IMS: Internet Age Telephony

IMS is the future architecture for Internet multimedia telephony. Because it is not constrained by legacy telephony, IMS can take full advantage of Internet technology and its environment.

By Denis Attal

Broadband Internet access is becoming ubiquitous. However, telecom operators recognize that today’s broadband subscribers are not simply interested in bandwidth, they are also looking for more and richer “user-centric” services that make life easy. Cable operators are moving quickly to offer services such as voice over IP (VoIP) and video on demand. In parallel, incumbent operators and their larger competitors face a growing threat from fixed-to-mobile substitution, and from smaller operators, such as Skype and Vonage, exploiting VoIP technologies in a simple overlay architecture.

Many of these operators have not had to fulfill universal service obligations such as emergency calls, legal interception, quality of service or lifeline services, and so can generate substantial traffic without a major investment in infrastructure.

To avoid becoming only low-margin “bit-pipe” providers, telecom operators have devised the Internet Protocol Multimedia Subsystem (IMS), their architecture for Internet telephony and multimedia communications, to compete against these newcomers. An important advantage of IMS is that it is independent of the underlying access technology, such as the 3G Radio Network Controller (RNC). Today, most fixed operators are adopting IMS (or the IMS-like Tispan) as the de facto convergence model for fixed, mobile and enterprise telephony. In contrast, competitors such as Microsoft MSN, Yahoo, AOL and Skype are all proposing proprietary Internet telephony solutions. Operators must rapidly push IMS before these proprietary solutions are widely adopted, or they face the prospect of becoming commodity access providers.

What is IMS?

IMS is based on the Voice over Internet Protocols (VoIP) defined by the Internet Society/Internet Engineering Task Force (IETF). Real-time Transport Protocol (RTP) transports the voice/video flows. The sender uses a codec to encode the flow in a format such as G711, MP3, Advanced Audio Coding (AAC), or Motion Picture Experts Group (MPEG). The flow of bytes is divided into packets, each of which is time stamped and then sent to the destination address. The destination receives the RTP messages, puts them in the correct order and plays them with a stable delay after the time stamp.

Session Initiation Protocol (SIP) is used to set up a call between hosts (typically phones) so that people can communicate by voice or video. The caller sends a SIP invite message to the called host with technical parameters for the RTP connection. The called host answers with a ringing message to the caller and starts to ring. SIP can also be used for other purposes, such as instant messaging. SIP messages are structured as plain English text, and carry the information regarding which codec to use as well as the parameters for the RTP connection. So, what do we notice about an RTP/SIP phone call?

• Voice/video telephony is just another Internet application
• There is no circuit: the voice is divided into packets by the phone, and the packets follow independent paths to the destination
• There is no switching, because there is no circuit to switch
• Voice/video can be exchanged directly between phones

How IMS Works

Let’s consider John and Betty, two subscribers of the same operator. John wants to call Betty using his SIP phone. The unresolved question is how John’s phone knows Betty’s phone address, since IP addresses change from one connection to another. The legacy telephone system ensures that a phone has the same fixed or mobile number, although this comes at the price of some additional complexity. However, in the case of Internet telephony, the constant element is the subscriber’s symbolic name, such as “betty@operatorB.com”, while the IP address changes. Each time a phone is connected, it sends its name to a directory, which can then update its name-address table.

John’s phone obtains Betty’s IP phone address by sending a request to the directory, which performs the “address resolution.” But where is the directory located? Microsoft MSN, Yahoo and AOL telephony take a centralized approach, with each using its own worldwide directory. In contrast, Skype distributes free software for the Windows, MacOS, Linux and PocketPC operating systems (with more to come), which works with a decentralized directory. Skype uses the FastTrack peer-to-peer technology as the well known file sharing application KaZaA. Updates and searches are done by cooperation—no operators are involved.

As the above approaches are incompatible, John and Betty must use a common provider. Operators prefer an architecture that shares the responsibility for the directory. Using this approach, each operator maintains a directory system, and mechanisms are put in place to enable a user to call a phone served by another operator. The invite message to the called phone is relayed by the network systems in the caller’s domain and in the called domain in a similar way to e-mail.

Advantages of IMS

Simplicity – RTP/SIP telephony is designed, like other Internet services, for host-to-host communications. IMS adds functions in the networks (i.e. between the hosts) only to make the services easier to use or to add functionality to the services. Examples are address resolution to translate “Betty” into her phone’s IP address, and roaming to enable access from a visited network.

Flexibility – It is easy to add a new application in the form of a SIP message processing function for each new service to be created. For example, call forwarding would be accomplished by “Betty” being replaced by “Betty’s assistant” in a SIP call message.

Integration with other Internet services – As an example, Betty wants to choose the ring tone John will hear when he calls her. In classic telephony, this is quite complex. While the phone call to Betty is in progress, another temporary circuit is set up to an audio announcement machine. When Betty off hooks, the circuit closes and another one is created to Betty’s phone. In the case of IMS, the ringing message that answers John’s phone’s invite message includes the Web address of the sound to be played. This address could equally refer to a photograph of Betty, a movie clip of her, or a Web page to be displayed on John’s phone.

Open standard for Internet telephony – Network operators are proposing to use IMS as an open standard in preference to the proprietary solutions being created by pure-play Internet competitors. The operators are committed to interoperability and to taking advantage of their unique relationship with users to deliver services. For example, the operator knows who has paid for the access, and can therefore certify the identity of a caller. In contrast, anybody can set up an account on MSN or Skype using any name.

Independent of the underlying network technology – IMS is based on the Internet, which can carry IP packets over fixed or mobile networks. IMS implements roaming, and much more in terms of mobility. A public phone can be seen by IMS systems as your phone as long as you identify yourself using a login-password sequence. The only access-specific function the PDF receives from the P-CSCF is information about the call which can be used to optimize the quality of service.

Basis for innovative multimedia services – The IMS architecture is not confined to supporting Internet telephony. It is also the basis for a wide range of new services that enrich a user’s interactions and experience. Because it is Internet based and controls the SIP sessions, IMS can embed the APIs needed to develop and invoke applications through high level APIs, there by enriching SIP multimedia communications. Examples are: “Where is the nearest taxi?” and “What is the best way to contact you?”.

The Future of Internet Multimedia Telephony

It is simple, flexible and can be integrated with other Internet services, such as web browsing and streaming applications. It offers operators the advantage of being independent of the underlying network technology, and providing a base for the development of many innovative multimedia services.

Technically, however, IMS is a disruptive architecture, as it introduces new concepts and new technologies. By opening up IP as the universal way to communicate, the Internet has intensified competition on IP services. Consequently, IMS will face stiff competition from innovative companies such as Microsoft and Skype. IMS is both a challenge and an opportunity as the foundation for the telephony, applications and services businesses over the coming decade.

Denis Attal is Vice President, Convergence Services, in the Chief Technology Office for Alcatel.