wireless Multi-Media) and WMM-SA(Wireless Multi-Media Scheduled Access) which are both standards based and help wireless networks to reliably enable real time and latency sensitive applications like voice and video by prioritizing them and transmitting them ahead of the normal data traffic like email, etc.
This is an IEEE standard (Subset of IEEE 802.11e) for QoS – Quality of Service in wireless networks. So, why is it so important, you may ask. It is, because it enables real time applications like voice and video on the wireless network by prioritizing those packets at the client level, Access Point level and Controller level – for both upload and download of wireless packets.
In a wireless network, the latency is high as the access points are a shared medium of connectivity. At any given point of time, only one computer/laptop can connect to the access point but the access point claims to simultaneously connect 15-20 clients (for a decent performance). How is that possible? Simple, it keeps switching between the clients, but fast enough that we feel that all the PC’s are connected simultaneously. So, which clients can associate to it, how long and which channel they associate to, is decided based on a number of factors.
Generally, IEEE 802.11 compliant products implement the DCF – Distributed Coordination Function. So, the clients using DCF would monitor whether someone else is transmitting. If no one is, then it will transmit, otherwise it will wait for a prescribed period and monitor again. While this avoids collision, it is vulnerable when the network load is high. That is, when some one is downloading a huge file, it might slow down the access time for real time applications like voice and video.
So, IEEE decided to implement a standard protocol (WMM) for all the Wi-Fi devices (Helps in multi vendor scenario where the different vendors certify their products for this standards with the Wi-Fi alliance) which would ensure that the real time applications like voice and video are processed before the other non-critical network traffic like mail traffic, etc. The IEEE 802.11e Enhanced Distributed Channel Access (EDCA) uses 802.1d user priority to classify traffic in to four categories:
¤ Voice (With highest priority of 7 or 6)
¤ Video (With next higher priority of 5 or 4)
¤ Best effort (3 or 0, for latency sensitive multimedia applications)
¤ Background (2 or 1 for batch data transfer like email)
These tags are generally set by the applications (like VOIP or Video application) and sent to the access point. When they reach the controller, the controller can either allow the respective tags to remain as such and process according to the priority, or change the tags (for applications which are non-critical but still want to be processed with the highest priority). If the data packets are not tagged by the applications, some controllers can identify the application and tag them accordingly.
WMM prioritizes both uplink and downlink traffic (If all the devices in the chain are WMM certified). There are also some proprietary protocols and methodologies to implement priority, but in a multi-vendor network, it is advisable to go according to the standard. Though it may seem quite normal, the WMM based priority queuing is very critical to enable real time applications in the wireless network. Some wireless controllers also help restrict the maximum bandwidth utilized per client/ group of clients so that the wireless network can perform optimally, especially for real time applications.
Wireless Multi-Media Scheduled Access (WMM-SA):
There is one more method of allocating the airtime connectivity to stations called the PCF (Point Coordination Function). In this, the controller/AP gives access to all the stations for equal durations. While this ensures equal sharing, it fails to address the priority queuing which is required for real time applications.
So, a standard called HCCA (HCF Controlled Channel Access) also called WMM-SA, implements parameterized QOS in such scenarios. Instead of giving equal access to all stations, it polls the stations as to what application and priority level data that it is transmitting. Based on this information obtained from all the stations, the Controller/Access Point decides how much time to give to which station and channel allotment, instead of the stations deciding for themselves. This technique requires the stations to know a lot about the application that it is transmitting, but nevertheless offers more control over real time applications. It is good in handling multiple high density voice queues and video streaming applications also as the data rate/access can be controlled. But this standard has not been adopted on a large scale by the industry, perhaps because of the complexity it introduces.
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