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Configuring Telephony and Conferencing

Microsoft Windows XP Professional provides support for telecommunications in a variety of environments, including Internet Protocol (IP) telephony networks. The following discussion includes installation and configuration details for traditional and IP-based telephony and conferencing, and technical details relating to modems and communications tools.

For information on how to obtain the Windows XP Professional Resource Kit in its entirety, please see


On This Page

Related Information
Overview of Telephony and Conferencing
Setting Up Telephony and Conferencing
Troubleshooting Telephony and Conferencing
Additional Resources

  • For more information about installing and troubleshooting hardware devices, see Chapter 9, “Managing Devices.”

  • For more information about configuration of telephony and conferencing services on a computer running Microsoft Windows 2000 Server, see “Telephony Integration and Conferencing” in the Internetworking Guide of the Microsoft Windows 2000 Server Resource Kit.

  • For more information about planning and deploying Group Policy in a Windows 2000 domain, see “Group Policy” in the Distributed Systems Guide of the Microsoft Windows 2000 Server Resource Kit.

  • For more information about deploying Group Policy in a Windows Server 2003 domain, see the Designing and Managed Environment book of the Microsoft Windows Server™ 2003 Deployment Kit.

Overview of Telephony and Conferencing

IP Telephony and conferencing allow you to converge data, voice, and video—communication traditionally implemented through separate networks—over the same IP-based network infrastructure. The Windows XP Professional telephony platform allows for both IP telephony and conferencing solutions, the use of IP over an existing computer network for telephony and conferencing, and computer-telephony integration (CTI), which is the integration of existing circuit-switched telephony equipment with computer-based Telephony Application Programming Interface (TAPI) applications.

Telephony Environments

Windows XP Professional can provide telephony and conferencing services within a variety of communications environments, including:

  • IP telephony

  • Client/server telephony

  • Public Switched Telephone Network (PSTN)

  • Integrated Services Digital Network (ISDN)

  • Private Branch Exchange (PBX)

IP Telephony

Using IP telephony and conferencing technologies, a personal computer (or other device) captures audio and video signals from the user by using, for example, a microphone attached to a sound card and a video camera connected to a video capture device. The computer compresses and sends this information to the intended receivers over the local area network (LAN) or the Internet. For the recipient, a computer restores the signals to their original form and plays back audio by using speakers attached to a sound card and video by creating a window on the display of the computer.

IP telephony in Windows XP Professional supports the following features:

  • Session Initiation Protocol (SIP)

  • H.323 protocol

  • IP multicast conferencing

You can integrate IP telephony systems with the public telephone system by using an IP-PSTN gateway, allowing users to place telephone calls from an enabled computer. Users can place audio and video calls to external users by using the Internet with an H.323 proxy, allowing administrators to control host access.

Session initiation protocol

SIP is a text-based application-layer signaling and call control protocol. The main function of SIP is to create, modify, and terminate SIP sessions. SIP supports both unicast and multicast communication. The main components in a SIP environment are SIP servers and SIP user agents.

There are two different types of SIP user agents, as shown in Table 26-1.

Table 26-1 SIP User Agents

SIP User Agent


User Agent Client

Initiates SIP requests

User Agent Server

Receives SIP requests

Most SIP-based applications act as both a user agent client and server. Each user agent is associated with a SIP address.

All SIP servers accept and reply to SIP requests. The function that the specific SIP server performs determines which SIP requests it processes. Table 26-2 lists the different SIP servers and their functions.

Table 26-2 SIP Servers

SIP Server


Proxy server

Acts as an intermediary between a SIP user agent client and a SIP user agent server. Depending upon the direction of the communication between client and server, the proxy server performs the functions of either a SIP user agent client or SIP user agent server. The proxy server can simply forward the SIP request or modify it before sending it.

Registrar server

Receives REGISTER requests, which contain both the IP address and SIP address (Uniform Resource Locator, or URL) of the user agent. This allows the Registrar server to keep track of the location of user agents, of which the Registrar server has received REGISTER requests.

Redirect server

Accepts initiation (a SIP INVITE request) of a SIP session from the calling User Agent, obtains the correct SIP address of the called User Agent, and replies to the calling User Agent with the correct SIP address. The calling User Agent then uses the correct SIP address to directly initiate a SIP session with the called User Agent.

The SIP servers can be developed as separate applications or as a single application with the functionality of all the servers. The combination of both a registrar and proxy server is sometimes referred to as a rendezvous server.

RTC client APIs

RTC (Real Time Communication) client APIs are included with Windows XP Professional. RTC client APIs are a set of Component Object Model (COM) interfaces and methods that create computer-computer, computer-phone, phone-phone audio/video calls, or text-only Instant Messaging (IM) sessions over the Internet. Application Sharing and Whiteboard can also be added to computer-computer sessions.

These features can be configured so that they are also available to users in an audio/video conference. Instant Messaging, where a text message is sent to a URL or IP address, allows users the ability to communicate with other Instant Messaging users in real time. Application Sharing allows for a user to give real-time access and control of an application to another user. Whiteboard support allows for real time creation, collaboration, and viewing of sketches or diagrams.

H.323 protocol

H.323, an application-layer signaling and call control protocol, is an International Telecommunication Union-Telecommunications (ITU-T) protocol that provides 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 made to conventional phones by using an IP-PSTN gateway and Internet audio/video calls made by using a proxy server.

H.323 gateway

You configure H.323 gateways as part of your enterprise’s IP telephony network. Using the configuration of H.323 gateways, IP telephony integrates data networks and information with the traditional Public Switched Telephone Network (PSTN). The H.323 protocol provides client support of H.323 gateways.

Figure 26-1 shows an example of an H.323 gateway.

