WINDOWS 1002000 PROFESSIONAL RESOURCE KIT [Electronic resources] نسخه متنی

Overview of Telephony and Conferencing

Windows 2000 Professional provides support for telephony and conferencing applications in a variety of telecommunications environments, including device support for modems, network adapters, and other telecommunications devices used by applications.

Function calls made by the Telephony Application Programming Interface, also known as Telephony API (TAPI), provide the foundation for the deployment of telephony and conferencing applications. TAPI abstracts the details of the underlying telecommunications network, allowing applications and devices to use a single command set. TAPI-enabled applications and devices can operate in a variety of telecommunications environments, including traditional analog switched networks, Private Branch Exchange (PBX) phone networks, and Integrated Services Digital Network (ISDN). TAPI further expands telecommunications support through the IP-based telephony, enabling telecommunications functions through a private intranet or over the Internet.

What's New

Telephony support has steadily evolved from its introduction in Microsoft® Windows® 95, which was the first operating system to bundle TAPI support. In Windows 2000 Professional, Microsoft builds upon this initial platform by making improvements in the following areas:

TAPI 3.0

NetMeeting 3.0

Phone Dialer

The Telephony Application Programming Interface, also known as Telephony API (TAPI), provides a common method for applications and devices to control the underlying communications network. TAPI was first released with Windows 95 as version 1.4, which provided a basic set of call control functions for analog modems and PBX systems, provided a common dialog for dialing rules, and introduced the Unimodem modem driver.

TAPI 2.0, released with Microsoft® Windows NT® 4.0, enhanced the basic features of TAPI 1.4. TAPI 2.0 was the first full 32-bit implementation of TAPI, providing improved processing capabilities. TAPI 2.0 introduced an expanded set of call control functions, designed specifically for call centers. TAPI 2.1 expanded the capabilities of TAPI 2.0 by adding support for client/server TAPI environments, and including support for both the Windows 95 and Microsoft® Windows® 98 platforms.

TAPI 3.0 provides the following enhancements to TAPI 2.1:

Support for IP telephony

Support for multiple-user conferencing

Support for NDIS 5.0–compliant devices

Unimodem 5 support

Media stream control

Compatibility with the Common Object Model (COM)

For more information about TAPI 3.0, see "Microsoft Support for CTI" later in this chapter.

NetMeeting 3.0

NetMeeting 3.0 is an application included with Windows 2000 Professional that provides conferencing capabilities within an intranet or to users on the Internet. NetMeeting provides point-to-point audio and video conferencing, data conferencing, text chat, whiteboard, and file transfer features. In addition, NetMeeting 3.0 provides the following improvements:

Improved user interface facilitates use, and provides additional features, such as picture-in-picture and compact views.

Remote Desktop Sharing allows viewing and control of a remote computer in a separate window.

Enhanced security protects data transferred during text-chat, whiteboard, and shared program or data conferences.

For information about configuring NetMeeting, see "Configuring NetMeeting 3.0" later in this chapter.

Phone Dialer

Phone Dialer in Windows 2000 Professional is a TAPI-enabled application that can be used to place audio and video calls. The version of Phone Dialer supplied with Windows NT 4.0 was limited to voice calling onto private and public switched telephone networks. Windows 2000 Professional Phone Dialer additionally supports point-to-point and multipoint audio and video conferencing over a private intranet or over the Internet.

Comparison of Telephony Features

Table 12.1 lists the new features provided in Windows 2000, in comparison with Windows 98 and Windows NT 4.0.

Table 12.1 Comparison of Windows 2000 Telephony Features

¹ TAPI 2.1 provided in Windows NT Service Pack 3.0.

² NetMeeting 3.0 available as download from http://www.Microsoft.com/Windows/netmeeting/download/.

Telephony Environments

Windows 2000 Professional can provide telephony and conferencing services within a variety of communications environments. The following section describes the telephony environments that Windows 2000 Professional can operate in, including:

Public Switched Telephone Network

Integrated Services Digital Network

Private Branch Exchange (PBX)

Client/Server Telephony

IP Telephony

Historically, most telephone connections in the world have been made through the public switched telephone network (PSTN). Most PSTN calls are transmitted digitally except while in the local loop, the part of the telephone network between the telephone and the telephone company's central switching office. Within this loop, speech from a telephone is usually transmitted in analog format.

