When comparing different forms of VoIP, one risk comparing “apples and oranges”. Broadly speaking, we can divide VoIP into two main categories. First, the service can be implemented as a faithful copy of circuit switched telephony; in a network with full control over performance and quality. Second, VoIP can be implemented as a standalone application used over the open Internet.
Originally published in NetworkWorld Norway.
3GPP and IMS
3GPP (3rd Generation Partnership Project) has played an important role when VoIP has become a recognised substitute for traditional telephony among telecom operators. 3GPP standardises the mobile technologies 2G, 3G and 4G, and they have done so based on the general IP technology standardised by IETF (Internet Engineering Task Force).
At first 3GPP concentrated on developing mobile networks as an evolving telecommunications architecture, following a vertically integrated model for provision of telephony. As the Internet revolution influenced the telecom market, the focus has shifted more towards IP-based services of various kinds.
IP networks and the Internet are not equivalent concepts. As IP technology was introduced in the mobile architecture, this was done in a way that maintained telecommunications networks’ support for QoS (Quality of Service). They had a clear view to continue provision of telecom services, as opposed to Internet applications, but based on a new IP-based network.
The service platform which was standardised as part of the mobile architecture was named IMS (IP Multimedia Subsystem). IMS is based on SIP (Session Initiation Protocol), the VoIP protocol from IETF, but extended with a comprehensive architecture for QoS. IMS has an “open” interface for service development, but requires a business agreement with the mobile operator. So this is a completely different kind of openness than the one found on the Internet where “everyone” can develop their own services.
VoLTE and RCS
The basic mobile architecture has undergone a tremendous development by 3GPP. Now we are in a phase where LTE (Long Term Evolution) is being adopted, often referred to as 4G despite the fact that it is not “real” 4G. LTE is the first 3GPP architecture that has eliminated the circuit switched domain, appearing as a pure IP network. Therefore there are great expectations for VoIP in this architecture, a functionality called VoLTE (Voice over LTE).
The transition from traditional telephony to VoIP has been going on for a long time. In mobile networks this has taken longer than expected. IMS has been around as a part of the mobile architecture for many years already. Furthermore, VoLTE includes options that could still delay this transition; LTE phones will initially combine LTE with older mobile technologies, allowing telephones to fall back to these older technologies. There is also a quasi-solution that transports traditional telecom protocols encapsulated in IP packets, so-called VoLGA (Voice over LTE via Generic Access).
The telecom industry also promotes advanced VoIP services that can stimulate the transition from traditional telephony and SMS to IP-based “equivalents” called RCS (Rich Communication Services). RCS provides services such as voice and video telephony, presence, instant messages and more, integrated in a unified user client for mobile phones that will provide seamless user experience of multimedia communication.
RCS is based on the IMS platform using SIP and SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions). Thus, the basis of this is IETF protocols, but implemented in an architecture that is intended to replicate the telecom network in the shape of an IP-based multimedia network. RCS is promoted by GSMA (GSM Association) and OMA (Open Mobile Alliance). OMA is the descendant of the WAP Forum, if there are still some who remember WAP.
QoS and Policy Control
RCS seems like an impressive technology, and what is the big deal? What distinguishes this from the applications that are already in use on the Internet? A major difference is that RCS can benefit directly from the mobile network built-in mechanisms for QoS. But it is difficult to predict what will give the best user experience, multimedia services integrated in the mobile architecture or free choice among different applications offered over the Internet.
A well-known characteristic of the Internet is that it is “best effort” and can’t guarantee the quality of the communication. In the mobile architecture, QoS is a key feature across the entire design. The underlying IP network will typically be based on DiffServ (Differentiated Services) and MPLS (Multiprotocol Label Switching), both well-known technologies from IETF supporting traffic management and QoS.
In the LTE architecture, QoS is policed by a function called PCC (Policy and Charging Control). As the name suggests, not unnaturally, management of QoS and charging are two sides of the same coin. PCC controls establishment of user sessions with various performance levels, and charging information is generated based to the capacity used by the different sessions.
Initially, IMS was specified for mobile networks, but in retrospect it has been found very useful extending the scope to include fixed networks, giving a combo solution which is often referred as NGN (Next Generation Networks). This facilitates convergence between fixed and mobile networks (Fixed-Mobile Convergence).
The traditional telcos are operating in a market that is completely changed because of the Internet. This leads to a situation where the business that telecom players envision, is facing strong competition from Internet players. The Internet model is based on decoupling of applications from the network layer, as opposed to the telecom model that relies on the services that are vertically integrated with the network.
Innovative solutions that can be used “over-the-top” without specific facilitation from telecom operators, enables virtually unlimited choices for end users. Internet applications, even real-time applications such as VoIP, work fairly well without the quality architecture of NGN. Congestion control mechanisms regulate traffic load of the Internet, sharing the available capacity between users.
However, users’ choice is not easy. Such innovative solutions in some cases evolve into isolated “islands” that are not compatible with each other. Major players are trying to create their own closed ecosystems consisting of operating systems or app stores for example. On the other hand, some traditional telecom operators introduce OTT solutions to meet the competition, making use of similar means.
The future will show which model is most adaptable. Net neutrality is tasked to ensure that the Internet model can develop freely. Meanwhile, the Norwegian guidelines for net neutrality are balanced, allowing the telecom model to evolve in parallel. This is often referred to as “specialised services”, as opposed to the Internet access service that works as a general electronic communication service.