Past, Present and Future of IP Telephony Vasco N. G. J. Soares 1,3 , Paulo A. C. S. Neves 1,3 , and Joel J. P. C. Rodrigues 2,3 1 Superior School of Technology, Polytechnic Institute of Castelo Branco, C. Branco, Portugal 2 Department of Informatics, University of Beira Interior, Covilhã, Portugal 3 Institute of Telecommunications, Networks and Multimedia Group, Portugal vasco_g_soares@est.ipcb.pt; pneves@est.ipcb.pt; joel@ubi.pt Abstract Since the late 90's IP telephony, commonly referred to as Voice over IP (VoIP), has been presented as a revolution on communications enabling the possibility to converge historically separated voice and data networks, reducing costs, and integrating voice, data and video on applications. This paper presents a study over the standard VoIP protocols H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and H.248/Megaco. Given the fact that H.323 and SIP are more widespread than the others, we focus our study on them. For each of these protocols we describe and discuss its main capabilities, architecture, stack protocol, and characteristics. We also briefly point their technical limitations. Furthermore, we present the Advanced Multimedia System (AMS) project, a new system that aims to operate on Next Generation Networks (NGN) taking the advantage of its features, and it is viewed as the successor to H.323 and SIP. 1. Introduction Since the late 90’s that terms like Voice over Internet Protocol (VoIP), Internet Telephony and IP Telephony invades the way we communicate over the Internet. These terms may seem equal at first glance, but they encompass some differences. The first term, VoIP, translates into passing phone calls over a packet data network, without bounds to network type or topology. As a result, VoIP allows phone calls from over Local Area Networks (LAN) up to the global Internet. The term Internet Telephony comes from passing calls across the Internet, and it may use VoIP, but also specific proprietary hardware or even computer software. Finally, the term IP Telephony denotes converged data and voice, and uses VoIP technology. Such integration can lead, for instance, to seamless use of both voice and data messaging. VoIP is basically the means to grab audio and video in digital form, divide it in small chunks that can be transferred through the network as packets. After, it reassembles the chunks on the other side in a convenient way so that people having a phone conversation have the idea of a circuit switching ordinary call, as shown in Figure 1. VoIP presents a shift on corporate voice communications where the traditional Private Branch eXchange (PBX) based systems were used to provide internal cost-free communications and sharing of external telephone lines. With VoIP, the PBX gives place to a gateway router and a server on the computer network that controls all calls. Figure 1. Basic VoIP functional architecture If we analyze the beginning of the telephony service back in 1876 when Alexander Graham Bell achieved the first phone call with his assistant, up to now there is not much difference for the end user in terms of voice communication. The main advances are in the way such information is transmitted, the quality of speech, introduction of wireless communication and smaller, faster and service-rich terminals. The first attempt to transmit voice over a packet network came with the Network Voice Protocol (NVP) [1] in the ARPANET. In 1995 a company named International Conference on Communication Theory, Reliability, and Quality of Service 978-0-7695-3190-8/08 $25.00 © 2008 IEEE DOI 10.1109/CTRQ.2008.12 19 International Conference on Communication Theory, Reliability, and Quality of Service 978-0-7695-3190-8/08 $25.00 © 2008 IEEE DOI 10.1109/CTRQ.2008.12 19 International Conference on Communication Theory, Reliability, and Quality of Service 978-0-7695-3190-8/08 $25.00 © 2008 IEEE DOI 10.1109/CTRQ.2008.12 19 International Conference on Communication Theory, Reliability, and Quality of Service 978-0-7695-3190-8/08 $25.00 © 2008 IEEE DOI 10.1109/CTRQ.2008.12 19