Evaluation of SIP signaling usability in networked operations

Latest development showed that VoIP-solutions are also going to expand into the area of safety critical voice communication systems. A variety of solutions has been developed for different fields of application, different scales and services. Signaling and media transports vary in voice communication architectures for LAN environments compared with those in global Internet telephony. Generally, multitudes of call-features as well as dedicated services are required. Besides also access to shared media resources, like analog or digital radio has to be controlled, together with coupling capabilities to virtually merge radio-channels. Additionally a voice communication system offers monitoring and recording capabilities and failover mechanisms for switching between redundant devices. Today's legacy voice communication systems (VCS), which meet these requirements, are based on circuit switching technology. They are well-proven and characterized by high reliability and availability. The vast majority of voice communication systems are single installations supporting dedicated resources. From there, a controller working on one VCS can not arbitrarily access the radios of another VCS, which substantially restricts the contingency capabilities of the system and may also be regard as a major cost factor. Development of new data services has shown that circuit switched systems lack of data integration. Also in deployable systems and mobility demands, such as in disaster operation or military applications, legacy systems have drawbacks, because configuration is complex to handle and interoperability between different vendors requires dedicated gateways. IP based voice communication generates a new momentum as IP based services inherently allow for networked operations. But due to the fundamental paradigms different to the VCS, it requires to reconsider the architectural framework. The proposed paper elaborates two aspects of the implications of introducing IP based communication systems on a system and architectural level. Separating the communication service, which is purely the IP packet from application services, which include A/G and G/G voice services, is one issue that shall be display. The second topic shall describe the evolution of system interfaces, which includes the working together with radio equipment and voice communication systems of other vendors. Finally a practical implementation using the session initiation protocol (SIP) evaluates the variants for implementation of the required call-features in terms of usability in a VCS. Further, it presents a model for signaling of (providing) additional VCS specific functions. SIP extensions meet the requirement for this variety of signaling, especially by using event-notification, which is done by SUBSCRIBE and NOTIFY requests. Event-packages already exist for conferencing, dialog-events and refer-events - additional event-packages for radio signaling (PTT and SQU) and radio-channel-coupling extend the signaling capabilities to provide a complete set of VCS-features