telephony fundamentals: an introduction to basic telephony...

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Telephony Fundamentals:An Introduction to BasicTelephony Concepts

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Telephony Fundamentals: An Introduction to Basic Telephony Concepts White Paper

Table of Contents

Executive Summary 1

Open Telephony Solutions 1

How Is Telephony Used? 1

Why Progress Was Rapid 1

Telephony Basics 1

What Is Signaling? 2

In-Band and Out-of-Band Signaling 2

CTI 4

First-Party and Third-Party Call Control 5

Application Programming Interfaces 6

Modular Media Processing Hardware 7

Web-Centric Voice Applications 7

VoiceXML and SALT 7

A Wealth of Options 8

Further Information 8

Standards 8

Intel® Telecom Products and Resources 8

Executive SummaryAs telecommunications has moved fromproprietary to open, standards-basedsystems, advanced voice solutions havegrown richer and more cost effective. Severalbasic telephony concepts are critical toworking with these solutions: call control,media processing, in-band and out-of-bandsignaling, and local, dedicated first-partycontrol versus shared, network-based third-party control. Once these concepts areunderstood, today’s modular, converged, andincreasingly Web-centric communicationstechnologies become easier to understand.

Open Telephony SolutionsOpen telephony solutions emerged with theintroduction of commercially-availablecomputer telephony technology in the mid-1980s, but the predominance ofexpensive, closed proprietary systems, whichwere used to handle functions such as voiceprocessing and computer-based fax, heldback their wide adoption.

Today open, standards-based telephonytechnologies are widely available and arerapidly becoming converged and Web-centric.

How Is Telephony Used?Basic telephony concepts are the foundationfor all advanced voice processing solutions.Here are just a few examples of suchsolutions today:■ Call centers

■ Self-service interactive voice response (IVR)systems

■ Unified messaging

■ Application media servers

■ Voice over IP (VoIP)

Why Progress Was RapidSeveral factors combined to simplify opentelephony systems significantly and acceleratetheir deployment:■ International standards for interconnecting

telephone and computer systems weredefined, such as the Computer-SupportedTelephony Application (CSTA) from the

European Computer ManufacturersAssociation (ECMA) for linking computers totelephone systems.

■ Application programming interface (API)specifications, which provide a set ofsoftware calls and routines that can be usedby an application to access communicationsservices, became widely accepted.

■ Voice processing technologies continued toadd advanced features and increased portdensity at attractive prices.

■ The deregulation of public networksencouraged new and innovative service andequipment providers to enter the marketsegment.

Today a new group of developers arebeginning to build voice applications using aWeb application infrastructure with standards-based interface languages such as VoiceXMLand SALT. A basic knowledge of how telephony works allows these developers toleverage their current skills to createconverged voice and data applications aseasily as they create data applications for thePC and Internet.

In addition, microprocessor technologycontinues to advance to the point whereprocessing no longer has to be performed onspecialized silicon but can take place on thehost processor of an off-the-shelf computer.This host-based media processing technologypromises to significantly reduce the total costof ownership of telephony equipment.

Telephony BasicsPublic and private telephone systems providereal-time information paths between two ormore parties. The wireline public system isgenerally referred to as the PSTN (PublicSwitched Telephone Network) and privatesystems are created with PBX (Private BrancheXchange) or KTS (Key Telephone System)switching technologies.

Traditionally, these public and privateinformation paths have taken the form of voiceconnections, originally through hardwiredanalog circuitry but later through an increas-ingly broad range of technologies such asradio transmission, digital signal encoding,


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and fiber. Over time, these transmission pathsalso came to be used for non-voice applica-tions such as fax and data transmission.

When equipped with the proper software andhardware, computers today send andreceive every kind of information thatpasses through the telephone network.Computers do all of the following: ■ Act as fax machines

■ Interact with human speakers through voicerecognition and synthesis

■ Provide gateways between different kinds ofnetworks, such as the PSTN and networksenabled with VoIP

It is this intersection, with the general-purposecomputing platform serving as the interfacepoint, which makes telephony so valuable.

What Is Signaling?The telephone network is a widelydistributed system of intelligentswitching nodes. Signaling is the process bywhich nodes communicate to establish andtear down calls so that two or more partiescan communicate via terminal equipment(such as a phone acting as an endpoint) andthe network.

The best-known scheme for terminal-equipment-to-network signaling is the DualTone Multi-Frequency (DTMF) protocol, underwhich the terminal equipment generatessimultaneous pairs of tones to represent eachdialed digit. Over time, more protocols weredeveloped to facilitate communicationsbetween switches and endpoints (known as“line connections”) and switch-to-switchcommunications (known as “trunkconnections”). Today’s signaling protocolstake the form of tones and digital messages,such as Channel Associated Signaling (CAS),Integrated Services Digital Network (ISDN),Signaling System 7 (SS7), and SessionInitiation Protocol (SIP).

