Note that the H.323 specification is oriented towards the use of “stupid” terminals, and all management functions and services are performed on central servers. At the first stages of the development of VoIP systems, this feature was perceived quite naturally, because of they, in fact, were the successors of conventional telephone services. But you will agree that in the Internet age such architecture already seems archaic. After all, there is no place for such a variety of opportunities to increase the functions and services that the World Wide Web provides today. And as the time dictates a new model for building VoIP systems, based on the open protocol Session Initiation Protocol (SIP) and Java.
The authorship of the SIP specification belongs to the Internet Engineering Task Force (IETF).
This protocol serves not only for organizing a voice connection in IP networks but also for data transfer and multimedia. SIP is a level protocol applications and is based on UDP, but can also use TCP. SIP, like H.323, refers to the class peer-to-peer, but unlike the latter, its basic structure are intelligent devices that perform functions and clients (User Agent Client – UAC), and servers (User Agent Server – UAS). UAC serves to initiate a call, whereas UAS is a program that works with SIP requests. To the number of servers SIP also refers to a Location Server – an analog of a DNS server.
Its purpose is to receive IP-address of the subscriber by his name, represented in a variety of ways. Exist also proxy servers and redirect servers (Redirect Server). The last redirects received calls to new addresses stored in their cards. And the fourth type – Register server, which allows you to process proxy requests or server reassignment so that they know the new location of the user.
Thus, SIP pays much attention to work with addresses. And this is not accidental. The fact is that each user in SIP is characterized by a unique identifier, represented in any convenient form for application. It can be built on the basis of its e-mail address and based on the phone number. For example, SIP: [email protected] or SIP: [email protected] And both of these spellings are equivalent.
As you can see, these addresses are very similar to HTTP URLs, only SIP is specified in them instead of the HTTP protocol. The advantage of this scheme is that there is no need for a new directory structure, and these addresses will be handled by existing IP servers. Yes, and users will be able to apply for their calls as email addresses and URLs, and ordinary phone numbers.
The main features of the protocol are its openness and simplicity. And this, according to experts, will allow filling it with new services and applications. As for simplicity, it is enough to say that all messages used in SIP are text-based and support the embedding of any data types. Therefore, a voice connection can be accompanied by the exchange of data and applications. So, the conversation over the SIP protocol is freely supplemented by the transfer of one subscriber to another, for example, an electronic business card, digital photos or an MP3 file.
The above advantages of SIP look very attractive, but still, the most important feature of this architecture is the use of intelligent devices in it that can perform a variety of applications. So, at a new round of its history, VoIP technology again returned to the same multimedia PCs with which it began its journey. But according to the plan of the developers, it does not have to be personal computers, the blessing that today it is no longer a problem to give any kind of computing power, including a phone.
In this regard, the high hopes of the developers of the SIP protocol are assigned to support in Java clients and interaction with various Web services. Among the next generation applications that fill this architecture with life, in particular, they are called Instant Messaging (IM) services and locating, as well as a variety of programs of the WTI class (Web-Telephony Integration). Possibilities of the latter include, for example, booking tickets and searching for the best deals in electronic stores.
Indeed, today it is still difficult to predict what exactly SIP will bring to openness and its ability to interact, which once became the key to the success of the Internet. However, all observers believe that the future of VoIP is precisely the development according to the SIP specification. True, now we are still at the initial stage. Although all the leading manufacturers have announced support for this specification in their products, there are as yet no major implementations in Europe. Therefore, it looks impressive that Microsoft has already included SIP in its new Windows XP operating system. Thus, Messenger, which replaced the Net Meeting and is intended for video conferencing organizations, is based on the SIP protocol. Therefore, we can expect that such an example will force the entire industry to move more quickly to SIP rails and in the field of IP telephony.