Figure 26-1 H.323 gateway

Figure 26-1 H.323 gateway

For example, a call from an IP telephony client to a conventional telephone is routed over the IP network to the H.323 gateway, which translates H.323 signaling to conventional telephone signaling, such as Signaling System 7 (SS7), and then routes the call over the conventional telephone network to its destination.

IP multicast conferencing

The Multicast Conference Service Provider included with TAPI 3.1 provides support for IP multicast-based audio and video conferencing between multiple participants. IP multicasts support multi-user conferences by using a single connection instead of multiple connections, which conserves network bandwidth.

All routers between the Windows XP Professional client and other conferencing participants must support IP multicasting. Windows 2000 Server and Windows Server 2003 provide a multicast-enabled Dynamic Host Configuration Protocol (DHCP) server that can allocate a unique IP address for the duration of the conference.

Client/Server Telephony

You can configure a computer running Windows 2000 Server or Windows Server 2003 as a telephony server, providing an interface between the PBX switch and workstations enabled to use TAPI. For example, a LAN server might have multiple telephone-line connections to a local telephone switch or PBX. An associated client starts TAPI operations and forwards them 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 26-2 shows an example of a PBX system configured with a telephony server.

Figure 26-2 Client/server telephony using PBX

Figure 26-2 Client/server telephony using PBX

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

  • Single-line device model.

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

  • Separate-line device model.

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

Windows XP Professional workstations locate the telephony server through auto discovery of the published telephony service object in Active Directory. After communication is established with the telephony server, users at the Windows XP Professional computer can perform basic and advanced call control functions, such as placing, answering, and terminating calls to the PBX switch or PSTN through the computer. Installation of third-party telephony services that conform to TAPI 3.1 standards can enable advanced functions, such as computer-telephony integration (CTI) functions.

For more information about client/server telephony, see “Configuring Client/Server Telephony Support” later in this chapter.

Public Switched Telephone Network

Historically, most telephone connections in the world have been made by using the 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 and 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 XP Professional provides basic telephony call support for modems using PSTN lines, such as dialing and call termination. Windows XP Professional provides device drivers for a number of internal and external analog modems, which can be automatically installed by using Plug and Play or manually installed by using the Add Hardware 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 uses existing copper telephone cabling that is provided by regional and national phone companies.

To use ISDN, you need either an ISDN modem or ISDN adapter. You might also need an NT1 (the equivalent of the phone jack into which you plug your device) and an ISDN line from your telephone company. Some ISDN equipment comes with the NT-1 built in.

ISDN modems are available in internal and external configurations. Internal ISDN modems are more commonly used and are installed in the same manner as a network adapter. External ISDN modems hook up to your computer by using a serial port, the same as regular modems. 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.

The same company that supports the PSTN typically supplies ISDN. 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

Data transfer rate

ISDN can provide data transfer rates of up to 128 Kbps. These speeds are slower than those of 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 used by 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.


ISDN is available throughout the United States.


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 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. In Windows XP Professional, TAPI supports computer call control, voice mail, Caller ID, and other advanced features in conjunction with a PBX.

TAPI 3.1

The Telephony Application Programming Interface, also known as Telephony API or TAPI, is a set of Microsoft Win32 function calls and Component 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 hardware needed by the telephony application. Windows XP Professional includes TAPI 3.1 and TAPI 2.2, which are compatible with TAPI 3.0 and TAPI 2.1, respectively.

The following are some enhancements to TAPI 3.0 found in TAPI 3.1:

  • New audio codec, DVI4 at 16-kilohertz (KHz) sample rate, in H.323 Service Provider

  • New audio codecs, DVI4 at both 8-KHz and 16-KHz sample rates, in Multicast Conference Service Provider

  • Acoustic Echo Cancellation (AEC) support in H.323 Service Provider and Multicast Conference Service Provider

  • Data encryption for Real-Time Transport Protocol (RTP) in Multicast Conference Service Provider

  • Improved jitter management in H.323 Service Provider and Multicast Conference Service Provider

  • Supplemental Services for H.323, which has the following features enabled:

    • Call Forwarding with the following options: unconditional, on busy, and on no answer

    • Call Diversion, which allows you to forward a call while it is ringing

    • Blind Transfer, which allows you to transfer a caller to a third person without consulting the called person

    • Consultative Transfer, which allows you to transfer a caller to a third person after consulting the called person

    • Call Hold/Unhold, which allows you to stop and restart the media on a call without dropping the call

Figure 26-3 shows the architecture of TAPI.

Figure 26-3 TAPI architecture

Figure 26-3 TAPI architecture

TAPI 3.1 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 ending a call. It also manages supplemental services such as hold, transfer, conference, and call park, which are 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 3.1 provides support for IP telephony—that is, telecommunications through IP-based networks. TAPI 3.1 supports user-to-user and multiparty audio and video conferencing through the H.323 and IP multicast. TAPI 3.1 interfaces with user directories to associate user and conference objects with call information, such as IP address and computer name.

Service Providers

TAPI 3.1 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.

For more information about media service providers in TAPI, see “Telephony Integration and Conferencing” in the Internetworking Guide of the Microsoft Windows 2000 Server Resource Kit.

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 telephone network communication.

The installation tool for a service provider registers the application 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 Conference TSP can both use the same network adapter. Existing applications can be associated with new telephony devices, or the function of existing devices can be extended by using the development and implementation of new service providers.

Table 26-3 lists the telephony and media service providers included with Windows XP Professional.

Table 26-3 Service Providers in Windows XP Professional

Service Provider


H.323 Telephony Service Provider

H.323 Media Service Provider

Provide voice and video services over data networks using the H.323 protocol. Support calling conventional phones through IP-PSTN gateways and Internet audio/video calls.

