WiFi QoS Support in Windows Vista: WMM

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In Windows Vista, a great deal of effort has gone into making it simple for network applications to take advantage of QoS capable networks. This post focuses on QoS for WiFi networks (both consumer and enterprise) and how the Vista network stack enables differentiated treatment of outgoing traffic. The WiFi Alliance has created a certification for Wireless Multimedia, or WMM, which identifies four access classes (WMM_AC from lowest to highest priority): background, best-effort, video, and voice. In abbreviated form, these classes are BG, BE, VI, and VO. If both the wireless station NIC and access point (AP) support WMM, it is possible to gain differentiated treatment for traffic sent. The good news is that many retail and enterprise APs support this feature, either out of the box, or in the most recent firmware available from the vendor. As for the wireless station adapter, the NIC driver must be WMM capable.

The WMM specification clearly states that traffic must be assigned to these four aforementioned WMM_AC (to receive differentiated treatment) based on either the MSDU 802.1Q UserPriority tag or the DSCP mark in the IP header of a packet. Importantly, from a Windows implementation perspective, the task of assigning outgoing traffic to the appropriate access class is carried out by the wireless NIC driver on the sending station. As such, this driver must be WMM capable. Like wireless APs, the good news is many 802.11g and 802.11a/b/g (and pre-n MIMO) wireless NICs support this feature, either out of the box, or in the most recent firmware available from the vendor.

Now that you have some background, the purpose of this post is to provide details on how (1) vendors writing Native WiFi (NWF) NIC drivers ascertain which access class to assign outgoing traffic to (create WMM capable drivers), and (2) how network sockets-based applications can gain differentiated treatment for traffic sent over the network.

1. In Windows Vista, the net buffer list (NBL) available to NDIS-6 drivers exposes a new field, WMMInfo, in the NDIS_NET_BUFFER_LIST_8021Q_INFO structure. A NWF driver need only examine the WMMInfo field to ascertain the correct WMM_AC. When sockets-based applications call QoS APIs or an enterprise QoS policy is enabled (see section 2 below), a QoS flow is created and maintained by the network packet scheduler (Pacer.sys). If 802.1Q UserPriority and/or DSCP are applied to this flow, Pacer.sys will automatically populate WMMInfo correctly. This means that the NIC driver does not have to incur a per-packet performance hit (an improvement over WinXP) from manually parsing each packet to determine the proper WMM_AC.

2. Windows Vista provides a simple set of APIs for adding QoS to sockets-based applications. These functions can be found in qos2.h. I’ll post a detailed description about Qos2 APIs shortly. Policy-Based QoS is also available, which enables IT administrators to specify Group Policies that indicate which applications/users/groups/etc should receive differentiated treatment from QoS capable routers and WiFi APs. Policy-based QoS can be used to easily enable and manage QoS on enterprise WiFi networks.

I am interested to hear any feedback or questions about the improved support for WMM in Windows Vista.

- Gabe Frost

Comments

  • Anonymous
    June 29, 2006
    My previous post on WiFi QoS (WMM) discussed the four access classes (BG, BE, VI, and VO) available for...

  • Anonymous
    July 21, 2006
    In my previous posts about WMM support in Vista, I described how Windows network applications can indicate...

  • Anonymous
    August 21, 2008
    which built-in Vista apps supporto WMM? Windows Media Center asks for packet marking while streaming TV to a render device. This means that at least the layer 3 DSCP marking is set. If the link to the next hop supports layer 2 tagging (such as WMM or 802.1p), then windows will set it. In this scenario, the hop to the rendering device is also interesting. If the router and device support it (such as Microsoft Xbox 360 Wireless Network Adapter) then the router will do WMM based on the DSCP marking. Through Group Policy mechanisms other applications and connections can be made to have similar behavior. -- Ari and the QoS Team

  • Anonymous
    September 26, 2008
    Hello, I noticed that installing the Responder, through the KB 929120, on XP SP2/3 wireless equipped machines, the WI-FI signal strenght is reported on the net topology map that Vista creates. The version of Rspndr.sys is of course different between XP and Vista (respectively 5.1.26 vs. 6.X) but I think, from what I read, that the Responder is not involved in he selection of the QoS services/functions available. Is not a demonstration of it, but forcing Vista to use the 5.1.26 release of the Rspndr.sys, in fact, doesn't solve the problem, and the WI-Fi signal strenght small display, keeps on be missing on the map. Is there a way to config Vista to behave like XP once LLTD is installed? What follows is a link that shows the map of a simple home network (mine), with some laptops (equipped with Intel Wireless boards) and the described issue. The WI-FI strenght indicators are the ones of the XP machines.... http://stefania.us/blogs/pictures/signal_strenght_display_missing_on_vista.jpg

  • Anonymous
    November 22, 2010
    Alot of good info about what...just no info about how?

  • Anonymous
    May 26, 2011
    Is there a way to get Wifi WMM support on Windows XP