The OpenPhone Project—Internet Telephony for Everyone!
We envision a basic application that provides a framework for Internet telephony using plug-in modules to accomplish the various tasks to be performed. For example, one plug-in module would provide the signaling tasks, and another would provide the audio tasks (including use of RTP and jitter buffers). We envision an H.323 module, a SIP module and an MGCP module—the user could select which one to use based on the interoperability requirements. New modules could be plugged in as needed to evaluate different RTP/jitter buffer techniques. As improvements are made in signaling or audio-transport modules, all the user has to do is drop in the new module.
All that is needed to make this approach viable is a common API for applications to use to perform basic high-level functions. The modules would all provide those API functions; the appropriate module would be used to provide the actual functionality. Since many people refer to the signaling and audio code as a “stack”, we call this the Stack Adaption Layer (SAL). The SAL is a commonly defined and adopted API that will allow the application developer to focus on the functionality of the program, and the stack developers to focus on the detailed lower-level implementation.
To extend the concept down a layer and provide platform and hardware independence, we envision a Hardware Adaption Layer (HAL) that provides a set of common functions for controlling and using the hardware. This allows the signaling/audio stacks and the SAL to work seamlessly on top of the HAL code, regardless of which Internet telephony card is in use at the hardware level. This approach will allow us to realize the goal of true cross-platform multi-vendor interoperability.
Design specifications for the SAL and HAL layers are in active development as of this writing, and by the time of publication there should be several white papers and some reference code available on our web site. We encourage active participation and have started a mailing list devoted to the project. You can join this majordomo-hosted list by sending a “subscribe” message to email@example.com.
Internet telephony has many intricate pieces that work together to make it function well. There are programs available now that work—but not at a level that is truly useful. Most of these implementations fall short because they don't use modern compressed-codecs, or because they don't use something like RTP and good jitter-buffer techniques to control the audio stream. Most also lack a standardized signaling protocol that provides interoperability with other programs. The OpenPhone Project aims to provide a new, highly flexible framework that uses plug-in modules to provide the components discussed above. It will be based on inexpensive, easily available hardware that can be used in normal, commonly available computers. OpenPhone will use modern techniques to provide near-toll-quality calls between any two phone-enabled computers, and hopefully will foster growth and acceptance of Internet Telephony to the point that all computers are phone-capable.
Greg Herlein (firstname.lastname@example.org) has been using Linux since the early 1.0 series kernels. He's built and run systems on remote mountains, on research ships on the high seas and in corporate high-reliability settings. He's now a member of the technical staff at Quicknet Technologies, Inc. in San Francisco and is leading a team of developers to create the next generation of Linux-based IP telephony software.