Station devices that CallManager supports, as shown in Figure 3-8, include H.323 stations, voice mail/unified messaging, IP phones, Unicast conference devices, transcoders, and voice applications such as Cisco IP Communicator. Chapter 5 discusses conference devices and transcoders in detail. In addition to explaining the supported devices, this section differentiates a user from a station and categorizes station devices by protocol and then subdivides them by device capabilities.
Figure 3-8. System Station Devices
The three basic protocols CallManager uses to communicate with station devices are as follows:
The CTI interface provides Telephony Application Programming Interface (TAPI) and Java TAPI (JTAPI) application layer support. Although stations can reside on the other side of a gateway, the stations discussed in this chapter are limited to those that communicate directly with CallManager. Refer to Chapter 4 for gateway communication and protocols such as Media Gateway Control Protocol (MGCP).
Role of CallManager for Stations
CallManager provides call control on behalf of stations or gateways. In the case of stations, stations indicate requests for service to CallManager, and CallManager acts on the requests for service. The requests for service include such tasks as device initialization, device configuration information, call origination, call acceptance, call termination, call information, media statistics, and feature activation. CallManager provides the call control engine for the devices that are configured in the CallManager database via CallManager Administration.
The signaling from CallManager to the station is the call signaling path. The media path for the exchange of media between stations does not pass through CallManager. The stations stream media directly between the stations or directly from a station to a media processing device, such as a Unicast conferencing device or a transcoder.
SCCP is a lightweight, simple, stimulus protocol. The signaling path is a TCP/IP connection that the station establishes to CallManager. The station interface message set encompasses three basic areas:
CallManager directs the establishment of the media connection, but the station and device to which the station is connected establish the media stream directly to each other. You can learn more about SCCP in Appendix C.
Computer Telephony Integration (CTI) Overview
CTI provides a communication interface into CallManager. Both JTAPI and TAPI make use of CTI communication with CallManager. The CTI interface is specific to communication between the Cisco TAPI service provider and CallManager. The TAPI service provider is a layer external to CallManager that is used by TAPI applications to communicate with CallManager without requiring knowledge of the CTI interface.
Microsoft's TAPI for Windows simplifies telephony application development. The interface abstracts the telephony services from the actual hardware and software infrastructure of CallManager. Applications developed with TAPI are more portable and less subject to change when the CallManager infrastructure changes.
JTAPI is a Java-based interface that provides similar abstraction for Java-based development. JTAPI also abstracts the application development from the CallManager infrastructure. JTAPI extends application portability to include not only independence from CallManager infrastructure, but also independence from any particular operating system.
Figure 3-9 shows the role of the CTI protocol in the application infrastructure of CallManager. You can learn more about the CTI protocol in Appendix C.
Figure 3-9. CTI Protocol
H.323 Endpoint Overview
An H.323 endpoint is a device or software application that communicates with CallManager using the H.323 protocol specification. The Microsoft NetMeeting software application is an example of an H.323 endpoint.
The H.323 Recommendation from the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) contains a set of very complex protocols that work together under the H.323 protocol umbrella. These protocols (for example, H.225.0 and H.245) manage the connection and the media for a communication session. The frame structure for the multiplex layer allows for a vast multitude of services. However, the complexity is more extensive than is required for simple voice communications. H.323 stations must maintain state information for a call and thus are relatively complex. By contrast, SCCP provides both features and services for relatively low-cost user stations that require no protocol state processing.
Figure 3-10 shows the relationship of the protocols under the H.323 umbrella specification. You can learn more about the H.323 protocol in Appendix C.
Figure 3-10. H.323 Protocol Specification