Multicast Conference TAPI Service Provider

Multicast Conference Media Service Provider

Provide multiple-user conference support over intranets and the Internet.

NDIS Proxy TAPI Service Provider

Permits TAPI applications to access wide area network (WAN) devices, such as ISDN modems and Asynchronous Transfer Mode (ATM) devices, using a standard Network Driver Interface Specification (NDIS) 5.0 interface.

TAPI Kernel-Mode Service Provider

Provides TAPI support for NDIS 4 WAN drivers.

Unimodem 5 Telephony Service Provider

Unimodem 5 Media Service Provider

Provide device abstraction and TAPI support for a wide variety of modem devices. The Unimodem 5 MSP is used when using full-duplex voice modems.

Wave Media Service Provider

Used with any TSP that provides an audio wave driver. For example, when Unimodem 5 TSP is used with half-duplex voice modems, Wave MSP is used.

Additional service providers can be obtained from hardware vendors for use with their hardware and existing telephony applications, such as a PBX hardware solution.

Note To install TSPs and MSPs from hardware vendors, follow the instructions provided by the vendor.

Quality of Service

Quality of Service (QoS) refers to a combination of mechanisms that cooperatively provide a specific quality level to application traffic crossing a network or multiple, disparate networks. QoS helps ensure a constant, reliable, steady data stream when using real-time communications, such as IP telephony and video conferencing, over packet-based networks.

Support for QoS in Windows XP Professional

Applications that use QoS can take advantage of the QoS infrastructure supported in Windows XP Professional.

QoS features in Windows XP Professional provide traffic shaping, smoothing bursts and peaks in traffic to an even flow. Packet marking (802.1p marking for layer 2, and Diff-serv Code Point [DSCP]marking for layer 3) helps achieve efficient traffic shaping. The QoS Packet Scheduler enforces QoS parameters for data flow. The QoS Packet Scheduler retrieves the packets from the queues and transmits them according to the QoS parameters. The marked packets then receive priority over nonmarked packets when processed by network devices (switches and routers) along the data path.

QoS Components for Windows XP Professional

The components for QoS support are built into Windows XP Professional. Windows XP Professional provides an interface (QoS API) so that applications can support QoS technologies.

The QoS Packet Scheduler is not automatically installed with Windows XP Professional. After selecting and installing QoS Packet Scheduler, you might also have to configure Windows XP Professional to use 802.1p. Select the option for 802.1p support on the properties page for the network adapter. The network adapter must support 802.1p.

Setting Up Telephony and Conferencing

IP telephony support is also installed during Windows XP Professional setup, including support for TAPI 3.1 and all telephony and media service providers. During installation, Windows XP Professional automatically detects, installs, and configures most Plug and Play modems, adapters, and other telephony devices. Use the Add Hardware Wizard to install and configure devices that are not automatically configured and require installation information, such as the driver location. Support for telephony devices added after initial Windows XP Professional installation can also be provided in this manner.

For a list of supported telecommunications devices, see the Hardware section of the Windows Catalog at

If Windows XP Professional is installed over a previous version of Microsoft Windows that included telephony services (such as Microsoft Windows 95, Microsoft Windows 98, or Microsoft Windows NT version 4.0), any previous versions of the TAPI programming interface are upgraded. Previous versions of Windows with TAPI 1.4 or TAPI 2.1 are upgraded to TAPI 2.2. Previous versions of Windows with TAPI 3.0 are upgraded to TAPI 3.1. In some instances, TAPI 2.2 and TAPI 3.1 will coexist on the same computer. For example, if the previous version of Windows included TAPI 1.4 or TAPI 2.1, TAPI is upgraded to TAPI 2.2. The upgraded computer would then have both the upgraded TAPI 2.2 and TAPI 3.1, which is installed with Windows XP Professional. The correct version of TAPI would be used depending upon the TAPI version for which the application was written.

Warning The default configuration of Windows Firewall in Service Pack 2 for Windows XP blocks the port needed (port 1720) by TAPI phone dialers. For information on this issue and how to resolve it, see article 841802, “Phone Dialer calls are immediately disconnected in Windows XP,” in the Microsoft Knowledge Base at

Configuring Modems

A modem is a communications tool that enables a computer to transmit information over a standard telephone line. Using Windows XP Professional, you can install a modem in one of three ways:

  • Plug in your Plug and Play modem or USB modem.

  • In Control Panel, use Phone and Modem Options.

  • In Control Panel, add a modem by using Add Hardware.

In each of these cases, the Add Hardware Wizard appears and asks whether you want Windows XP Professional to automatically detect the modem or whether you want to manually select a modem from the list of known manufacturers and modem models. If you choose the detection option, the wizard detects and then queries the modem to configure it. If it cannot detect the modem, it prompts you to select one.

After the modem is selected, you can, if necessary, adjust its properties, such as the volume for the modem speaker, the time to wait for the remote computer to answer the call, and the maximum data transmission speed. These adjustments are made by using Phone and Modem Options in Control Panel.

Depending on the type of modem you have, installing and configuring it might vary slightly as follows:

  • If the modem supports Plug and Play, make sure it is configured to respond as a Plug and Play device, rather than manually configured for resource settings. This is normally accomplished by using a configuration application provided with the modem.

  • If you install a modem that does not support Plug and Play, you must configure its built-in COM port by using the Add Hardware Wizard before it is installed by using Phone and Modem Options in Control Panel. In most cases, the Add Hardware Wizard does this automatically for you.

  • If you are using Personal Computer Memory Card International Association (PCMCIA) drivers with Windows XP Professional, Windows XP Professional detects and configures PCMCIA modems automatically when they are first inserted.