Two functions of a voice application describehow it interacts with the rest of the telephonenetwork:

■ Call control – An application must

control how calls are established, reconfig-ured, and “torn down” (the telephony termfor concluding a call). Session control in adata network is similar. Picking up the tele-phone handset, pressing dialing digits, andlistening for the tones signaling the success-ful completion of a call are examples of“human” call control.

■ Media processing — An application mustsend and receive information through thecall endpoint interface, generating andreceiving the appropriate informationformats such as fax, voice, tones, or data.The human equivalent of media processingis speaking and listening to the other partyonce the call is established.

A basic voice mail system provides a simpleillustration of how call control and mediaprocessing interact. The system answersincoming calls, presents a greeting, andthen records the caller’s message. Sucha system consists primarily of media processing functions, with call controlfunctions limited to detecting a ring,answering the call, and hanging up after themessage has been taken.

More sophisticated applications such asautomated attendant require more complexcall control functions (call transfer, the abilityto initiate calls, and conferencing) andadvanced media processing such as speechrecognition and speech synthesis.

Call center applications typically utilizethe most advanced call control andmedia processing functions. These includespecial call control functions to monitorcalls as they pass through holding queues ontheir way to their ultimate destinations, andcomprehensive media processing functions inan IVR system, which allows some callers tocomplete their transactions without everspeaking to a human service representative.

In-Band and Out-of-Band SignalingAn accurate and reliable signaling connectionbetween telephone and computer systems isessential to successful voice applications.

Signaling on circuit-switched networks can

Whitep Paper Telephony Fundamentals: An Introduction to Basic Telephony Concepts

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take place “in-band,” that is, on the samechannel with voice communications (known asthe “talk path”), or “out-of-band,” thatis, through some communicationschannel other than the talk path. In today’stelephone network, terminal equipment or lineconnection signaling is generally in-band(except for ISDN devices), whilesignaling between telephone switches or trunkconnections is often done out-of-band forsecurity and performance reasons.

The most reliable way to implement signalingbetween telephone systems is to use out-of-band signaling, which creates a direct,message-based digital informationlink between the telephone switch and thecomputer-based application platform. Thisapproach is much more accurate than in-bandsignaling, under which the application mustattempt to generate and recognize widelyvarying and ambiguous analog signals inthe call’s talk path.

Out-of-band signaling is available in severalforms:■ The data channel (D-channel) associated

with basic and primary rate interfaces (BRIand PRI respectively) in ISDN lines

■ The proprietary digital signaling betweenPBXs and digital telephone sets

■ The switch-to-switch signaling protocolcalled SS7 used in public and largeprivate telephone networks

■ The Computer Telephone Integration (CTI)links available for many modern PBXs andsome public exchange switches

■ H.323 and SIP protocols used in VoIPnetworks

For an illustration of out-of-band signalingusing SS7 technology, see the call routingapplication in Figure 1. In this example, PartyA calls a special “virtual” telephone numberassociated with an endpoint, such as an 800number, triggering a number translationrequest from the switch through the SS7signaling network to the application system.The application system responds with anumber translation giving the actual telephonenumber of Party B. The switch then appliesstandard routing rules to make a connectionbetween Parties A and B. The talk paths arededicated to carrying the voice signal only.

Signaling protocols on VoIP networks (e.g., SIP, H.323, MGCP/Megaco) evolved


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ApplicationSystem

TelephoneNetwork

Party BParty A

SS7Subnetwork

TelephoneSwitch

TelephoneSwitch

TalkPaths

SS7Signaling

Paths

Figure 1. Out-of-Band Signaling Using SS7 Technology

from out-of-band, message-based protocolsof the PSTN. CTI links, now offered on mostmodern PBXs, offer a signaling mechanismthrough which a computer system can receiveconsolidated signaling for groups oftelephone extensions.

Telephone networks have existed fordecades. As a result, a wide variety ofstandard and proprietary protocols exist. Thishas made it difficult for systems developers toeffectively connect to telephone networks.

The good news is that today’s telephony andWeb-oriented development environmentsabstract application developers from many ofthe underlying complexities of the telephonenetwork. As long as developers understandthe capabilities and limitations of commonsignaling protocols, they can designsophisticated applications on a high level.

CTICTI links are also used for out-of-bandsignaling, but on a scale more suitable for thesmaller and relatively simpler environment of acustomer premises PBX or a single public

telephone exchange switch rather than in thecore of the PSTN.

CTI links often offer a broader range of callcontrol functions than commercial customer-premises SS7 services, including call initiationand hang-up as well as call routing. CTI linkscan operate using either a proprietary protocolor a standard protocol such as CSTA.

