Packet Based Telephony Boˇ stjan Vlaoviˇ c, Zmago Brezoˇ cnik Faculty of Electrical Engineering and Computer Science, University of Maribor Smetanova ul. 17, SI-2000 Maribor, Slovenia e-mail: bostjan.vlaovic,brezocnik @uni-mb.si Abstract This paper describes methods used to place telephone calls over packet-based networks with emphasis on Inter- net point-to-point communication. Two approaches will be described — H.323 and SIP. The first is ITU standard and is already widely deployed by commercial vendors. The second, SIP, was developed by IETF activists and rep- resents “Internet approach” to telephony. First quick overview of reasons for growing importance of IP telephony will be given. It will be followed by quick introduction to voice transmission over packet networks and overview of the existing standards. In the second part of the paper short example of H.323 and SIP call setup will be given. 1 Introduction Since the initial deployment of the Internet, the architec- ture of the network has been changing. It has evolved in response to technological progress, experimentation, ex- plosive growth, and new services. For quite some time a dream of many developers and users is to use multi- media applications over the Internet. One of the first ob- jectives, to transfer voice over Internet Protocol (IP) in real-time, is already available in many commercial and freely available products. First successful VoIP (Voice over Internet Protocol) connection was made by Informa- tional Sciences Institute (University of Southern Califor- nia) and Lincoln’s Laboratory (Massachusetts Institute of Technology) in the late 1974. In 1976 first RFC (Request For Comment), RFC 741 (Specifications for the network voice protocol), was published. Due to various reasons work on VoIP stagnated till the 1990. Vocaltec’s Inter- net Phone was a break through in VoIP software develop- ment and a real motivation for other developers and gen- eral public. Nowadays most telecommunication equipment devel- opers have formed their own vision of new technology deployment. German giant Siemens (SURPASS), Cisco Systems (Architecture for Voice, Video and Integrated Data), Lucent, Telcordia, Nortel, Alcatel, Ericsson, and Iskratel are all actively involved in IP telephony develop- ment. Is the market ready for global IP telephony? Are great expectations justifiable or is all the hype produced with active marketing and Internet revolution? Overview of Internet growth in the past and assumptions for the future can help us in the search for the answer. In the year 1950 world community witnessed first trans-atlantic telephone call, in 1960 digital switching be- came popular, in 1970 we started to program telephone exchanges and software became its expedition to become the most important part of modern telephone exchange. In 1980 Common Channel Signalling System number 7 (SS7) was introduced and made new services, like toll- free numbers, possible. It introduced packet-based trans- port of Call Control to traditional CSN (Circuit Switched Network). With fast deployment of Internet packet based networks became widely available and it was a dream of many to use them for all their daily voice communication. Next, importance of IP availability for different VoIP scenarios will be discussed. It will be followed by ex- planation of packet-based transfer of voice and short ex- ample of its usage. In conclusion our views for the close future development topics will be presented. 2 IP availability For VoIP IP availability is essential. There are many uses of the new technology. If VoIP is not used by end users, IP availability does not present a serious problem. The most frequent use in current telecommunication market is long distance transport of voice streams using packet technol- ogy. It enables better exploitation of existing infrastruc- ture, thus considerate cost savings. Most such connec- tions use private IP network which guarantees satisfactory QoS (Quality of Service) provision. Nowadays Internet is not ready to be used for toll quality voice communica- tions. For the global usage of VoIP — which could be com- pared to traditional CSN — all regular users would have to have VoIP-enabled terminal. VoIP terminals come in different forms and shapes. They can be specially de- signed telephones connected to packet network (already available on the market) or properly equipped comput- ers. Most IP telephones are vendor specific. They de- pend on special software and sometimes even use their own non-standard call control protocols. However, all of them also support standard protocols so they can inter- act at least with some computer programs. There is a lot of activity in CTI (Computer Telephony Integration) and VoIP in Open Source community, too. In the year 2001, great freely available client and server programs can be