Telephony and Conferencing

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 companys 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.

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:

• Data transfer rate

• Available channels per call

• Availability of service

• Cost of service

• Quality of connection

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.

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.

PSTN is available throughout the United States.

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

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.

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 companys 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.

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 clients 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 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 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 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 enterprises 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.

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.

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 .