We have implemented PSTN-originated crossover services for the wireline [GUR03a] and cellular [GUR04d, GUR05a] components of the PSTN. Our implementation will be discussed in detail in the next chapter, where we will highlight its applicability to ongoing research in the area of pervasive computing.

It is important to note that the ontology we have described is not limited to PSTN events culled from the BCSM. The methodology presented here is independent of any call model; just an agreement is needed to specify the points where a call model is amenable to interruption. Once this is done, an extension schema can be constructed and the framework we discussed here used to transport the events between networks. For example, Kozik et al. [K0Z03] use our proposed architecture and protocol extensions to transport encoded Parlay events between the PSTN and the Internet.

A key requirement of PSTN-originated crossover services will be third-party programmability of such services. Arguably, the service creation framework for the World Wide Web (WWW) infrastructure has thrived because it enables third parties to provide value-added services over a common transport, namely, Internet Protocol (IP). The most important factor for the success of WWW services has been a common lingua franca (HTTP/Hypertext Markup Language (HTML)) and an extensive service creation tool set (Web Common Gateway Interface (CGI), Active Server Pages, Java scripts, servlets, Service Object Access Protocol (SOAP),1 etc.). Telephony, on the other hand, has traditionally been an environment where

1 SOAP [W3C03] is a lightweight protocol intended for exchanging structured information in a decentralized and distributed environment. It uses XML to represent a message construct that can be exchanged over a variety of underlying protocols.

the inner workings of the protocols and services, although not entirely secret, were not subject to as much public access and scrutiny as Internet protocols have been. We believe that the Web model of allowing open, well-defined protocols needs to be replicated for PSTN-originated crossover services. To that extent, the work presented in this chapter contributes to an open and extensible architecture for crossover services based on standard protocols to help third parties in developing such services.

We believe that establishing a taxonomy of PSTN-originated crossover services is extremely important, so that implementers can quickly identify various techniques for rapid implementation. Thus, we have proposed a taxonomy of PSTN-originated services.

And finally, we presented a case for using SIP as a distributed middleware.

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