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Wednesday, February 23, 2011

3G Mobile Service Performance: End-to-End QoS concept in 3GPP & 3GPP2

I do recall a few years ago data telecommunication networks caused an important revolution worldwide, allowing quick and easy communication between parties. At that stage, very basic services like file transfer or e-mail were supported, and service requirements were not an issue.
However, one of the most significant achievements along the years has been the support for a richer variety of services and a higher level of service personalization.

The different nature of the services implies particular quality requirements that need to be fulfilled in order to satisfy the end-user expectations. For instance, listening to the radio through the Internet requires a lower delay compared to an e-mail service, while the e-mail service requires higher reliability compared to the radio audio information. This basic service classification is just the starting point for different research areas and the development of a wide set of mechanisms and protocols oriented to classify, differentiate or provide a special treatment to each data flow. These mechanisms are the principles of Quality of Service (QoS) management.

As wireless data services makes use of both the cellular networks and the IP-based Internet for providing the service, both set of protocols and functionalities must cooperate to provide a seamless end-to-end QoS. Both IETF and 3GPP/3GPP2 are contributing to the standardization of QoS-oriented mechanisms and protocols in IP and cellular networks.

The figure below represents the scope of 3GPP and IETF in the end-to-end path of a mobile network. It can be observed how 3GPP is initially covering the whole wireless network, towards the interfaces to Internet. However, as IP-based Radio Access Networks (RANs) will be developed in the near future, IETF mechanisms and procedures will become more relevant in the wireless side, and the coordination between both systems will be critical.

The gateway interconnecting the mobile network with the external IP network (e.g. Gateway GPRS Support Node, GGSN, in 3GPP architecture) is responsible for the mapping between the cellular QoS parameters and the IP QoS parameters, and plays a very important role in maintaining a consistent service level between both domains.

The concept of QoS has been created and defined in order to provide different treatments to traffic from different services and/or users. QoS is a broad concept that can be decomposed in the QoS provided in the different domains and layers of the system, but that has significance only when considered end-to-end, as the final QoS provided is always determined by the weakest layer

More specifically, QoS mechanisms are responsible for the traffic management and service differentiation from the network point of view, i.e. according to certain quality requirements specified by the application, the network functionalities treat each service according to their own requirements. However, the perception of service quality from end-user point of view is a wider and more subjective issue, also defined as Quality of Experience (QoE). In any case, from the cellular network perspective the best way to try to ensure a good QoE is to provide a good end-to-end QoS. QoS management architecture defined in the different standardization forums such as 3GPP and 3GPP2 have defined the end-to-end QoS as a combination of QoS provisioning in different layers of the subsystem. I demonstrate this common approach as below;

End-to-end QoS is achieved by mapping the end-to-end requirements onto a proper configuration along the different mechanisms available in the \ network. For instance, the RAN includes QoS-aware mechanisms like AC, channel allocation, packet scheduler, power control or handover control that may differentiate among services and/or users. This is illustrated below;

In the core network, there exist many other mechanisms such as congestion control, authorization of resources, RR for real-time services, etc. Additionally, the operator may include any IP-based QoS method within its own intranet as well as a proper mapping between mobile and external networks.

The end-to-end service is defined on top of UMTS/GPRS bearers and external bearers. The UMTS/GPRS bearers follow a bottom-up philosophy, being the upper bearers defined on top of the capacity provided by the lower bearers. As an example, UMTS bearer is compound of radio access bearer and core network bearer; while the radio access bearer is compound of a radio bearer and Iu bearer. Each of the bearers shown in the figures above, have their own QoS mechanism; therefore QoS provisioning can be layered or classified also by QoS mechanisms per bearer, being end-to end QoS the result of the combination of the different QoS mechanisms through the different system bearers.

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