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), 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:

  • 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 24-1.

Table 24-1: SIP User Agents

SIP User Agent

Function

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 24-2 lists the different SIP servers and their functions.

Table 24-2: SIP Servers

SIP Server

Function

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 back 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 24-1 shows an example of an H.323 gateway.

click to expand
Figure 24-1: 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 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 provides 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 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 24-2 shows an example of a PBX system configured with a telephony server.

click to expand
Figure 24-2: 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:

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, 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 using existing copper telephone cabling that is provided by regional and national phone companies.

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. 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

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.

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

ISDN 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 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 of the enhancements TAPI 3.1 provides to TAPI 3.0:

Figure 24-3 shows the architecture of TAPI.

click to expand
Figure 24-3: 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, 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 24-3 lists the telephony and media service providers included with Windows XP Professional.

Table 24-3: Service Providers in Windows XP Professional

Service Provider

Function

H.323 Telephony Service Provider

H.323 Media Service Provider

Provides 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

Provides 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

Provides 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 non-marked 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.




Microsoft Windows XP Professional Resource Kit 2003
Microsoft Windows XP Professional Resource Kit 2003
ISBN: N/A
EAN: N/A
Year: 2005
Pages: 338
BUY ON AMAZON

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net