Communication by telephone as we know it today could be considerably upgraded by providing a picture with the sound. The Integrated Digital Services Network (ISDN) offers enough bandwidth to make this vision come true. Still, implementing a visual telephone system requires powerful hardware and flexible software.

1. Goal

   Our goal was to use the hardware developped by our predecessors and to set up all components for proper operation. This required firmware which had to support full compatibility to normal ISDN telephone services as well as a visual telephone system. We based our work on the standardisation documents of ITU-T and the data sheets of the manufacturers of the chipset used. The starting point for our Firmware was a C port of some existing code in Modula which implemented the most basic ISDN telephone functions.

2. Procedure

   In order to reach our goal, we split up the work into several parts. The first step was to debug the C port of the existing firmware to a stage where it had the same functionality as the original Modula code. We then extended this version of the code to support full ISDN telephone services, i.e. calls on two separate B-channels with caller identification and channel switching. Also included in this extension was the interface for the future video call enhancements.
   Our predecessors were unable to alpha-test the video codec chip due to AT&T shipment problems. Therefore we set our next task to take the AVP chip into operation and to test the video interface. During this we encountered various problems which delayed video operation considerably. Parallel to working on the video interface we started studying the ITU-T standards on visual telephone calls and this allowed us - together with the AT&T documentation - to write the code for interaction with the system controller. As a last step we fired up the audio interface and the audio codec chip. These two chips require close interaction with the system controller for transfering audio data.

3. Results

   Our Firmware satisfies almost all requirements. It implements the complete ISDN protocol stack which allows signalisation for two B-channels. We are thus compatible to common ISDN telephones. By adding an additional protocol layer which takes care of the H.320 standard series we can setup up connections with other videotelephones. They recognize us as a peer and allow video connections to be started and torn down again. The hardware has - with exception of the AVP - been fully integrated and modules to control all components have been written. Jade, Seco and the System Controller support all common audio compressions and allow mode changes according to the standards. The difficulties with the AVP delayed the whole video path. As the 16 bit bus interface did not work properly we developped an add-on board which allows 32 bit access to the AVP chip. This and some other minor patches finally allowed us to download the necessary software to the video codec and to execute it. Lack of time and documentation prevented us from integrating full video capability. However we are able to output the signal received from the other station.

4. Outlook

   The video transmit path still requires some work. Once the video input board works together with the AVP it should be a small matter to add the last small function calls in the code. Comments have been inserted in the proper places.

5. H.320 Protocol Stack Family

   

Urs Beeli, Daniel Wesemann
Graduate Students in Electrical Engineering, Swiss Federal Institute of Technology, ETH Zürich