Digital data from a computer must first be converted to analog by a modem. The modem is installed in the computer, connected to the computer by the serial port, or by a Universal Serial Bus connection. The data is converted at the receiving end by another modem, which changes the data from audio to its original data form.

Windows 2000 Professional provides basic telephony call support for modems using PSTN lines, such as dialing and call termination. Additionally, computer-based support is provided through the Hayes AT command set, as well as vendor-specific commands. Windows 2000 Professional provides device drivers for a number of internal and external analog modems, which can be automatically installed through Plug and Play, or manually installed using the Install New Device Wizard in Control Panel.

Integrated Services Digital Network

The need for high-speed telecommunications support within the existing telecommunications infrastructure has led to the development of new technologies, such as Integrated Services Digital Network (ISDN). ISDN is a digital phone service that is provided by regional and national phone companies, using existing copper telephone cabling.

To use ISDN, you need either an ISDN modem or an ISDN adapter. You might also need an NT-1 (the equivalent of the phone jack into which you plug your device) and an ISDN line from your telephone company.

ISDN modems are available in internal and external configurations. Internal ISDN modems, the more common of the two, are installed in the same manner as a network adapter card. External ISDN modems hook up to your computer through a serial port, just as regular modems do. Thus, because a serial port cannot exceed 115 kilobits per second (Kbps) (which is lower than the total effective bandwidth of the ISDN line), some throughput is lost if you are using the maximum ISDN bandwidth. An ISDN adapter, which operates at bus speed, provides the higher rate that ISDN needs. With most ISDN modems and adapters, you also need an NT-1. Some ISDN equipment comes with the NT-1 built in.

ISDN is typically supplied by the same company that supports the public switched telephone network. However, ISDN differs from analog telephone service in several ways, including:

Sata transfer rate

Available channels per call

Availability of service

Cost of service

Quality of connection

Data Transfer Rate

ISDN can provide data transfer rates of up to 128 Kbps. These speeds are slower than those of local area networks (LANs) supported by high-speed data communications technology, but faster than those of analog telephone lines. In addition to the difference in data transfer rates, ISDN calls can be established much faster than analog phone calls. While an analog modem can take up to a minute to set up a connection, you usually can start transmitting data in about two seconds with ISDN. Because ISDN is fully digital, the lengthy process of analog modems is not required.

Channels

PSTN provides a single channel, which can carry either voice or digital communications, but not both simultaneously. ISDN service is available in several configurations of multiple channels, each of which can support voice or digital communications. In addition to increasing data throughput, multiple channels eliminate the need for separate voice and data telephone lines.

Availability

PSTN is available throughout the United States.

Cost

The cost of ISDN hardware and service is higher than for PSTN modems and service.

Connection Quality

ISDN transmits data digitally and, as a result, is less susceptible to static and noise than analog transmissions. Analog modem connections must dedicate some bandwidth to error correction and retransmission. This overhead reduces the actual throughput. In contrast, an ISDN line can dedicate all its bandwidth to data transmission.

Private Branch Exchange

A Private Branch Exchange (PBX) is a private telephone switching system owned by a company or organization. The PBX is connected to a common group of PSTN lines from one or more of the telephone company's central switching offices to provide service to a number of individual phones, such as in a hotel, business, or government office. PBX solutions are available in a number of third-party hardware and software configurations, ranging from large dedicated switches, to server-based solutions, to internal cards that can be inserted into individual workstations. TAPI in Windows 2000 supports computer call control, voice mail, Caller ID, and other advanced features in conjunction with a PBX.

Client/Server Telephony

Within an organization, a computer running Windows NT Server can be configured as a telephony server, providing an interface between the PBX and TAPI-enabled workstations. For example, a LAN-based server might have multiple telephone-line connections to a local telephone switch or PBX. TAPI operations invoked at any associated client are forwarded over the LAN to the server. The server uses third-party call control between the server and the PBX to implement the client's call-control requests. Figure 12.1 shows an example of a PBX system configured with a telephony server.

