The Linux Telephony Kernel API

A year ago, Internet Telephony was a curiosity, and many people thought it would never work for real phone calls. Now, with services like Net2Phone, Deltathree.com and DialPad providing free or extremely low-priced phone calls delivered via the Internet, Voice over IP (VoIP) has reached near-mainstream status. While Linux clients for those services are not yet available, Linux is not being left behind. With the 2.2.14 kernel, Linux has taken a bold lead in the area of computer telephony integration: we have the first modern operating system with a defined kernel-layer application programming interface (API) for telephony support. To sweeten the pot, excellent quality open-source telephony software is already using this API. You can call around the world using Linux and the Internet—and the call is free!

This article will explain the basics of how the telephony device drivers are integrated into the kernel in a way suitable for creating a common API across vendors. Then we'll discuss the basic ideas behind the design and function of the API, and how data and event information are dealt with separately. Finally, we'll discuss how telephony events (like ringing or picking up a handset) are handled in a process called “asynchronous event notification”.

There are many people who've asked why we needed a new telephony support API for the kernel. Even Alan Cox had to be convinced! After all, most of the Internet Telephony software out there can use a sound card, and if that card supports full-duplex (simultaneous playing and recording) and the user has a decent microphone-speaker headset, the quality of the call can be adequate. Why add complexity and a new API?

The answer is quite simple: sound cards are not telephone devices! Sound cards can't generate dial tone, rings, wink, flash-hook or caller-ID—all of which are needed to operate a normal phone device properly. Yes, sound cards are functional for limited telephony use with a headset and microphone, but real telephony cards let you plug that nice cordless phone in and be free from the short cord to your computer...or plug into a sophisticated business phone system. Sound cards also have no chance of providing the ability to plug into the phone line provided by your local telephone company, therefore, losing the ability to do inbound and outbound call control, hop-on and hop-off toll bypass applications or use any of the other new generation telephony software applications being written today.

Also, for any real VoIP use over the Internet, it is necessary to compress the audio data. To be compatible with other VoIP applications and equipment you must support the commonly accepted compression codecs like G.723.1 or G.729a. Unfortunately, if you want to do this in software you have to license the codecs, and this can cost in the six-figure range (easily!) these libraries are most certainly not open source. Telephony cards (like the Quicknet cards mentioned below) have pre-licensed these codecs and provide them built into the hardware. This avoids the mess of paying per-copy royalties for the codecs (it's part of the hardware cost) and lets you develop code with your own choice of license, not one imposed on you by the codec developer. In other words, you can do open-source VoIP software and still use the advanced codecs! These codecs are most definitely not something you would find on a sound card.

Additionally, sound cards can't interface with phones or the phone system, and they can't support hardware-based audio compression codecs. Sound cards are simply different from phone cards.

To be fair, telephony cards are not sound cards either. Sound cards have audio capabilities that far exceed what telephones can do. For example, sound cards are stereo; phones are mono. Sound cards can sample and playback sounds at music frequencies (20Hz-20kHz); phones are limited to voice frequencies (300Hz-4kHz typically). Sound cards have advanced music capabilites and often support MIDI and extensive sound effects. Phone cards cannot do any of that. The hardware and functionality are very different. The device drivers and the API need to be different, too.

But wait, you say, “what about all the great software out there that can work with sound cards, like voice recognition and text-to-speech processing?” Wouldn't it be nice to be able to use that software on phone devices with a minimum of effort? You can. The API for Linux telephony was designed to work in a way that does not preclude the use of software originally intended for a sound card with a telephony device. Yes, some minor code changes will be needed, but it's not all that hard. I'll explain more about that towards the end of the article.

The push for this new API was spearheaded by Quicknet Technologies, Inc., which manufactures a line of phone cards. In November of 1999, Ed Okerson of Quicknet and myself (I was an employee of Quicknet at the time) proposed the precursor to today's API to Alan Cox. After several weeks of intense e-mail and a pile of code by Ed and Alan, the Linux telephony API was born.

So, how does it work? Let's get started.