  • If you are using a USB modem, it is automatically detected when installed.

    Note This procedure is for both internal and external modems. PCMCIA and USB modems automatically install when inserted. Before you install a modem, see “Modems” in Windows XP Professional Readme.txt.

To install a modem by using Phone and Modem Options
  1. In Control Panel, open Phone and Modem Options.

  2. Select the Modems tab, as shown in Figure 26-4, and then click Add to start the Add Hardware Wizard. The Add Hardware Wizard leads you through the steps for installing a modem.

In most cases, let the Add Hardware Wizard detect the modem for you. If it cannot detect the exact manufacturer and model, the wizard picks a standard configuration that is usually compatible; your modem still functions at its maximum speed and according to factory default settings. A few advanced features—such as enabling and disabling compression, error control, and flow control—might be disabled.

Figure 26-4 Phone and Modem Options dialog box

Figure 26-4 Phone and Modem Options dialog box

For information about installing a modem or about finding a better match than a standard modem type, see “Troubleshooting Modems” later in this chapter.

Windows XP Professional automatically makes COM port assignments to communications ports, internal modem adapters, and PCMCIA modems according to their base input/output (I/O) port addresses. For more information, see Chapter 9, “Managing Devices.”

Defining a Location

A location is information that the modem uses to analyze telephone numbers in international format and to determine the correct sequence of numbers to be dialed. It does not need to correspond to a particular geographic location, but it usually does. For example, a user on a portable computer might require a dialing prefix of “9” to dial an external number from an office location or require a dialing prefix of “*70” to disable a call waiting feature when placing calls from home. A location would be created for each dialing prefix and selected when dialing from each environment. Table 26-4 shows the information associated with a location.

Table 26-4 Location Information

Location Property


Location name

A recognizable name that identifies the location.


The country or region for the dialing location.

Area or city code

The calling prefix for the area code.

Dialing rules

Specifies which prefixes, if any, need to be dialed prior to dialing the area code and number, whether call waiting needs to be enabled or disabled, and if tone or pulse dialing method needs to be used to place the call.

Area code rules

Determines how phone numbers are dialed from the area code used in the current location to other area codes, or within the area code. For example, if your current location is area code 425 and all calls to area code 206 require the 206 area code to be omitted, you can create a rule to enforce this whenever phone numbers are passed to TAPI.

Calling card information

Specifies the calling card type, account number, and personal identification number (PIN) to be used for the location.

The first time you set up a modem, the Add Hardware Wizard prompts you for the default dialing information about the location from which you usually call (My Location), including your area code and country/region code.

To set default dialing location information
  • Run the Add Hardware Wizard, and then type the area code and country/region code information in the New Location dialog box.

    – or –

    In Control Panel, open Phone and Modem Options. Select the location (for example, My Location) to modify, and then click Edit. The Edit Location dialog box appears, as shown in Figure 26-5. Type the dialing information in the appropriate boxes, and then click OK.

    Figure 26-5 Edit Location dialog box

    Figure 26-5 Edit Location dialog box

After you install the modem, you can enter more specific location information, such as calling card numbers or rules for dialing outside your local area code, by editing the fields in the Edit Location dialog box. Additional dialing rules can also be created from this location. For more information about configuring dialing properties, see Windows XP Professional Help and Support Center.

Setting Modem Properties

In Phone and Modem Options, you can globally change default modem settings for all communications applications and tools created for Windows XP Professional. For example, if you do not want to listen to the modem speaker, you can turn it off for all tools and applications that use that modem. You can also adjust these settings individually within each application.

Note For Microsoft Windows 3.1–based or Microsoft MS-DOS–based applications, you need to configure the modem settings within each application.

To view general properties for a modem
  1. In Control Panel, click Phone and Modem Options.

  2. Select the Modem tab.

  3. Select the modem or device you want, and then click Properties.

Modem settings are listed on the General, Diagnostics, and Advanced tabs. Table 26-5 describes the General settings.

Table 26-5 General Modem Settings




A port is either a COM port or an LPT port to which an external modem is attached, or it is a COM port name that identifies an internal or PCMCIA modem. Windows XP Professional automatically assigns a port name (COM1, COM2, COM3, or COM4) to any device it detects. Usually, the port name is adjusted only if you move an external modem from one COM port to another. For PCMCIA modems, the port cannot be changed.

Speaker volume

Sets the volume for the telephone speaker, which broadcasts the dial tone, modem connection, and voices, if applicable, on the other end. To change the volume, move the slider bar to the right or left.

Maximum speed

Sets the speed at which Windows XP Professional communicates with the modem. It is limited by the central processing unit (CPU) speed of the computer and the speed supported by the communications port. Windows XP Professional selects a conservative default speed so that slower computers do not lose data during transfers.

You can set the speed lower if the faster rate causes data errors. Set it higher for faster performance. For example, 57,600 might work better than the Windows XP Professional default setting of 38,400 for v.32bis (14,400 bps) modems on fast computers. If applications report data errors, set a lower speed. (For example, change it from 38,400 to 19,200 for v.32bis modems.)

Dial control

Clear the Wait for dial tone before dialing check box if you are making calls from a country or region other than where your modem was purchased and your modem fails to properly detect the dial tone.

Tip If you have a slower computer and an external modem, you can install a 16550A Universal Asynchronous Receiver/Transmitter (UART)–based COM port adapter to increase speeds. Some internal modems have an integrated 16550A UART adapter.

The Diagnostics tab provides hardware information that can be used in hardware configuration and problem determination. Table 26 -6 describes the Diagnostics settings.

Table 26-6 Diagnostics Modem Settings



Modem Information

Displays manufacturer-specific information identifying the modem.