Figure 12.1 Client/Server Telephony

The server can be connected to the switch using a switch-to-host link. It is also possible for a PBX to be directly connected to the LAN on which the server and associated clients reside. Within these distributed configurations, different subconfigurations are possible, such as:

To provide personal telephony to each desktop, the service provider can model the PBX line associated with the computer (on a desktop) as a single line device with one channel. Each client computer would have one line device available.

Each third-party station can be modeled as a separate line device to allow applications to control calls on other stations. (in a PBX, a station is anything to which a wire leads from the PBX.) This enables the application to control calls on other stations. This solution requires that the application open each line it wants to manipulate or monitor.

Windows 2000 Professional workstations with TAPI support specify the telephony server in their configuration using the tcmsetup command. After it is configured, users at the Windows 2000 Professional computer can perform basic and advanced call control functions, such as placing, answering and terminating calls to the PBX or PSTN through the computer. Advanced functions, such as computer-telephony integration (CTI) functions, can be enabled through the installation of third-party telephony services that conform to TAPI 3.0 standards.

IP Telephony

IP telephony and conferencing technologies are built around simple, core concepts: A personal computer (or other device) is used to capture audio and optionally, video signals from the user (for example, by using a microphone attached to a sound card, and a video camera connected to a video capture device). This information is compressed and sent to the intended receivers over the local area network (LAN) or the Internet. At the receiving end, the signals are restored to their original form and played back for the recipient. Audio can be rendered by using speakers attached to a sound card and video by creating a window on the display of the computer.

IP telephony integrates audio and video stream control with legacy telephony functions. IP telephony in Windows 2000 is supported through the H.323 protocol and IP multicast conferencing. H.323 provides point-to-point audio and video conferencing. Support for multiple-user conferencing is provided through IP multicast conferencing. IP telephony systems can be integrated with the public telephone system through an IP-PSTN gateway, allowing users to place telephone calls from an enabled computer. Users can place audio and video calls with external users through the Internet with an H.323 proxy, allowing administrators to control host access. This IP telephony infrastructure is described in the following sections.

H.323 Protocol

H.323 is an International Telecommunication Union–Telecommunications (ITU – T) protocol that is used to provide voice and video services over data networks. At the most basic level, H.323 allows users to make point-to-point audio and video phone calls over an intranet. H.323 also supports voice-only calls to be made to conventional phones through a IP-PSTN gateway, and Internet audio-video calls to be made through a proxy server.

IP-PSTN Gateway

IP telephony permits the integration of data networks and information with the traditional public switched telephone network (PSTN) through the configuration of IP-PSTN gateways. IP-PSTN gateways are configured as part of an enterprise's IP telephony network. Client support of IP-PSTN gateways is provided through the H.323 protocol.

Figure 12.2 shows an example of a PSTN gateway.

Figure 12.2 A PSTN Gateway

For example, a call from an IP telephony client to a conventional telephone would be routed on the IP network to the IP-PSTN gateway, which would translate H.323 signaling to conventional telephone signaling and route the call over the conventional telephone network to its destination.

H.323 Proxy

In enterprises where firewalls have been implemented for IP security, but IP telephony through the H.323 protocol is desired, an H.323 proxy can be used. Any IP telephony client needing to connect to users outside the firewall must specify the name or IP address of the H.323 proxy server.

IP Multicast Conferencing

The Multicast Conferencing Service Providers shipped with TAPI 3.0 provide support for IP multicast-based audio and video conferencing between multiple participants. IP multicasts support multi-user conferences using a single connection instead of multiple connections, which conserves network bandwidth.

TAPI 3.0 provides additional interfaces that TAPI-aware applications can use to access directory services, such as the Windows 2000 Site Server ILS Service.

All routers between the Windows 2000 Professional client and other conferencing participants must support IP multicasting. Windows 2000 Server provides a multicast-enabled DHCP server that can be used to allocate a unique IP address for the duration of the conference.

For more information about the installation and configuration of the Windows 2000 Site Server ILS Service, see "Telephony Integration and Conferencing" in the Internetworking Guide.

