The “Certified for Windows Vista” logo is comprised of requirements for myriad networking features and scenarios, one of which being network Quality of Service (QoS). What is QoS and why is it important you ask? QoS is an overloaded term, but in short, it is a capability which enables priority handling of differentiated traffic such as voice, video, and typical web surfing. Mathias did a good job describing QoS at a high level in his post about QoS fundamentals. QoS is becoming increasingly important for no-new-wires home networks like wireless because they are shared network mediums. A shared network means that all hosts connected to the network share the available bandwidth as equally as possible. If your wireless network has 16Mbps available, that means total – each device shares that 16Mbps. Each device does not get its own dedicated 16Mbps. In a network where each device shares the available bandwidth, it’s important to differentiate the more important traffic so it receives priority; otherwise your large Internet download or multi-megapixel photo upload may cause your video stream to glitch. Nobody likes a poor experience; especially our families and significant others who don’t know (and shouldn’t have to know) how to fix the problem. A router reboot should never be an acceptable answer.

In order to enable this differentiation, your wireless router has to support QoS. The logo requirements not only ensure that products support QoS, but that common implementation bugs which break connectivity or hinder performance are not present. Each QoS requirement is validated using both TCP and UDP, and from both wireless-to-wired and wired-to-wireless directions (there are currently no requirements for traffic destined for the Internet). Without going into great detail, the QoS requirements ensure:

  1. End-to-end connectivity is never broken because layer-2 802.1p tags are added to traffic
  2. Traffic priority (indicated by DSCP or 802.1p) is never unexpectedly modified by network devices
  3. Traffic differentiation via the WMM function actually works for each WMM priority, i.e. higher priority traffic receives a greater share (1.5x throughput of a concurrent lower priority stream) of the shared available bandwidth
  4. During network congestion (regardless of concurrent priority traffic), voice traffic is never interrupted

We also ensure a high expectation of network hygiene by validating bridge function behavior, e.g. round trip time (RTT) for packets in congested and non-congested situations is sane, and that available bandwidth estimates do not vary beyond reasonable thresholds.

Devices that carry this logo will work great for your digital home scenarios. For example, both Windows Media Player (library sharing) and Windows Media Center streaming to Xbox 360 in Windows Vista make use of QoS if your network supports it; ensuring a great experience.

:: Gabe Frost