Figure 2 shows how CTI links can be used ina call center. The caller dials a number associ-ated with the call center’s T-1 circuit. Thepublic network delivers the call with associat-ed calling-party and dialed-number informa-tion. When the call arrives, the Automatic CallDistributor (ACD) sends a message to the CTIserver via the CTI link including this informa-tion. The CTI server passes the message tothe application system, which responds with arouting instruction. The CTI server passes thisrouting instruction to the ACD via the CTI link,and the ACD completes the call to thedesignated call center agent.

Because they provide access to sharedresources, both SS7-based connections and


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ApplicationSystem

TelephoneNetwork

Caller

Agent A

T1 CircuitACD

CTI Link

CTIServer

Agent B

Agent C

LAN

Figure 2. Out-of-Band Signaling Using a CTI Link

CTI links typically terminate in a server rather

than a specific application computer.

This allows multiple applications to

influence calls flowing through a common

telephone domain, and provides greater

flexibility for the computer systems on which

these applications can be installed.

First-Party and Third-Party Call Control

First-party and third-party call control are ways

of defining how an application handles a call.

In first-party call control, an application

performs call control functions locally. Here are

some examples:■ A computer equipped with a telephony

voice board and connected to a telephone

line senses a ring signal, answers the call,

and initiates a voice mail application to greet

the caller and record a message.

■ An incoming call is routed to a desktop

where an application on the desktop

computer uses the Caller ID information

accompanying the call to look up data aboutthe caller in a database on the PC. Theapplication displays the information as the callrings on the desktop phone or headset. SeeFigure 3. Calls can also be re-routed after aset number of rings to a pager, anotherdevice, or another location.

■ A user accesses a personal call list on adesktop PC and tells the application to initiate a call to someone on the call list.

In third-party call control, an application on aserver or switching device handles calls for agroup of users. Call control is handled on thenetwork and not locally, and media resourcesare shared. Here are two examples:

■ A server-based application monitoringseveral users’ telephone lines, without anactual physical connection to each of thoselines, sends an incoming call to one of thetelephones on the network based on pre-determined criteria. The application canalso initiate a conference among severalusers on the network or transfer calls fromdesktop to desktop.


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PBX

PSTN

Figure 3. First-Party Call Control Example

■ A server-based application uses the CallerID information that comes in with a call tolook up information on the caller in a net-work database and send that informationalong with the call to be displayed as thecall rings at a desktop. See Figure 4.

Third-party call control usually implies out-of-band signaling, since there is no direct con-nection between the computer system run-ning the application and the telephone linebeing controlled.

Third-party call control is far more efficient andless expensive. Since only a small number ofdesktop phones in an organization are in useat any given time, the application on thenetwork can manage a pool of incoming linesand provide service to a large number ofusers on demand.

Application Programming InterfacesA telephony API is the mechanismthrough which application softwaremanipulates telephony resources. APIsare necessary for both call control and mediaprocessing functions.

In the spring of 1993, Microsoft and Inteljointly introduced the Telephony ApplicationProgramming Interface (TAPI), whichstandardized the interfaces to telephonyapplications on the Windows* operatingsystem and removed the difficulties inherent inaddressing a variety of switches individually. Asimilar API was the Telephony Services API(TSAPI) developed by AT&T and Novell.

Although both TAPI and TSAPI wereimportant steps towards standardization, bothAPIs were oriented towards first-party use onthe desktop, that is, for local, non-sharedresources.

Today the industry is working towardsstandardized XML-based APIs such asCCXML and ECMA323 for speech and callcontrol and MRCP for media resourcemanagement. And as telephony movestowards a Web-centric model, XML-basedAPIs such as WSDL and SOAP are gainingimportance.


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PBX

PSTN

LAN

ApplicationServer

Figure 4. Third-Party Call Control Example

Modular Media Processing HardwareTraditional, proprietary systems useddedicated hardware. A first step towardsmodularizing telephony systems came withthe development of voice processing boardsimplemented in standard computing formfactors such as ISA and PCI. In addition,standards were created for pooling theseresources using a common bus architecture.Examples include SCbus, MVIP, and CT Bus(H.100/H.110).

Voice processing boards utilize digital signalprocessors (DSPs) as the engines for voiceprocessing. Thanks to Moore’s Law,microprocessor technology has advanced tothe point where many of these tasks can nowbe implemented on the host processor of anoff-the-shelf computer instead. Intel refers tothis technology as host media processing.

