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Special Focus
January 2003

Welcome To The SIP Spot

Presented By:

The SIP Center is a portal for the commercial development of the Session Initiation Protocol. Serving both the SIP community and the wider industry, the SIP Center offers comprehensive technical and market resources as well as an environment for the testing of SIP implementations. Visit The SIP Center online at www.sipcenter.com.

Every month, in this space, the editors of Internet Telephony will bring you the latest commentary, views, and news you can use from the world of the Session Initiation Protocol, in the hopes of generating a greater understanding of this important standard. This month we have invited industry experts to share their opinions, guided by a simple set of questions regarding the significance of SIP:

  • Why is SIP so important?
  • What is the value of SIP when it comes to VoIP?
  • What impact will SIP have on the future of communications?
If you wish to comment, congratulate, or carp, or in any way contribute to the conversation, please drop a line to the Editor at ggalitzine@tmcnet.com, and reference The SIP Spot in the subject line. If you have a technical question, feel free to ask, and we will attempt to have our industry experts answer it for you. We look forward to your feedback.

SIP promotes voice (and multimedia) services from being a stovepipe application to a fully integrated part of the Internet architecture and an equal participant to other core Internet applications, such as Web and e-mail. From a user perspective, it adds synchronous interaction to the semi-synchronous (IM) and asynchronous (e-mail) modes available today.

In the instant messaging space, it offers a capable infrastructure that might help to finally achieve interoperability across IM users.

Also, with the adoption by 3G, it has the potential to overcome the division of the telecom world into landline and mobile services. If things go well, users no longer have to be conscious of the type of wire (or the lack thereof) they use to communicate, different numbers and different services, and so on.
It provides a uniform infrastructure for services that is open to edge-based innovation as well as network-provided services. SIP is designed to offer as much information as possible to the end application, allowing it to make appropriate decisions. The traditional division between network-to-user and network-to-network protocols disappears. In a sense, every end user or end user organization gets access to the same capabilities as a carrier peering via SS7.

As for SIPs impact on the future of communications, it fills (the) two major missing pieces in the service spectrum for the Internet, namely low-latency messaging and event notification as well as session setup.
Henning Schulzrinne, Columbia University


Unless youve been vacationing in Antarctica for the last year and a half, you have undoubtedly noticed the slowdown in the market, and in telecom specifically. These difficult times have forced companies to scrutinize everything that they do, so that money is spent only on the things that are truly important. The Session Initiation Protocol (SIP) has been a familiar technology to those in the VoIP industry. But, is it still something thats important for struggling telecom vendors and operators to invest in?

The answer is definitely, yes.

At its most basic level, using SIP saves money. Its a technology, which is widely supported amongst vendors, with good interoperability and deployment results. It is always cheaper to roll out a new product or service using a technology where there are plenty of vendors that you can integrate with, OEM products available to cut development costs, testing tools to reduce development cycles, and consultants that can speed up design. Indeed, there is a huge community of intellectual talent working on advancing SIP, defining new features and capabilities all the time. When you invest in SIP, this community becomes your R&D laboratory, and you can reap the benefits of what they produce.

That of course, leads to the second benefit of SIP -- a story for new services, and therefore, new revenue. In this area, SIP is the unquestioned champion. Presence, instant messaging, Web integration, and multi-modal applications are all among the enhanced capabilities SIP can offer. New innovations continue to happen, and the scope of the new services SIP can offer continues to grow.

Even if these new capabilities arent important to you today, they may be tomorrow. Thats the double-win with SIP. It provides a sound, safe choice for solving basic telecommunications problems, and leaves the door open for you to build on that choice, adding new capabilities as soon as you are ready.

The net result is that SIP is a safe investment. In a tough market, nothing is more important than that.
Jonathan Rosenberg, Chief Scientist, dynamicsoft, Inc.; Co-author of the original SIP specification


Why is SIP important? In a nutshell, SIP is the protocol that will transform VoIP into MoIP (or Multimedia over IP). Think of it as expanding the horizon of a one-dimensional entity -- that of transporting voice over the Internet into a three-dimensional scope of being able to transfer voice, video, and data (the three dimensions of communication).

SIP technically is very attractive to developers, deployers, and business heads, as well as consumers. For developers it offers ease of use and programmability. For deployers, its flexibility offers infrastructure reuse and ease of deployment of new services without overhauls. For business heads, its likeliness to the WWW protocol HTTP offers possibilities of new revenue models combing those of the Web and that of telecom (e.g., ASP model for telecom services) as well as reducing OpEx considerably. And for consumers, its simplicity accounts for less expensive devices and its feature expandability means customers can drive service rollouts rather than switches.

