For historical reasons, different nets emerged for speech and for data communication. Today these nets should converge, i.e. it is planned to integrate speech, fax and video into the existing data networks. To implement this convergence gateways will have to be developed, which will allow to mediate between different protocols and networks.

The goal of this work was to implement an ISDN2SIP-gateway that would allow to transmit digital calls form a certain ISDN-net to a predefined IP network and build up a voice connection between them. Of course this process should also be possible in the other direction. Furthermore, the gateway should allow an ISDN call to trigger a certain action inside the IP network.

To realize the aforementioned the Gateway must fulfill the following criteria:

• The software should not limit the maximal number of parallel connections.
• The packet delay of the dataflow within the gateway should be minimized.
• The signalisation within the IP-net should be solved with the Session Initiation Protocol (SIP) [RFC 2543].
• The design of the gateway should allow to implement other services easily.

In this study a software was developed, that allows to transform ISDN D-channel signalling information into SIP messages as well as vice versa. Also, the problem was solved to code and decode ISDN PCM A-law into the PCM A-law or ?-law used in IP networks. Therefore all the requirements discussed in the definition of the goals could entirely be met and implemented.

The design of the gateway allows to easily implement further services, such as the ability of an answering machine. Furthermore, it would be possible for the ISDN2SIP-gateway to take over the function of a SIP-gateway. This would greatly enhance the possible applications.