Dual Tone Multi Frequency Signalling - Specialty · PDF fileDual Tone Multi Frequency, or DTMF as it is popularly known, is the technical term for the sound frequencies produced when

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© Specialty Answering Service. All rights reserved.

Contents

1 Introduction .......................................................................................................................................... 2

2 History and Evolution ............................................................................................................................ 3

3 Technology ............................................................................................................................................ 5

3.1 DTMF Keys ..................................................................................................................................... 5

3.2 DTMF Frequencies ........................................................................................................................ 6

3.3 Special Tones ................................................................................................................................. 7

3.4 DTMF Transmission ....................................................................................................................... 8

3.5 DTMF Programming ...................................................................................................................... 9

4 Technical Standards ............................................................................................................................ 11

5 Applications ......................................................................................................................................... 12

5.1 Applications in the Call Center Industry...................................................................................... 13

6 VoIP and DTMF ................................................................................................................................... 14

7 Advantages of Using DTMF ................................................................................................................. 15

8 Conclusion ........................................................................................................................................... 16

9 References .......................................................................................................................................... 17

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1 Introduction

DualToneMultiFrequency,orDTMFasitispopularlyknown,isthetechnicaltermforthesoundfrequencies produced when a telephonic key is pressed. DTMF also known as touch tone wasprimarilyusedfortelephonesignalingtoandfromthelocalexchange,thoughtodayitfindsseveralapplicationsinthefieldoftelecommunicationsandcallcenters.

Adifferent frequency isassignedtoeachkey in thetelephoneandtherearetwotones–one lowfrequencyandanotherhighfrequency–thatareplayedsimultaneouslywhenakeyispressed.Thiscombinationoftwotonesmakesitnearlyimpossibletoreproducebythehumanvoice.

Eachofthefourrowsofkeysinatelephoneisassignedalowfrequencytoneandeachofthethreecolumns is assigned a high frequency tone. A fourth column of keys labeled as A, B, C, andD isoptionalandismostlyusedinmilitarynetworks.

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2 HistoryandEvolution

Before DTMF was introduced, telephone networks were dependent on pulse dialing. In pulsedialing, pulses are generated by rapidly connecting and disconnecting the telephone connection.Typically,eachnumbercorrespondedtothesamenumberofclicks,thus1wasrepresentedbyoneclick,2bytwoclicksandsoon.Thenumbersweredialedwithashortgapbetweeneachdigit inordertoavoidthedigitbleedingintothenextdigit.Pulsedialinghadthedisadvantagethatitwaslimited to the local exchange connections and required an operator for connecting longdistancecalls.Itwasalsoeasytotrickthephonesystembytappingthehook,whichispusheddownwhenthetelephoneishungup.

ResearchonDTMFwasinitiatedbyBelllabsinthelatefiftiesinanattempttoallowtonesignalsforlongdistancedialing.DTMFwasbeingdevelopedasthefutureofelectronictelecommunicationsasopposedtothemechanicaltelecommunicationnetworksthatwereprevalentatthattime.

DTMFwasintroducedonNovember18,1963underthetrademarkof‘TouchTone’byBellSystems.ThefirstTouchTonetelephonewastheWesternElectricmodel1500withtenbuttons,whichwasintroducedonthesameyear.UsingaudiotonesforsignalingwasnotnewandtheMulti‐Frequencysignaling(MF)whichwasusedbytelephoneexchanges forcommunicatingwitheachotherusingin‐band signalingwas already around.Multi‐frequency signalinguses a combination of twopuresine wave frequencies for signaling. Both CCITT and Bell System devised various MF signalingprotocols.Thein‐bandsignalingbetweenexchangeswasbasedonasixteen‐digitkeypad,whichatelephoneoperatorusedtoinputthenextlegofthedestinationtelephonenumberforconnectingtothedownstreamtelephoneoperator.Thissemi‐automatedsignalingandswitchingtechnologyhadthetwinadvantageofcostandtimeeffectivenessasitwasfasterandcheapertouseMF.

