Quality of Service (QoS) is a way of shaping the data traffic on a network so that the finite amount of bandwidth can be utilized most effectively. The idea is to prioritize certain packets of data that are more important, over the less critical data. What is more important is up to the individual user of course, but the typical use case scenarios are to reduce latency in gaming, or to keep HD video playing smoothly. What both of these usage scenarios have in common is that the data for the game or video needs to arrive in a certain order for an ideal experience.
Users often focus on the download and upload speeds of their connections, known as the bandwidth. While this is important, there is also the issue of latency, often synonymous with ping, a measure of how fast your computer through the network can communicate with a website.
For the home user, QoS is generally done at the level of the router. It requires a higher end router, but unfortunately most routers ship with it disabled by default. With QoS properly configured, it can make a significant difference. For example, this will allow stutter free playback of Netflix, while downloading a large Windows update file.
It is also useful for VoIP, which while low bandwidth, is important to prioritize the voice communication for a stable connection without artifacts. While QoS was once a luxury for a network, it has evolved to become a necessity with multiple users, and multiple devices connected to the router, all competing for internet bandwidth, and all desiring a seamless experience.
Without QoS, all the users and their devices get the same level of priority. Therefore the downloads that can wait get the same priority of those that can wait. Routers can implement QoS via two methods. The first method is to have QoS implemented via the device. The router will present a list of connected devices, which can then be prioritized. While this does provide some certain traffic shaping, it is less than ideal as gaming may not always happen on the same device, or the video viewing may shift from a higher priority notebook to a lower priority tablet, which then would force the user to have to redo their QoS settings on a frequent basis with each session to maintain the data priority. D-Link routers often choose this type of QoS method as seen in the screenshot above.
However, it is still limited as the list is limited to a handful of applications and games, and if your app is not listed, it cannot be prioritized other than by the client, which may not need the consistent priority depending on the users on the network, and their devices use of these apps. (As an aside, this Linksys router is really sold for open firmware DD-WRT use, so many users will not use the Linksys factory firmware anyway).
Unfortunately, in the three examples above, it should be clear that QoS has not finished its evolution. Any list provided in the firmware of apps that can receive potential priority will be limited as there are literally millions of potential apps and games that users may wish to prioritize. No list included in the router, no matter how complete at the time the router ships, will ever be complete over the changing landscape of the internet. What would really be ideal would be to prioritize by both device and application, but that the applications could be added by the user. Perhaps a browser plugin, that could be turned on to give that site priority, and a more dynamic list with a hierarchy or more than just priority or no priority. Router manufacturers, are you listening?
At any rate, feel free to discuss your thoughts, opinions and experiences with router QoS below.
Source : http://www.dslreports.com/shownews/Networking-101-Quality-of-Service-QoS-138390981