And finally, technology itself does not make a protocol succeed. SIP has gained immense market acceptance and has been adopted as a protocol of choice for wireline, wireless (3G), Instant Messaging, and presence networks to name just a few. Using one protocol across all communication networks of the world means converged services -- anytime, anywhere!
Priya Rattan, Hughes Software Systems Ltd.


SIP and the capabilities that SIP encompasses are becoming the basis of a whole new world of communications.

SIP allows the telecommunications business model to change to become an Internet business model. How many features do you use on your PSTN-connected phone today? Not many, Ill bet. Last number redial? Yes. Caller ID? Probably. Voice mail retrieval? Likely. On the other hand what did you do on the Internet? Read e-mail, surfed the net, bought products, played games, corresponded via Instant Messaging/chat, watched a video, listened to music? All of these.

SIP integrates voice with all of these Internet type features and by adding presence makes for a powerful communications capability.
Because of SIP when you play an Internet game you can actually talk to your opponents online or you can have a private chat with your own team. By looking at a buddy list you can see whether others are available for a game and if they arent online you can send them an IM that will hook up with their cell phone to tell them to get online right away.

As SIP shifts power to the edge of the network in the same way as a PC in a data network, new SIP applications can be quickly developed and tried out without having to worry about testing out a whole switched infrastructure as with the PSTN.

The incumbent business model for telephony will have to change because of SIP as more consumers begin to use it and take advantage of the ability to make phone calls anywhere in the world for free.
Trefor Davies, Director, SIP Forum; Founder, PurplePacket Ltd., SIP consultants


SIP is an incredible, powerful infrastructure protocol for a vast array of voice and video over IP applications from IP telephony and video conferencing to wireless and Instant Messaging.

However, the S in SIP does not stand for simple and this is an important fact sometimes lost on equipment vendors who are going it alone in developing their SIP stack.

Although once touted as an easy to develop and easy to implement protocol, it has become increasingly complex, as both the baseline SIP standard (RFC3261) and its multiple extensions (through complementary IETF Internet-Drafts and RFCs), continue to evolve to cope with the vast requirements of the public Internet, the PSTN, 2.5G and 3G wireless networks, and other types of real-time media environments.

What is not so well known is the fact that continuing standardization changes in the new SIP standard are not necessarily backwards compatible with previous versions of SIP, creating serious problems for equipment vendors who are building their own SIP-based solutions and trying to keep up with the evolving specs of the standard.

While most equipment developers will turn to off-the-shelf stacks from third-party vendors, for a large part because these third parties provide maintenance contracts to address SIP interoperability/backwards compatibility and update their SIP Stacks as the standard evolves, if the equipment developer does choose to continue to go it alone they must be very aware of the challenges ahead as SIP moves from revolutionary to evolutionary.

While we see SIP as having a huge future, building SIP-based products today is a huge task that requires considerable investment not only in initial development but also in constant follow-up on dozens of draft standards, continuous development of the protocol stack to sustain standard compliance, interoperability issues, and testing. This situation will continue and even become more complex as more actual SIP-based deployments are fielded, providing true-life experience with the protocol and leading to subsequent changes.
Amir Zmora, Product Marketing, SIP Products Technology Business Unit, RADVISION


SIP is important because it enables a new era of communications flexibility over IP networking infrastructure. With the shift of real-time communications to the IP domain, entire communications solutions can now be built that are more scalable, flexible, and cost effective than any legacy TDM-based communications solution. By itself, SIP represents an efficient and scalable IP-based signaling protocol to establish a communications session; however, when combined with Web access, database access, real-time media transmission using data streaming protocols, media conversion, and other IP-based protocols and APIs -- highly flexible communications services can be built that target specific market requirements. And because SIP separates signaling interfaces from hardware and media, a variety of communication solutions can be assembled quickly and cost effectively using off-the-shelf hardware and network components.

SIP is a key component of VoIP technology. For voice calls in the pure-IP domain, SIP represents the signaling protocol used to establish a connection between two voice-enabled devices, whether they are SIP-enabled IP phones, adapters for analog phones, or PC-based voice communications software. So SIP is an integral part of end-to-end IP-based voice communications.

SIP will play a vital role in the future of communications. Today, SIP allows communication services to interoperate seamlessly between the PSTN and IP networks. By using PSTN-to-IP media gateways in conjunction with SIP-based service platforms, carriers can cap-and-grow proven revenue generating services onto next generation IP-based services infrastructure. This approach does not require that a carriers entire network get SIP-enabled, but only those components that support this service delivery. In the near term, SIP will enable instant messaging, chat, interactive gaming, voice/video conferencing, and multimodal sessions that combine these various communications capabilities. In the longer term, SIP will play a key part in providing a common voice services infrastructure for both wireline and wireless networks, since it is the designated IP-based signaling protocol for 3G wireless networks.
Ken Osowski Vice President of Product Management/Marketing, Pactolus 

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