TheMFtechnologywasagreatsuccessamongspecialistsforestablishinglongdistancetelephonecalls.TheTouchTonewasanextensionofthistechnologytotheendconsumerandwasofferedasa‘modernway’ofplacingcallswhereconsumerscandialthedestinationtelephonenumberdirectlywithout having to talk to a telephone operator. AT&T described the product as "a method forpushbutton signaling from customer stations using the voice transmission path”. EngineerswhodesignedtheTouchToneexperimentedwiththekeypadlayoutwhichwouldreplacetherotarydialontelephonesetsandthecurrentlayoutwith1intheupper‐leftand0atthebottomwaschosenaftertestingnearlyfifteendifferentlayouts.Duringtheinitialdays,mechanicalswitcheswereusedinthekeypadstoselectdifferenttapsontunedcoilsandthemicrophonewasdisconnectedeverytimeakeywaspressedinordertopreventthebackgroundnoisefrominterferingwiththeDTMFtones thatwere transmitted.Eachbuttonactivated twocontacts, and the systemwas calleddualtonemultifrequencybecauseofthetwotonesandmultiplefrequenciesgenerated.Thetoneswerethendecodedbytheswitchingcentertoidentifywhichkeywaspressed.

WhenitcametoselectingthefrequenciesforDTMF,theengineerswantedasetoffrequenciesthatwould not interfere with the MF based switching between telephone exchanges. Accordingly,

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DTMF’s frequenciesweredifferent fromallpre‐existingMFsignalingprotocolsat that time–R2,R1,CCS4,andCCS5.

DTMFasusedinconsumertelephonepush‐buttontonedialingwasknownbythetrademarkTouchTone.OthervendorswhomanufacturedequipmentcompatiblewithDTMFcalledthefeatureTonedialingorusedtheirownregisteredtrademarks(asinthecaseof‘Digitone’ofNorthernElectric).

The initial research team had also examined the possibility of accessing computers throughtelephonelinesandsurveyedseveralcompaniestoidentifytheirneedsforthis.The#(hash)and*(star)keyswereaddedbasedontheinputsreceived.AsetofletteredkeysA,B,C,andDwerealsoaddedtofacilitatemenuselection,thoughitwaslaterdroppedfrommostphones.However,itwasseveralyearsbeforethesekeyswerewidelyusedforsuppressingcallerIDsorforenteringservicecodes.Itwasalsousedbypublicpayphonesthatwouldacceptcreditcardsforsendinginformationfromthemagneticstrip.

Yetanotherusefortheletterkeyswasbythemilitary,whereitwasusedtoprioritizethecalls.Therelabeledletterswereusedtodefinethepriorityofthecallsbeforedialing,inordertocutcurrentlyactive calls, if required, allowing themore important traffic to flow through uninterrupted. ThelevelsofpriorityavailablewereFlashOverride(A),Flash(B), Immediate (C),andPriority(D), indecreasingorderofpriority.Pressinganyofthesekeysprioritizedthecall,overridingothercallsonthenetwork.Forexample, ifBwaspressedbeforedialinganumber, the telephoneswitchwouldfirst look for available lines and if unavailable, try to disconnect any no‐priority calls. If alsounavailable,thenitwoulddisconnecttheDandCprioritycallsinthatordersoastofreealineforthis call. A Flash Override would stop all other calls on the trunks between the origin anddestinationofthecurrentcallandhenceitwaslimitedtotheWhiteHouseCommunicationsAgency.However, this method of precedence dialing is largely obsolete today as military networks usenumbercombinationstoprioritizecalls.

OnceDTMFwaslaunched,pulsedialinglostitssignificance.Asaresult,DTMFcontrolledswitchingsystemswereintroducedintelephoneexchangesinlieuoftheequipmentcircuitsthatwereused.Customer telephonesetswerealsoupgradedtouseDTMFcircuitsandtonedialing.Several testswereperformedduringthesixtiesontheDTMFsystemanditwasmadeofficialduringthattime.Sincethen,DTMFhasbeenusedasthedefactotelecommunicationsdialingandswitchingsystem.

