SOTAs: The Telephony Code: Page 4 of 12

PSTN/Carrier APIs: These interfaces allow applications to communicate with devices and services on modern public networks, such as digital cellular and ISDN networks. Due to the limited-access nature of these networks, these interfaces are gen-erally only used by companies that provide services on those networks--the telephone companies themselves, or third-party firms that provides things like stock quotes and alerts to customers. With a few exceptions, PSTN/Carrier APIs are not commonly used by organizations. The two main interfaces in this category are Parlay/OSA from the Parlay vendor consortium, and the Java APIs for Integrated Networking (JAIN).

Standardized Network Protocols: Data-network telephony protocols such as H.323 and SIP provide a variety of control services, such as call setup and management, as a necessary part of their functionality; these control services can be incorporated into data-centric applications if the application developer is willing and able to implement the protocol stack into the application directly. In this scenario, the applications do not use APIs to interface with telephony devices and services, but instead act as direct peers to the devices and services on the voice network.

Vendor-Proprietary Interfaces: Apart from the common interfaces described above, there are also some vendor-specific proprietary interfaces, which are also sometimes supported in software packages and development tools. Two examples are Cisco's Skinny Call Connection Protocol (SCCP), which is widely used on Cisco VoIP gear, and Avaya's Communications Manager API (CMAPI), which is used for connecting third-party gear to Avaya's soft switch.

The Problem With Today's APIs
While these low-level CTIs are widespread use, they tend to be platform specific and hence are generally not useful for distributed applications. Meanwhile, SIP ostensibly offers a platform-neutral control syntax and provides raw access to many critical functions. But while SIP does indeed work well for many things, the protocol also requires integrating data-centric applications into the voice network--a non-starter for many developers.

Web-service CTI interfaces, on the other hand, provide an abstract service-control layer that is distinct from the application and voice service layers. It therefore provides a generalized interface that data-centric applications can use to tap into the voice ser-vices (and vice versa) without forcing them to become peer devices. Through a single API, applications can talk to devices on SIP, H.323, possibly even PSTN networks, without having to program to each of those networks. Instead, applications need only communicate with front-end devices, such as an IP PBX or a gateway, and let those devices perform network-specific functions.