“QoS is one of the most important issues in networks in general, and particularly so in the Internet and other IP networks.” CodeIdol Labs (2009).
The four pillars of Quality of Service (QoS) are additional components that are added to the existing Internet Protocol (IP) infrastructure to support “perfectly synchronised transfer of packets of different applications” according to Bharadwaj (2005) and Kurose & Ross (2010, p665-669), are: [Traffic Characterisation], [Isolation, Scheduling & Policing], [High Resource Utilisation] & [Call Admission]. Essentially the four pillars ensure that based on the type of network traffic there are sufficient resources available to guarantee a given quality. The type of network application greatly dictates the required quality and therefore which quality pillar, if any, can be relaxed.
The first pillar, traffic characterisation, establishes the quality guarantee required in order to allow the device routing the data to calculate whether there are sufficient resources to meet the quality need. The second pillar, isolation, scheduling and policing, ensures that other network needs do not impinge on guarantees provided. The third pillar, high resources utilisation, allows the use of spare capacity and finally the fourth pillar, call admission, ensures that the data can complete its journey through the reserved route before traffic is allowed to proceed.
Given that QoS offers a level of guarantee that is ensured by the pillars, it is not possible to simply remove a pillar from the system. However it is possible to change the degree at which each pillar operates and affects the system to deepen or heighten the level of the guarantee so that different aspects of QoS support are considered fundamental to the guarantee. For example, if we examine one of the most important Internet uses for QoS, real time media streaming, then it is possible to see where the pillars can be removed (or rather, unused). Traffic characterisation in this respect is generally required as without this a determination of network resources cannot be made and similarly network isolation, scheduling and policing ensures that the media has sufficient resources available, so both are required. In this example the application is not concerned with efficient network resources utilisation (although parts of the network would be) so this pillar can be relaxed and although call admission would be welcomed, it may be acceptable to the application that this guarantee is not necessary and that some congested packets could be lost.
Bharadwaj, P (2005) Quality of Service in the Internet [Online]. Available at http://www.ias.ac.in/resonance/Mar2005/pdf/Mar2005p57-70.pdf (Accessed 13 May 2011).
CodeIdol Labs (2009) IP QoS [Online]. Available at http://codeidol.com/telecommunications/telecommunications-essentials/The-Internet-and-IP-Infrastructures/IP-QoS/ (Accessed 13 May 2011).