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3 Technology

DTMFisaveryreliablemeansofsignalingusedbytelecomcompaniestoprocessinformationfromcustomers.Wheneveranumber isdialed, theDTMFisdecodedbythe localexchange inorder toroutethecall.

DTMFcanbetransmittedovertelephonelinesaswellasovertheinternet.Thetonesaredecodedonthereceivingendandusedforpracticalapplicationssuchasinteractingwithcomputersystemsandansweringmachines.TheinteractionwithacomputersystemisachievedusinganIVRsystem.Over a regular landline, DTMF is sent as audio signals. DTMF tones are transmitted through thesamewiresthatcarrythevoicesignals.Inthecaseofmobilephones,DTMFtonescanbegeneratedonly after the connection is established. This feature is often used by call centers for gatheringinputs fromcallers for selectionof IVRmenus, capturingaccount information forphonebankingfacilities and so on. In a VoIP phone, DTMF is transmitted as a data packet over the internet,howeverthisispronetoerrorssuchasechoorpacketlossduringtransmission,distortingthedataandmakingitdifficulttorebuildthekeypresscombinationatthereceivingend.

DTMFtechnologysupportsacoustictransfer.ThismeansthatDTMFtonescanbetransmittedbyastandardspeakerandreceivedbyastandardmicrophone.

Speech recognition technologies are another alternative used by call centers to offer the callersmoreflexibilityforprovidinginputsduringacall.However,speechrecognitiontechnologieshavestilla longway togo toachieveahighdegreeofaccuracyand thereforemostcall centerspreferDTMFforuserinputsasitismorereliable.TheflipsideofusingDTMFisthatwithjust16distincttones,therearealimitednumberofpermutationsthatcanbeusedtotransmitinformation.

3.1 DTMFKeys

When the DTMF enabled telephone setswere introduced, AT&T attempted to remove the lettercodesthatwereaffixedtothedigitsintheolderpulsedialphones.Thus,theTouchTonedevicesinthe early sixties did not have letters above the digits. However, the use of named telephonenumbersinadcampaignsforcedAT&Ttobringbackthelettercodesonthedigitbuttons.Typically,telephonesets(bothrotarydialandtonedialones)donothaveanylettersassociatedwiththedigit1.SomesetsdidhavethelettersQandZonthe1digit,butthisismoreofanexceptionratherthantherule.SeveralcellularphonesputQonthedigit7andZonthedigit9,butthisagainisdifferentfrom land phones. As a result, marketing campaigns with named telephone numbers are bestadvised to avoid the use of the letters Q and Z. The letter codes assigned to each of the twelvenumerickeysonastandardkeypadisshowninthetablebelow:

Key Character

1 1

2 A,B,C,2

3 D,E,F,3

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4 G,H,I,4

5 J,K,L,5

6 M,N,O,6

7 P,Q,R,S,7

8 T,U,V,8

9 W,X,Y,Z,9

0 (Zero)

* (Clear)

# (Enter)

3.2 DTMFFrequencies

There are sixteen DTMF signals, each of which is made up of two tones from eight differentfrequencysignals.Twelveofthesearecommonlyusedbyconsumerswithfourbeingreservedformilitaryuseorusebyexchanges.ThekeysA,B,C,Dareusuallyabsentfromtelephonesetsusedinhomesandoffices.Thesekeysaresystemtonesused forconfiguringtelephoneexchangesandtocarryoutspecialfunctions.TheDTMFkeypadforconsumeruseisdesignedinafour‐rowbythree‐columnmatrix.Eachdialrowisrepresentedbya lowtone frequencyandeachcolumnbyahightonefrequency.

Thefrequenciesusedare697Hz,770Hz,852Hz,941Hz,1209Hz,1336Hz,1477Hz,and1633Hz.Thefrequencieswerecarefullychoseninsuchawayastopreventharmonics.Thus,onecannoticethatnofrequencyisamultipleofanotherandthedifferenceorsumbetweenanytwofrequenciesisnot equal to any other frequency. The frequencieswere initially designedwith a ratio of 21/19,whichisslightlylessthanawholetone.

