Written by: Mårten Ericson, Lotta Voigt and Stefan Wänstedt

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The architecture of the IP Multimedia Subsystem (IMS) defined by the Third Generation Partnership Project (3GPP) provides a solid base for solutions for interconnecting fixed and wireless networks and supplying new services over them, such as voice over IP (VoIP).

Compared with circuit-switched voice, the main end-user benefit of VoIP is the ease with which end users can enrich basic voice service with multimedia services during an ongoing session. Improved end-user flexibility will also probably lead to an increase in traffic and operator revenues. VoIP also has the potential to help operators lower their operating costs by converging their networks into a single all-IP network.
The challenges associated with deploying VoIP over mobile systems include matching the efficiency and quality of circuit-switched voice service while maintaining the inherent flexibility of IP-based services. In the process, one must consider every aspect of the VoIP realization, from compression of IP header overhead to radio access bearer, including quality enhancements and the handling of jitter.

The authors provide an overview of VoIP and its transmission over mobile radio links with available packet-based access (GPRS/EDGE, WCDMA) and dedicated bearers (HSDPA/E-UL). They also describe the capabilities a radio network must have to provide a flexible packet-switched voice service with adequate quality and capacity.

Introduction
The notion of offering VoIP service over mobile networks is not new. The 3GPP has been discussing this topic for years, but has yet to define an actual implementation, in part due to a lack of suitable radio access technology.

Interest in this technology generally falls into one of two main groups or drivers:

• rapid growth in the number of VoIP users on the internet; and
• desire on the part of mobile operators to make their businesses more efficient and profitable.

Compared with circuit-switched voice service, the main end-user advantage of VoIP is the ease with which one can enrich the basic voice service with multimedia services, such as messaging and video during a VoIP session. A continuously growing number of services will influence and possibly change the way people communicate. The addition of presence information, for example, will help determine the form of communication that users employ. Moreover, in the middle of an ongoing session, end users might change from one form of communication to another by adding video to a voice session. Improved end-user flexibility will also probably lead to an increase in traffic and operator revenues.

The advent of VoIP enables established operators to converge their networks into a single all-IP network and to lower their overall operating costs. Similarly, new-comer operators solely need one network for every type of communication (voice and data), which means smaller investments. Being able to deliver every media component over a unified IP infrastructure changes the way one creates and deploys services. Indeed, it changes the very nature of the IP networks themselves.

One price to pay for having this vastly more flexible service is the requirement to transfer IP headers all the way to the recipient (receiving client), which results in a larger total bit rate. Making VoIP over mobile systems into an efficient service with the quality and coverage end users have come to expect from circuit-switched voice service is thus a challenging task. Doing so entails more than merely optimizing radio network performance. In fact, reaching competitive efficiency affects every node in the associated chain.

One other challenge is the prioritization of media flows to guarantee a quality presentation of real-time media. The ability to differentiate between media flows allows operators to provide, guarantee, and charge for different kinds of services.

The introduction of high-speed downlink packet access (HSDPA) and the enhanced uplink (E-UL) will greatly increase transport capabilities in terms of capacity, quality and flexibility. These enhancements of the radio interface are a prerequisite for a resource-efficient deployment of flexible, real-time IP services.