The OpenPhone Project—Internet Telephony for Everyone!
In the traditional PSTN world, the phone companies have a private dedicated network to do all the signaling. Every phone system in the world that wants to interoperate with the rest of the world must use the SS7 network and play by its rules. On the Internet, though, there is no separate dedicated network—control and data signals use the same network. How does signaling work, then? It's not very simple. There are several different ways to get the job done, and several different “standards” in place to provide guidance. This is a simplification of the issues and standards. I refer you to the Resources section for places to obtain more detailed information.
Several standards for Internet telephony are currently in use: H.323, SIP and MGCP are the predominant ones in use now. I cannot hope to provide more than a quick introduction to these protocols, but can certainly provide an overview and pointers to where to learn more.
H.323 is a family of protocols established by the International Telecommunications Union (ITU). It is highly complex, but covers most of what one would want to do with audio-visual conferencing or calling over networks. H.323 arose out of the telecommunications world, not from the Internet world. It is a binary protocol that uses ASN.1 notation/encoding for message passing. This protocol is in wide use and presently has the highest level of use. Many commercial packages use H.323. H.323 is so widely deployed that many feel new VoIP applications must support the protocol. However, H.323 is so complicated that interoperability between different implementations is not good. For better or worse, Microsoft's NetMeeting product seems to be a common benchmark for measuring interoperability. Since Microsoft makes NetMeeting available for free, it's easily available, and if a product can interoperate with it, you are assured a wide user base. There are several excellent open-source projects working to provide this protocol to the community—most notably the OpenH323 Project. These folks have a working protocol stack now capable of making a call to a NetMeeting client on a Win32 machine. This is tremendous success, and I hope to see even more improvements as this code is used in more and more projects. The OpenPhone Project will use the OpenH323 libraries and the slimmer “Simple Endpoint Terminal” code that Vovida Networks derived from it. See Resources for more information and places where you can obtain that code.
The Session Initiation Protocol (SIP) is described in RFC-2543. This IETF protocol arose from the Internet community; it has a feel not too different from HTTP and similar protocols. It is a text-based protocol that uses fairly simple commands to get the job done. It is much more oriented towards telephony services, whereas H.323 provides details on full multimedia audio-visual services. SIP is growing in popularity because of its relative simplicity and ease of implementation. A few links to more information on SIP are in the Resources section. At the time of this writing, I am unaware of an open-source implementation of SIP, although there has been much talk about starting such a project. The OpenPhone Project would very much like to see this project, and perhaps we can facilitate getting it started. We will certainly provide a home for it if anyone is interested.
The Media Gateway Control Protocol (MGCP) is a protocol and API for controlling voice gateways from a centralized server or “Call Agent”. The protocol is text-based and relatively straightforward. The complete protocol is described in an IETF draft (see Resources for link information).
This protocol is finding wider acceptance and is thought by many to be superior to H.323. However, it is interoperable with H.323, since the Call Agent can act as an H.323 Gatekeeper. This is probably where the future of VoIP call control is going, and the OpenPhone Project intends to use this protocol as the default wherever possible. An open-source implementation of MGCP is available from Vovida Networks (see Resources), and several commercial versions are available. However, since the specification is not ratified and is changing slightly (but frequently), there is no assurance at this time that two different MGCP implementations will work together. This is an area where open source can make a huge contribution to Internet telephony. Vovida's MGCP implementation is released under the LGPL license, which allows for commercial use without releasing the associated source code for the non-open portions of the application. With polish, such an open-source MGCP could be used in a wide variety of applications and systems, assuring interoperability by virtue of using the same base code for the protocol. The OpenPhone Project will be using the Vovida code for its MGCP control with the hope of extending and enhancing the protocol to stay current with the ratified MGCP protocol.
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