Additionally,thefrequenciesgeneratedhavetobewithinanerrortoleranceof1.5%andthehigherfrequencyistransmittedat3dBloudertocompensateforanyhighfrequencyroll‐off.Thepairofsignalsrepresentsthedigitorsymbolattheintersectionoftherowandcolumn.Forexample,ifthedigit 5 has to be sent, the frequencies transmitted are 1336 Hz and 770 Hz in a sinusoidalcombination.

1209Hz 1336Hz 1477Hz

Optional

Consumer Military

1633Hz

697Hz 1 2 3 A FO

770Hz 4 5 6 B F

852Hz 7 8 9 C I

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941Hz * 0 # D P

Itisnecessarythattransmissionpathsarelinearanddistortionfreeinordertopermittheaccuratetransmission of DTMF signals. Any distortion between the source and the decoder can result ininter‐modulation products and consequent unreliable decoding. An example is the 2A‐B inertmodulationcalculationwhereadistortion in the transmissionpathcancauseastar (941Hzand1209Hz)tobedecodedasapound(941and1477).HereA=1209andB=941andtheformulawillgive2A‐B=1477whichrepresentsapound.

Whenwetalkabouta‘DTMFsignal’,itdoesnotrefertoanelectricalsignaloranaudiosignal.Itisacombinationoftwosinewavesthatcanberepresentedinanyform.Forexample,whentransmittedfromamobilephone,theDTMFstartsasanelectricsignalgeneratedfromtheaudiosignalthatisthenencodedonanelectromagneticwaveduringtransmissionandisagainconvertedbacktoanelectricsignalatthereceivingend,whichisfinallyconvertedbacktoanaudiblesoundsignal.

A DTMF Decoder is an Integrated Circuit (IC)which can decode the incoming DTMF signal andshowthecorrespondingkeyasafourbitdataoutputwhichisalsoknownasanibbleorhalfbyte.

3.3 SpecialTones

Other than the tones for thedigitsdescribed in theprevious section,national telephonesystemsusuallydefine special tones todenote the status of the equipment, the lineor the result of calls.Thesetonesmayconsistofasinglefrequencyortwofrequencies.Thesearestandardizedforeachcountry. Most special tones in the US are based on a dual frequency systemwhereas Europeannationsuseasingle frequency formostevents.Thetablebelowindicatesthe frequenciesused inUS:

Event Lowfrequency Highfrequency

Busysignal 480 Hz 620 Hz

Ring‐backtone 440 Hz 480 Hz

Dialtone 350 Hz 440 Hz

TheEuropeantonesaregiveninthetablebelow.MostEuropeannationsusethestandard425Hzfrequencytorepresentthebusysignal,thering‐backtone,andthedialtone.

Event Lowfrequency Highfrequency

Busysignal(UK) 400 Hz ‐‐‐‐

Busysignal(RestofEurope) 425 Hz ‐‐‐‐

Ringbacktone(UK&Ireland) 400 Hz 450 Hz

Ringbacktone(Rest ofEurope) 425 Hz ‐‐‐‐

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Dialtone(UK) 350 Hz 440 Hz

Dialtone(RestofEurope) 425 Hz ‐‐‐‐

3.4 DTMFTransmission

DTMFwasoriginallydesignforcontrolsignalsandnotfordatatransfer.Asaresult,thefocuswasonchoosingfrequenciesinsuchawaythatfrequencyfilterscanbeeasilydesignedfortransmissionthroughstandardtelephonelines.Thebandwidthavailableforatelephonelineisbetween300Hzand 3.5 KHz and the frequencies chosen were within this range. With the help of anencoder/decoder,aDTMFcanbeusedfordatatransfer,albeitataveryslowrateoftentonespersecond.Ideally,aDTMFtoneshouldbeplayedforat least50millisecondswitha50millisecondsintervalbetweentwosuccessivetones.