Today, host media processing software ismaking great strides in reducing costs.Although DSP price-performance hasimproved over the years, it has not kept pacewith the price-performance of standard siliconsuch as the Intel® Pentium® processor. Intel®

NetStructure™ Host Media Processing (HMP)software, introduced in 2002, moves mediaprocessing from the specialized DSP to astandard host processor, such as the Pentiumchip. This move offers key advantages:■ Lower cost of inventory and startup —

Initial capital investment is smaller

■ Lower development costs —Development systems do not requirespecialized hardware

■ Lower deployment costs — Software isless expensive to install and configure thanhardware

■ Lower sparing costs — Hardware can beused for multiple functions

■ Lower maintenance costs —Maintenance is easier and less training isneeded when system configurations arestandardized

Intel is actively working to migrate its broadportfolio of DSP-based voice processingproducts to host-based solutions.

Web-Centric Voice Applications

With the advent of the Internet over the pastten years, a new architecture, with new software tools, has emerged to support thedevelopment and deployment of Webapplications. “Web services” refers to aspecific aspect of Web applicationstechnology involving structured transactions,utilizing XML encoding, and flowing betweenWeb servers.

Increasingly, a Web-centric model is beingapplied to voice applications. Communicationssolutions have traditionally been standalonesystems that had little interaction with otherapplications in an enterprise. In contrast, aWeb-centric model provides importantintegration advantages. Data andcommunications systems can share physicalinfrastructure, development staff, and supportresources. Intel refers to this capability as“communications web services” (CWS),defined as the provisioning of communicationsservices so that they can be controlled andmanaged by Web-style applications.

VoiceXML and SALTJust as the growth of the Web was catalyzedby the creation of the HTML scriptinglanguage, the acceptance of standards forspeech services is propelling the adoption ofthis technology in both traditional and Web-based applications. Two emerginglanguage standards, Voice Extensible MarkupLanguage (VoiceXML) and Speech ApplicationLanguage Tags (SALT), enable developers todo the following:■ Write platform-independent applications that

handle synthesized speech.

■ Recognize spoken input and dual-tonemulti-frequency (DTMF).

■ Record spoken input.

■ Allow telephony control.

VoiceXML markup tags are specificallyintended for defining speech user interfaceswhile SALT markup tags can define “multimodal” user interfaces involving bothspeech and a range of devices such as agraphic display, mouse, keyboard, or pen.


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(Note that a recent extension of VoiceXMLcalled “X+V” is also intended to define multi-modal user interfaces.) VoiceXML defines acomplete standalone language with markupelements for defining a speech interface alongwith data and control flow. SALT defines asmall set of tags for creating a speechinterface within various markup environmentssuch as HTML, Wireless Markup Language(WML), Synchronized Multimedia IntegratedLanguage (SMIL), and others.

VoiceXML and SALT represent two differentapproaches to a speech markup language.Each provides open specifications and sup-ports the industry preference for known toolsand programming models.

A Wealth of OptionsOpen telephony solutions have grown fromsimple applications that used computingtechnology to enhance proprietary systems toflexible, diverse, and low-cost applicationsinvolving high-density, high-availabilitycommunications systems based on opentelephony standards, and, increasingly, Web-centric models.

Today telephony is at an important turningpoint. The basic elements of the technologyhave been developed and standardized.Drawing on its heritage in driving modularityand standards in the computer industry, Intelis applying the same principles to telephonyand communications and anticipates similarbenefits for solution providers and applicationusers alike.

Intel’s unique telephony experience allows it tosupply the same types of modular hardwareand software building blocks for thecommunications infrastructure as it does forthe computing infrastructure at several different levels.

■ Silicon — Intel is a very well-known siliconprovider. At this level in the communicationsindustry, Intel provides Intel® Architectureprocessors, specialized network processors,Ethernet interfaces, and specialized forms ofcommunications silicon.

■ Boards — In the computing industry, Intelsupplies reference motherboard designsand actual motherboards for companieswho do not want to create their own for avariety of reasons. In the communicationsworld, Intel can provide motherboards forswitches and routers, boards of variouskinds in different form factors such as PCIand CompactPCI, and design guides forcompanies that want to create their ownboards with Intel’s silicon. Intel is also at theforefront of work on new architectures suchas AdvancedTCA*.

■ Reference systems — In the computingindustry, Intel supplies complete servers orchassis with no branding on them so thatother companies can place theirapplications on the servers and their ownbrands. In the communications industry,Intel also provides “white box” communi-cations servers along with enablingsoftware, specialized switches and routerboxes, and design guides for companieswho want to design their own systemsaccording to what has worked well in the past.

Because Intel can provide many differenttypes of building blocks at many levels,companies can work with these at any levelthey wish in putting together converged andmodular communications solutions.

Further Information

Standards For information about standards and Intel’srelationship with various standardsorganizations along with links to the Web sitesof many of these organizations, visithttp://developer.intel.com/communications/corecomp/stdcomm.htm.

Intel® Telecom Products and ResourcesFor access to Intel telecom productinformation and resources such as whitepapers, design guides, and applicationsnotes, visit http://www.intel.com/go/telecom.


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