Definition

The advent of Voice over IP (VoIP) has given a new dimension to Internet and opened a host of new possibilities and opportunities for both corporate and public network planners. More and more companies are seeing the value of
transporting voice over IP networks to reduce telephone and facsimile costs.

Adding voice to packet networks requires an understanding of how to deal with system level challenges such as interoperability, packet loss, delay,density, scalability, and reliability. This is because of the real time constraints that come into picture. But then the basic protocols being used at the network and transport layer have remained unchanged. This calls for the definition of new protocols, which can be used in addition with the existing protocols.

Such a protocol should provide the application using them with enough information to conform to the real-time constraints. This paper discusses the significance of Real-time Transport Protocol (RTP) in VoIP applications. A brief introduction to VoIP and then a description of the RTP header are given in sections 1-5.

The actual realisation of the RTP header, packetisation and processing of an RTP packet is discussed section six. Section 7, called 'Realising RTP functionalities', discusses a few problems that occur in a real time environment and how RTP provides information to counter the same. Finally, sample codes that we wrote for realising RTP packetisation, processing and RTP functionalities, written in 'C', for a Linux platform are presented.

Please note that RTP is incomplete without the companion RTP Control Protocol (RTCP), but a detailed description of RTCP is beyond the scope of this paper.

VoIP, or "Voice over Internet Protocol" refers to sending voice and fax phone calls over data networks, particularly the Internet. This technology offers cost savings by making more efficient use of the existing network.

Traditionally, voice and data were carried over separate networks optimized to suit the differing characteristics of voice and data traffic. With advances in technology, it is now possible to carry voice and data over the same networks whilst still catering for the different characteristics required by voice and data.

Voice-over-Internet-Protocol (VOIP) is an emerging technology that allows telephone calls or faxes to be transported over an IP data network. The IP network could be

" A local area network in an office
" A wide area network linking the sites of a large international organization
" A corporate intranet
" The internet
" Any combination of the above

There can be no doubt that IP is here to stay. The explosive growth of the Internet, making IP the predominate networking protocol globally, presents a huge opportunity to dispense with separate voice and data networks and use IP technology for voice traffic as well as data. As voice and data network technologies merge, massive infrastructure cost savings can be made as the need to provide separate networks for voice and data can be eliminated.

Most traditional phone networks use the Public Switched Telephone Network(PSTN), this system employs circuit-switched technology that requires a dedicated voice channel to be assigned to each particular conversation. Messages are sent in analog format over this network.

Today, phone networks are on a migration path to VoIP. A VoIP system employs a packet-switched network, where the voice signal is digitized, compressed and packetized. This compressed digital message no longer requires a voice channel. Instead, a message can be sent across the same data lines that are used for the Intranet or Internet and a dedicated channels is no longer needed. The message can now share bandwidth with other messages in the network.

Normal data traffic is carried between PC's, servers, printers, and other networked devices through a company's worldwide TCP/IP network. Each device on the network has an IP address, which is attached to every packet for routing. Voice-over-IP packets are no different.
Users may use appliances such as Symbol's NetVision phone to talk to other IP phones or desktop PC-based phones located at company sites worldwide, provided that a voice-enabled network is installed at the site. Installation simply involves assigning an IP address to each wireless handset.

VOIP lets you make toll-free long distance voice and fax calls over existing IP data networks instead of the public switched telephone network (PSTN). Today business that implement their own VOIP solution can dramatically cut long distance costs between two or more locations