DTMFsignalscanbe transmittedeithermanuallyorautomatically throughaDTMFencoder. It isfaster to use an automatic encoder, which presses the digits much faster than manual DTMF.AutomaticDTMFencoderscanbeprogrammedtostoretheDTMFcodeinthedevice.ThiscanlaterbeforwardedtosendanentireDTMFstringofalphanumericcharacters.Hence,thesedevicesarealso known as ‘Store and Forward’ encoders. Typical examples of a ‘Store and Forward’ DTMFencoderincludeaprogrammablemicrophoneoratwo‐wayradio.

Atthetransmitterend,thesignalstrengthoftheDTMFlowfrequencytoneshouldbebetween+1dBmand‐10.5dBmandthatofthehighfrequencytoneshouldbebetween+1dBmand‐8.5dBm.Thefrequenciesgeneratedbythetransmittedshouldnotvarybeyond1.5%ofthedefinedvaluesandthereceivershouldnotacceptanydeviationthatisgreaterthan3.5%fromthedefinedvalues.Anyleakageoftonemustnotexceed‐55dBm.

Atthereceivedend,thereareseveralchecksperformedbeforeinterpretinganincomingsignalasaDTMFsignal.

1. Twosimultaneoustones–onefromlowfrequencyandonefromhighfrequencyhastobepresentandthetonesshouldpersistforatleast40milliseconds.

2. Otherfrequenciesmustbeabsentorlessthan‐55dBm.

3. Theremust be an interval of at least 40milliseconds between two signals duringwhichthereisnoDTMFfrequency

4. Theminimumdutycycleis85milliseconds.(sumoftonedurationandintervalduration)

5. Thesignalstrengthshouldbebetween‐25dBmand0dBm

6. Thehighfrequencytonemustbe‐8dBmto+4dBmrelativetothelowfrequencytone.

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ThedurationofaDTMFsignalisknownasMarkandtheintervalisknownasSpace.Thedifferenceintransmissionlevelsbetweenthelowandhighfrequenciesisknownastwist.Errorscausedduetoincorrectimplementationofthisareknownastwisterrors.Iftheamountoftwistisgreaterthanthereceiver’stolerancelevels,thenthesystemwillfailtodecodethebuttonspressed.Thisisoftenmisdiagnosedasaproblemwith the repeater controller, though theactualproblem lieswith thereceiver.Moderndecodersaredesignedinsuchawayastoeasilyovercomethisanddeciphertherightdigitsdespitetwisterrors.

The tonedurationand the inter‐digit intervalmentionedaboveareasperAT&Tstandards.ANSIstandardT1.401‐1988definesminimumtonedurationas50milliseconds,interdigitintervalas45milliseconds and duty cycle as 100 milliseconds. Further, popular telephone equipmentmanufacturers and modem manufacturers typically generate DTMF tones of 70 milliseconds ormoredurationinordertoavoiddialingtroubles.

3.5 DTMFProgramming

There are severalways inwhichDTMF sequences canbeused toprogramcharacters. The tablebelowshowshowalphabetsandnumeralscanbetransmittedusingastandardDTMFsequence.

Character DTMFSequence

0 0

1 1

2 2222

3 3333

4 4444

5 5555

6 6666

7 77777

8 8888

9 99999

A 2

B 22

C 222

D 3

E 33

F 333

G 4

H 44

I 444

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J 5

K 55

L 555

M 6

N 66

O 666

P 7

Q 77

R 777

S 7777

T 8

U 88

V 888

W 9

X 99

Y 999

Z 9999

Clear,Reset,Back,Exit

*

Enter,Ok,Next #

However,thesequencevarieswiththeequipmentorapplicationthatisbeingused.Theabovetableshows the sequenceona standard3X4keypadwhere thedigit1hasnocorrespondingalphabetvalue and every other digit has three or four alphabets and one numerical digit associatedwiththem.However,ifakeypadweredesignedinsuchawayastoassignanalphabeticvaluetothe1key,thenallothervalueswouldchangeaccordingly.

When programming with DTMF, the tones are usually repeated until the specific character isdisplayed in the display panel of the equipment. Once a character is entered a * or # is enteredbeforethenextcharacterisprogrammed.The*keyisusedfordeletingcharactersand#isusedtoindicateanewcharacter,thoughthiscanvarywiththecharacteristicsofthereceivingequipment.