Query Modem

Click Query Modem to display your modem’s responses to standard AT commands sent to it. This information can be used to assist in troubleshooting.

Append to Log

Windows XP Professional records commands and responses to and from the modem in the Modemlog.txt file in the Windows folder. If the box is not checked, Windows XP Professional erases the old log and records a new log at the beginning of each call. If the check box is selected, Windows XP Professional appends new call logs to this file.

View log

The modem log is a powerful tool for diagnosing problems, particularly with connection problems. However, the interpretation of the contents of the file requires modem documentation, technical support, or experience with modems. The problems diagnosed might be in the local modem, its configuration, the telephone system, the remote modem (for example, the Internet service provider [ISP]), or in some combination of these items.

The Advanced tab of the Phone and Modem Options dialog box allows you to override the hardware and connection settings that were configured for the modem and serial port. The Extra initialization commands text box allows you to append to the standard initialization commands used to set up the modem at the start of a communications session. These can be standard AT-type commands or commands specific to your modem or communications device. Refer to the manufacturer’s documentation for a description of available commands.

The Advanced Port Settings button allows you to change the default configuration for the communications port used by your modem. The advanced COM port settings are available only for certain brands of modems. If you do not have one of these modems and want to change the assigned COM port, you must reinstall the modem and choose the desired COM port during the installation procedure. Table 26-7 describes the advanced port settings.

Table 26-7 Advanced Port Settings



Use FIFO buffer

A serial port containing a Universal Asynchronous Receiver/Transmitter (UART) chipset allows inbound and outbound information to be stored in associated first-in, first-out (FIFO) buffers until it can be received by the computer or dispatched by the modem.

The sizes of the inbound and outbound buffers can be enabled by checking the Use FIFO buffers (requires 16550 compatible UART) option. The sizes of the inbound buffer (Receive buffer) and outbound buffer (Transmit buffer) can be modified by using the slider bars.

Increasing the buffer sizes in 16550 UART-compatible serial ports can improve performance in high-speed modems. However, if you experience data loss or overrun errors, try lowering the buffer sizes or disabling the FIFO buffers.

COM Port Number

If Windows XP Professional detected your modem automatically, it assigned it to an available serial communications (COM) port. If you want to force the COM port assignment, select the available port here. For example, if you have three serial devices that are never used simultaneously, you can change the port settings and have all three devices share the same serial port.

Click Change Default Preferences to modify the default settings for call handling and data connection preferences. Table 26-8 describes the available settings.

Table 26-8 Change Default Preferences



Disconnect a call

Change the number of minutes listed in the Disconnect a call if idle for more than x minutes field if there is no activity on the line—for example, increase the number if you want to stay connected to a computer bulletin board even though there is no activity.

Cancel a call

Change the number of seconds listed in the Cancel a call if not connected within x secs field if it takes a long time to make a connection—for example, this might occur when you are making an international call and there are long delays before the call is connected.

Port speed

Determines the speed of the flow of data from the modem to the serial port. The speed is normally set correctly during modem installation; however, some modems can transfer data at a rate faster than the 115.2 Kbps supported by the standard serial ports for most computers. For more information, see your modem documentation.

Data Protocol

Enables error correction, allowing your modem to negotiate the error correction that is to be used for a communications session with another modem. Available error correction protocols are V.42, MNP4, MNP3, MNP2, or None.


Select Enable to allow hardware-based compression. Compression boosts transmission speeds by compressing data between the modems. This feature is available on most modems. When it is enabled, modems sometimes have trouble connecting. If this occurs, select Disable and try again. Using modem compression can sometimes reduce performance if the data being sent is already compressed by the application.

Flow control

Select Hardware for all external modems to avoid loss of data. If your modem cable has RTS (Request To Send) and CTS (Clear To Send) wires connected, you can use hardware flow control; otherwise, select None to use software flow control.

Default hardware settings can be changed by selecting the Advanced tab of the Default Preferences dialog box.

With hardware settings, connection settings usually correspond to what the computer on the other end is using. Therefore, do not change connection settings by using Phone and Modem Options. Rather, use a specific tool or application, such as HyperTerminal, to change these settings connection by connection.

Preferences include Data bits, Parity, Stop bits, and Modulation. For information about these values, see Windows XP Professional Help and Support Center.

If you have installed an external ISDN modem, an additional ISDN tab appears in the Phone and Modem Options dialog box. The ISDN settings must be configured before the modem can be used.

Configuring ISDN Support

Windows XP Professional provides built-in support for ISDN. Before configuring ISDN on a computer running Windows XP Professional, you need the following:

  • Installed internal or external ISDN adapter

  • ISDN telephone line service at the location where you use dial-up networking to connect to the Internet

  • ISDN telephone line service at the remote location to which you want to connect, usually either your ISP or a remote access server

If your ISDN adapter supports Plug and Play, Windows XP Professional automatically installs the required support. If the ISDN adapter is not automatically installed, use the following procedure to install the device support.

To install your ISDN device
  1. In Control Panel, open Add Hardware.

  2. In the Add Hardware Wizard, click Next.

  3. If Windows XP Professional does not automatically detect the ISDN adapter, select Yes, I have already connected this hardware and then click Next.

  4. Scroll down to the bottom of the list of installed hardware, select Add a new hardware device, and click Next.

  5. If you want Windows XP Professional to attempt to find the ISDN adapter, select Search for and install the new hardware (Recommended).

    - or -

    If you want to manually select the ISDN adapter, select Install the hardware that I manually select from a list (Advanced), and then follow the instructions.

