How QoS Improves Performance
source: http://support.netgear.com/app/answers/detail/a_id/972/kw/qos Quality of Service (QoS) is a feature of routers and switches which prioritizes traffic so that more important traffic can pass first. The result is a performance improvement for critical network traffic. QoS equipment is useful with VoIP phones or in LANs with high volumes of local traffic. Routers FVS338, FVX538, and FVS124G have QoS. They prioritize traffic according to QoS information sent, and also can override the default QoS for selected services. QoS varies by switch, the higher the level switch, the higher network application layer it works with. The number of queues differ, as well as the kind of information used to prioritize. Most unmanaged switches do not use or change QoS. The traffic passes unaltered, except with GS605, GS608, and GS116, which honor priority tags. Smart switches allow setting QoS by port to a “high” or a “normal” queue. In addition, L2 managed switches both set and honor DiffServ packet tagging. In addition, L3 switches allow defining access control lists which recognize traffic types. QoS for NETGEAR products is “best effort”, meaning that bandwidth for high priority services is not 100% reserved, so low priority services cannot be locked out of network access. A key to optimizing networks is to fix any severe bottlenecks. When the bottleneck is a router or a switch, QoS is likely to help. Here are guidelines to decide whether QoS might be useful: The speed of the Internet is lower than LAN speeds a typical user can achieve. Therefore, if the worst bottleneck is your Internet connection — outside of your equipment — QoS will not help it. Bandwidth and traffic going from your LAN to the Internet is usually less than amount coming to your LAN. ISPs give less outgoing bandwidth intentionally, for economy, since most users upload less than they download from the Internet. QoS on routers, therefore, is most useful for traffic when outgoing from your LAN. For incoming traffic, any useful QoS has probably already been applied by your ISP. NETGEAR switches pass traffic so quickly, that in high volume situations, a router to the Internet is swamped with traffic. Adding a QoS switch to work with your existing QoS router will not improve your Internet performance. On the other hand, if your LAN has many heavy users sending traffic within your LAN, then a QoS switch may very well help local performance. Just as a chain always has a “weakest link”, a network always has a bottleneck. Users find some bottlenecks more important than others. If you don’t mind a slow printer connection — then for you it’s not a problem. The bottleneck that QoS solves is a high volume of traffic coming through a router or switch, where some kinds of traffic are always more urgent than other kinds. If you are sending or receiving data from a slow computer or with a slow network adapter, network performance will be affected — sometimes significantly. Read Optimizing a Slow Computer. For the best results, use network analysis tools with QoS. Since NETGEAR routers do not show you traffic’s existing QoS priorities, varying them from the default might take some experimentation; a packet sniffer would help, here. Although intended for expert users, this sniffer (NOT supported by NETGEAR) can easily be installed and run: Ethereal sniffer.