Microsoft Support for CTI

Support for computer-telephony integration (CTI) is built into Windows 2000 Professional. CTI support is provided in Windows 2000 Professional in the following areas:

TAPI 3.0

Service Providers

NetMeeting 3.0

Phone Dialer application

The Telephony Application Programming Interface, also known as Telephony API (TAPI), is a set of Microsoft® Win32® function calls and Microsoft Common Object Model (COM) interfaces used by telephony applications. These function calls are processed internally by TAPI, and result in calls to service providers, which control the telephony and media hardware needed to provide the function required by the telephony application. Windows 2000 includes TAPI 3.0 and provides backward compatibility with TAPI 2.1, which was included with earlier versions of Windows. Figure 12.3 shows the architecture of TAPI.

Figure 12.3 TAPI Architecture

TAPI 3.0 provides a standard method for communications applications to control telephony functions for data, fax, and voice calls. TAPI manages all signaling between a computer and a telephone network, including basic functions such as dialing, answering, and hanging up a call. It also supports supplemental services such as hold, transfer, conference, and call park, found in PBX, ISDN, and other telephone systems. The support of supplemental services varies by service provider.

In addition to support for conventional telephony, TAPI 3.0 provides support for IP telephony; that is, telecommunications through IP-based networks. TAPI 3.0 supports user-to-user and multiparty audio and video conferencing through the H.323 and IP multicast. TAPI 3.0 interfaces with user directories, such as the Site Server ILS Service, to associate user and conference objects with call information, such as IP address and computer name.

Service Providers

TAPI 3.0 supports two classes of service providers: telephony and media. Telephony Service Providers (TSPs) provide implementation of telephony signaling and connection control features, and Media Service Providers (MSPs) provide access to and control the media content associated with those connections, such as the audio and video streams of a videoconference. MSPs use the DirectShow API for efficient control and manipulation of streaming media.

For more information about Media Service Providers in TAPI, see "Telephony Integration and Conferencing" in the Internetworking Guide.

A Telephony Service Provider (TSP) is a dynamic-link library (DLL) that supports communications over a telephone network to one or more specific hardware devices through a set of exported service functions. The service provider responds to telephony requests sent by TAPI, and completes the basic tasks necessary to communicate over the telephone network. In this way, the service provider, in conjunction with TAPI, shields applications from the service-dependent and technology-dependent details of the telephone network communication.

The installation tool for a service provider registers the software with TAPI and associates that service provider with the hardware devices it supports. Multiple service providers can share the same device: for example, the H.323 TSP and Multicast Conferencing TSP can both use the name network interface card. Existing applications can be associated with new telephony devices, or the function of existing devices can be extended through the development and implementation of new service providers.

Table 12.2 lists the telephony and media service providers included with Windows 2000 Professional.

Table 12.2 Service Providers in Windows 2000

Additional service providers can be obtained through third-party vendors to enable the use of their hardware with existing telephony applications, such as a PBX hardware solution.

NOTE

---

To install TSPs and MSPs from third-party vendors, follow the instructions provided by the vendor.

NetMeeting 3.0

Windows 2000 Professional includes the NetMeeting 3.0 application. NetMeeting uses the NetMeeting Application Programming Interface (API) to provide telephony functions to the application. NetMeeting supports the H.323 protocol, providing point-to-point audio and video conferencing features. NetMeeting can be used separately from, or in conjunction with, TAPI 3.0 applications such as Phone Dialer. However, the two environments must be separately configured.

In addition to point-to-point audio and video call support, NetMeeting provides conferencing support. NetMeeting provides the following features for audio, video, and data conferencing:

Multi-user whiteboard

Text-based chat

Program sharing

Remote desktop sharing

File transfer

Windows 2000 also ships with the Phone Dialer, which is a TAPI application. This can be used for basic telephony functions as well as audio and video conferencing services.

The Phone Dialer makes TAPI function calls to utilize Microsoft's Telephony Service Providers, including the H.323 and the Multicast Conference Service Providers. The Phone Dialer can also be used with Telephony Service Providers supplied by other vendors.