After the device support for the ISDN adapter has been installed, you are prompted to provide the information necessary to configure ISDN support. Table 26-9 shows the information required to configure ISDN in Windows XP Professional.

Table 26-9 ISDN Configuration Information



Switch type

Most ISDN hardware adapters need to know the type of switch to which they are connected. The switch type simply refers to the brand of equipment and software revision level that the telephone company uses to provide you with ISDN service. The switch types listed are ESS5 (AT&T), National ISDN1, and Northern Telecom DMS 1000.

Service Profile Identifier (SPID)

The SPID usually consists of the telephone number with some additional digits added at the beginning and end. The SPID helps the switch understand what kind of equipment is attached to the line. If multiple devices are attached, it helps route calls to the appropriate device on the line. The SPID is generally used only within the United States and Canada.

Telephone number

In some cases, each B channel on an ISDN line has its own number, while in other cases both B channels share a single telephone number. Your telephone company tells you how many numbers are in your ISDN line. Separate numbers might be useful if you plan to take incoming calls on your ISDN line.

You can change the ISDN configuration information by performing the following steps.

To configure an ISDN adapter
  1. In Control Panel, double-click System, and then select the Hardware tab.

  2. Click Device Manager.

  3. Right-click the ISDN device whose settings you want to change, and then select Properties.

  4. Select the ISDN tab.

    To change the switch type, select an item in the list. To change the telephone number and SPID information, click Configure.

Configuring Client/Server Telephony Support

Windows XP Professional supports access and control of telephony features on a PBX by using a telephony server. An example of the architecture of a client/server TAPI implementation is shown in Figure 26-6.

Figure 26-6 Client/server TAPI architecture

Figure 26-6 Client/server TAPI architecture

The TAPI client is installed with Windows XP Professional. To specify the Windows 2000 or Windows Server 2003 TAPI servers to be used by the TAPI clients, you must use the Tcmsetup tool on Windows XP Professional. The Tcmsetup tool allows you to specify the servers that provide the telephony services used by the network.

The following section describes the configuration of a Windows XP Professional client to access the telephony server.

To identify telephony servers to a TAPI client
  1. Log on to the client computer with an account that is a member of the Administrators group.

  2. In the Run dialog box, type:

    tcmsetup /c telephonyserver1 [telephonyserver2]... [telephonyserverx]

    The variable telephonyserver is the name of the TAPI server.

  3. Click OK.

The parameters for the tcmsetup command are described in Table 26-10.

Table 26-10 Tcmsetup Command Parameters




Suppresses message boxes during setup.


Specifies connection-oriented callbacks. (The default is connectionless.)

/c telephonyserver

Sets the telephony server to be used by this client to telephonyserver. Multiple servers can be listed, with each name separated by a space.


Deletes the current telephony server list and disables TAPI services on this client.


Disables automatic server discovery.

The telephony client must be in the same domain as the telephony server, or it must be a member of a domain that is fully trusted by the domain of the telephony server.

The servers specified in the tcmsetup command override any previous telephony servers specified previously in the tcmsetup command. All servers required by the client must be specified in a single instance of the command.

The tcmsetup command can be performed only when you are logged on to the client with an account that is a member of the Administrators group. Alternately, if you logged on by using an account in the Users or Power Users group, you can use the runas command to perform the tcmsetup command as an administrator, as shown in the following line:

runas /user:mydomain\adminacct "tcmsetup /c servername"

Type the password for the administrative account when prompted.

Configuring TAPI IP Telephony

This section discusses the procedures necessary for configuring a Windows XP Professional–based client to access IP telephony services by using the H.323 protocol in an environment where an H.323 proxy or gateway is present. If your Windows XP Professional–based computer connects directly to the Internet, or an H.323 gateway is not used, this configuration is not required. Microsoft Internet Security and Acceleration (ISA) Server provides an H.323 proxy and gatekeeper.

Specifying the H.323 Gateway

The H.323 protocol incorporates support for placing calls from data networks to the switched circuit PSTN network, and vice versa by using an IP-PSTN gateway. The H.323 Telephony Service Provider provides support for gateway calling by using a static configuration option, accessible through Phone and Modem Options in Control Panel.

To specify the address of the IP-PSTN gateway
  1. In Control Panel, click Phone and Modem Options.

  2. Click the Advanced tab, and then select Microsoft H.323 Telephony Service Provider.

  3. Click Configure. Select the Use H.323 gateway check box, as shown in Figure 26-7, and then type the computer name or IP address of the IP-PSTN gateway in the text box.

    Figure 26-7 Configure H.323 Service Provider dialog box

    Figure 26-7 Configure H.323 Service Provider dialog box

The telephony application running at the gateway must conform to ITU-T H.323 v1.0 standards.

For information about the installation and configuration of an IP-PSTN gateway, see “Telephony Integration and Conferencing” in the Internetworking Guide.

Specifying the H.323 Proxy

The Microsoft H.323 TSP incorporates support for firewall traversal. Use Phone and Modem Options to specify the inner IP address of the firewall computer. This allows calls to be made and received across the Internet.

To specify the IP address of the H.323 proxy
  1. In Control Panel, click Phone and Modem Options.

  2. Click the Advanced tab, and then select Microsoft H.323 Telephony Service Provider.

  3. Click Configure. Select the Use H.323 proxy check box, and then type the computer name or IP address of the inner edge of the H.323 proxy or firewall computer in the text box.

Specifying the H.323 Gatekeeper

The Microsoft H.323 TSP incorporates H.323 gatekeeper support. A gatekeeper is a server in a network that manages client access to telephony services. A gatekeeper provides address resolution, call routing, call logging, and other services to other computers within the local communications network, or to external users. Use Phone and Modem Options to specify the name or IP address of the gatekeeper, log-on credentials (phone number or account name), and timeout and port values.

