What is QoS?
In a general context, quality of service is a set of methods and processes a service-based organization implements to maintain a specific level of quality. In the context of networking, Quality of Service (QoS) refers to a combination of mechanisms that cooperatively provide a specific quality level to application traffic crossing a network or multiple, disparate networks. Implementing QoS means combining a set of IETF-defined technologies designed to alleviate the problems caused by shared network resources and finite bandwidth.
QoS provides two distinct benefits:
A mechanism for applications to request service quality parameters, such as low network delay.
Higher levels of administrative control over congested subnet bandwidth resources.
Implementing QoS enables administrators to make the most efficient use of subnet bandwidth when deploying resource-intensive applications. A QoS-enabled network provides guarantees for sufficient resources, giving a congested, shared network segment the level of service approaching that of a private network. Different classes of applications have varying degrees of tolerance for delay in network throughput. A QoS guarantee ensures the ability of an application to transmit data in an acceptable way, in an acceptable time frame so that the transmission is not delayed, distorted, or lost.
To uphold such guarantees, QoS requires cooperation from the sending and receiving hosts (end nodes), the link layer (OSI model layer 2) devices (switches), the network layer (OSI model layer 3) devices (routers), and any wide area network (WAN) links in between. Without QoS, each of these network devices treat all data equally and provide service on a first-come, first-served basis. In addition, for an application to make use of QoS it must have some level of QoS awareness, so that it can request bandwidth and other resources from the network.
The efficient use and allocation of bandwidth is critical for productivity. Real-time applications, media-rich applications, and Enterprise Resource Planning applications require a large amount of uninterrupted bandwidth for transmission to be successful, and therefore can strain existing network resources. When traffic is heavy, overall performance degrades and results in traffic delay (such as latency and jitter) and packet loss. This degradation causes problems with video conferencing, real-time audio, and interactive communication, causing distortion of voices and images. Because media-rich applications use large quantities of bandwidth, traditional mission-critical applications suffer from the lack of available resources. QoS provides a delivery system for network traffic that guarantees limited delays and data loss.
It is important to realize that QoS cannot create bandwidth; it can only efficiently partition bandwidth based on different parameters.