To configure the H.323 gatekeeper
  1. In Control Panel, click Phone and Modem Options.

  2. Click the Advanced tab, and then select Microsoft H.323 Telephony Service Provider.

  3. Click Configure. Select the Use H.323 gatekeeper check box, and then type the name or IP address of the H.323 gatekeeper in the corresponding text box. If they are needed, you can enter nondefault values in the text boxes for both the H.323 Call Alerting timeout and H.323 Call Listening Port.

Using Windows Directory Service

A directory service can be used to facilitate the process of making H.323-based IP telephony calls. The directory service provides transparent translation from either a user name or computer name to the associated IP address. For example, if you enter a known user name or computer name when making a call through an H.323-based IP telephony application, the directory service translates the information to an IP address. You can also use the directory service to publish IP multicast conferences.

Windows XP Professional uses a combination of the Lightweight Directory Access Protocol (LDAP) directory service and Active Directory for H.323-based IP telephony calls. The Internet Locator Service (ILS) of Microsoft Site Server is another directory service that can be used with TAPI applications on Windows 2000–based or Windows Server 2003–based computers. This service is distinct from Active Directory in that it is less scalable and does not provide persistent, centrally administered data storage.

Troubleshooting Telephony and Conferencing

You can use the following techniques and procedures to determine and resolve problems within telephony applications and in telephony device configuration, as well as problems with H.323 and multicast conferencing.

Troubleshooting Modems

The following sections detail troubleshooting procedures for analog and ISDN modems.

An analog or ISDN modem is not listed

If your modem is not on the Windows XP Professional Hardware Compatibility List (HCL) or is not detected by the Add Hardware Wizard, use one of the following procedures to install it:

  • Check the modem. If it is an external modem, make sure it is turned on and that all cables are tightly connected. If the modem is internal, verify that it is properly installed.

    If the modem is a Plug and Play device, open Device Manager in Control Panel and select Scan for hardware changes on the Action menu to reinstall the modem. To open Device Manager, open Control Panel and then double-click System. Click the Hardware tab, and then click Device Manager. If the modem is not a Plug and Play device, reinstall the modem using Add Hardware.

  • Obtain an .inf (installation) file from the modem manufacturer specifically for Windows XP Professional. Follow the manufacturer’s instructions for installing the modem in Windows XP Professional, or contact the modem manufacturer for assistance.

  • Install your modem as a standard modem by using the Add Hardware Wizard. This option provides basic dialing and connectivity support for the modem, although manufacturer-specific features might be unavailable.

Application cannot dial selected modem

If you cannot use an application to dial your modem, test the modem to verify that Windows XP Professional can connect to it. In the Phone and Modem Options dialog box, select the Modems tab, click Properties for the modem you want, and then select the Diagnostics tab. Click Query Modem to send a set of AT commands to the modem. If the modem response is not displayed in the Response area, perform the following steps to diagnose the problem:

  • If an external modem is experiencing problems, make sure that the serial cable connection between the computer and the modem is secure and that the cable is not broken or frayed.

  • Verify that Windows XP Professional recognizes your COM ports by displaying Device Manager. Verify that the COM port is not experiencing a hardware or resource problem (identified by an exclamation point icon next to the device listing) or has been disabled (identified by the international “No” symbol). If the connected port is listed without any additional icons, the COM port is recognized and available.

If the COM port is disabled in Device Manager, a hardware or configuration problem is likely. Use the following steps to troubleshoot the problem for an external modem:

  • Verify that the port is not disabled in the BIOS (also called the CMOS) setup of the computer. Refer to the documentation for your computer to obtain information about configuring options in the BIOS setup.

  • Make sure there are no other adapters or devices that are configured for the same base I/O address or interrupt request (IRQ) as the COM port to which the modem is attached.

  • Verify that the serial port is not defective. If the modem and any other serial devices fail on the COM port but work on other COM ports, and you have verified the two preceding steps, the serial port might be defective.

If the modem experiencing problems is internal, perform the following steps to diagnose and resolve the problem:

  • If the internal modem is not Plug and Play–compatible, it might use jumpers to specify the COM port. Make sure the jumpers on the modem are configured properly. There might or might not be jumpers that allow you to set the base I/O address and IRQ to be used by the modem as well. Verify that they are properly set. Some modems use a configuration application to change these settings.

  • If the modem is configured for a COM port number that is assigned to a COM port on the motherboard or a serial card (physical port), you must either set the modem to use a different COM port, or use the BIOS setup to disable the COM port that has the same number as the internal modem.

  • Make sure that no other adapters or devices are configured for the same base I/O address or IRQ as the internal modem.

  • Verify that the internal modem is not defective. Also, check with the vendor of your modem to see whether there is an upgrade available for your modem.

Troubleshooting PSTN Telephony

The following sections outline common problems and solutions for conventional (non-IP) PSTN telephony deployment.

Computer cannot find the telephony server

If the telephony server cannot be reached by means of the network—for example, a user cannot “ping” the telephony server—the following scenarios are possible:

  • The telephony server is not available or has not been correctly set up. Contact the administrator of the telephony server.

  • The Tcmsetup tool has not been run. Run the Tcmsetup tool with the /c parameter to specify the correct servers.

  • The Tcmsetup tool has been run, but an incorrect telephony server was specified. Run the Tcmsetup tool with the /c parameter to specify the correct servers.

  • The Tcmsetup tool has been run multiple times, overwriting the original telephony configuration. Run the Tcmsetup tool with the /c parameter, listing all telephony servers in the single command.

  • The Tcmsetup tool has been run with the /d (delete) parameter. Run the tcmsetup tool with the /c parameter to enable telephony services and to specify the correct server(s).

One or more clients cannot find a line for the telephony server

If one or more client computers cannot find the lines for a telephony server, it might be because they cannot be authorized for access to lines on the telephony server. When a TAPI application first accesses lines on the telephony server, the user context associated with the application process is authenticated. This means that those lines must have been configured on the server to allow access by that client. Contact the system administrator for the server.

After the lines have been configured, the new settings are not available until TAPI restarts on the client. Stop all client TAPI applications, and restart Windows XP Professional. When the client applications restart, they can find the newly assigned lines.

An application fails to start after you have canceled the Location Information dialog box

If an application fails to start after you have canceled the Location Information dialog box, the problem might be that address translation required by TAPI applications has not been specified. Use the Location Information dialog box to enter your country/region code, local area code, and pulse or tone and external line access settings.

A client cannot find a new line even though the server administrator has assigned the line to the client

When you assign a currently running client to a line on the telephony server, the new settings are not available until TAPI restarts on the client. Stop all client TAPI applications so that TAPI shuts down. When the client applications restart, they can find the newly assigned lines.

Troubleshooting Conferencing Applications

Users of H.323 or multicast conferencing might encounter problems connecting with other users or receiving audio or video.

Audio problems in conferencing applications

If audio problems occur in H.323 or multicast video conferences, the microphones or sound cards on the clients might be incorrectly configured or malfunctioning.

To diagnose sound hardware on Windows XP Professional–based computers, start the Sound Recorder application. In Accessories, point to Entertainment, and then click Sound Recorder. You can also open Sound Recorder by typing sndrec32 at the command prompt. Make a recording of your own voice using Sound Recorder, and then play it back. If there is no sound, make sure that the microphone is properly plugged in.

If the Sound Recorder test works properly but you continue to have audio problems, verify the sound settings by using Volume Control.

To verify sound settings by using Volume Control
  1. In Accessories, point to Entertainment, and then click Volume Control.

  2. In the Options menu, click Properties, and then click Playback. Make sure that the Wave and Microphone check boxes are selected. You might have to scroll the list to see these settings.

  3. Click OK.

  4. Select the Mute check box in the Microphone column if it is not checked. This prevents speech from being echoed locally (played back on the speaker’s computer).

  5. If the voices of all other conference participants are too loud or too quiet, adjust the Volume Control or Wave sliders downward or upward as needed.

  6. On the Options menu, click Properties, and then click Recording. Select all the check boxes in the list at the bottom of the dialog box. You might have to scroll the list to see these settings.

  7. Click OK.

  8. Select the Mute check boxes in all the columns except for the Microphone column if they are not already checked. Make sure that the Mute check box in the Microphone column is not selected. This allows your speech to be sent to the conference, but doing so also prevents other sounds, including those of other conference participants, from being transmitted from your computer.

  9. If other conference participants are dissatisfied with the level of sound, adjust the Microphone slider downward or upward as needed.

    Note A single incorrectly configured computer can cause audio problems or echoes for all other conference participants.

If you continue to encounter audio problems after adjusting the sound settings, make sure that the affected computers have full-duplex sound cards. Full-duplex sound cards can capture and play audio simultaneously, while half-duplex sound cards can capture and play only one at a time. Most modern sound cards are full-duplex, but many older sound cards are only half-duplex.

To check whether the sound card on your computer supports full-duplex audio, start Sound Recorder and record a speech sample for approximately 30 seconds. After this is complete, open a second instance of Sound Recorder. Play the sample you recorded using the first instance of Sound Recorder, and while this is playing, attempt to record a sample using the second instance of Sound Recorder. If the second instance of Sound Recorder cannot record a sample while the first instance is playing, the sound card does not support full-duplex audio, and thus does not work with TAPI.

If sound is distorted or otherwise continues to malfunction after you attempt the aforementioned procedures, the problem is most likely with the microphone, sound card hardware, or sound card driver. Contact the manufacturer of your sound cards to ensure that you have the most recent Windows XP Professional drivers. Also, replace the microphones and sound cards on affected computers and attempt these tests again.

Eliminating audio echo

Audio echo is a common problem with audio-conferencing systems. Audio echo is often more detectable when using a microphone and speakers, as opposed to using a headset, which has an integrated microphone and speakers. For example, echo can originate in the local audio loopback that happens when a user’s microphone picks up sounds from the user’s speakers and transmits them back to the other participants. Normal conversation can become impossible for other participants in the conference when sensitive microphones are used, speaker level is high, or the microphone and speakers are placed in close proximity to each other.

TAPI 3.1 applications written to use the acoustic echo cancellation capabilities of either the H.323 Service Provider or Multicast Conference Service Provider allow for the elimination of acoustic echo when using a microphone and speakers.

Another way to completely eliminate audio echo is to use audio headsets. These eliminate the possibility of a user’s microphone picking up sound that is being received from other conference participants.

A more expensive solution is to use special microphones with built-in echo-canceling capabilities. These microphones detect and cancel out echo. The main advantage to these is that users do not have to wear headsets. Echo-canceling microphones are also a necessity for conference rooms because using headphones is not a practical solution.

Video problems in conferencing applications

If the video image of an H.323 conference participant cannot be seen by the other party, or if the image of a multicast conference participant cannot be seen by all other endpoints, the computer’s video capture device might not be working properly. See the camera troubleshooter in Windows XP Professional Help and Support Center.

Unable to publish multicast conference invitations

If you cannot publish multicast conference invitations, confirm with your network administrator that a Windows directory service is available at your site. The directory service provides the ability to publish IP multicast conferences.

Additional Resources

These resources contain additional information and tools related to this chapter.