Apartfromprogrammingwords,DTMFstringscanbealsousedtoperformcertainfunctionsonanapplicationorequipment.

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4 TechnicalStandards

Inordertoensurethatdevicesandapplicationsmanufacturedbydifferentvendorsarecompatiblewith each other, several technical standards have been defined for DTMF technology. DTMF isstandardizedby ITU‐TRecommendationQ.23 titled ‘Technical featuresofpush‐button telephonesets’ and it provides a complete set of standards for all applications that intend to use DTMFsignaling.

The ETSI Standard ES 201 235 is a fairly detailed standard which has been developed inconformationtotheITU‐TRecommendationQ.23specifieshowDTMFsignalingsystemcanbeusedfor transmitters and receivers. The standard applies to DTMF signaling over LAN (local areanetwork)where the transmissionpath is a two‐wire analogue subscriber line aswell asover anend‐to‐end transmissionpath.This allowsmanufacturers of telecomequipment todesign robustequipmentthatfacilitatesreliableDTMFsignaling.

Other technicalstandardswhichapply toDTMFsignaling includetheEuropeanStandardEN300659whichhasanannexthatdetailsDTMF‐basedsubscriberlineprotocolforthesupportofPSTNdisplayservicesattheTerminalEquipment.

Intheinitialdays, just likeothermulti‐frequencyreceivers,DTMFwasdecodedusingtunedfilterbanks. Later, thesewere replacedwithdigital signalprocessors.Althoughany frequencydomaintransform such as the fast Fourier transform can be used for decoding DTMF, the GoertzelalgorithmisapreferredchoiceduetoitshighperformanceforDTMFsignals.

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5 Applications

DTMFdecodersaremuchslowerthanagoodcommunicationmodem.Whiletheformercanprocessnearly tentonespersecond(or fivebytesofdata),amodemcantransmitnearly3000bytespersecond.Still,becauseof itsdurabilityundernoisyconditions,DTMFsignaling is stillpreferred inseveralcases.DTMFtonescanalsobeusedtotransfercallerIDinformationintelephonesystems.

The primary use of DTMF tones is by a telephone exchange for the identification of the dialednumber.DTMFtoneshaveawidevarietyofapplicationsinareassuchasvoicemail,phonebanking,helpdeskservices,andothertypesofinboundcallcenters.DTMFtonesarealsousedbyradioandcableTVnetworkstosignalwhenlocalinsertions(localnewscontentorads)canbemade.Itcanalsobeused for station identification aswell as for switching the remote transmitters onor off.RadiochannelsalsouseDTMFtonestotransfertheaudiotolocalairwaves.DTMFtoneswereoftenheardduringastationidentificationprocess,whichtypicallyprecededalocalinsert.

TheA,B,C,andDkeysarerarelyusedonpresentdaytelephonenetworks,butsomenetworksstilluse them for network control purposes. For example, the A key is used to listen in on calls bycyclingthroughdifferentcarriersatwill.(Thisisprohibitedbymostcarrierstoday).Thesekeysarealsousedinradiophonepatchandrepeateroperationstocontroltherepeaterwhileconnectedtoanactivephoneline.Thesekeys,alongwiththe#and*keys,arealsowidelyusedbyamateurradiooperatorsforrepeatercontrol,VoIPbasedcommunications,andremote‐baseoperations.

EarlierVHStapes(VideoHomeSystem)usedDTMFsignalingtoencodeinformationonthemastervideo tape thereby providing information on settings such as format, duration, volume etc. toautomaticduplicationmachines.Thisallowedbetterreplicationofthemastervideotapesandthesesignalingtonescansometimesbeheardatthebeginningandendofsometapes.

DTMFsignals canalsobe transmittedover a radio to control turning thingsonandoff.Practicalapplicationsincluderemotecontrolofhomeappliances,activationofwarningsystems,openingandclosingofdoors,startingandstoppingofmotorsandingeneralanybinaryaction.

Using a two‐way radio, a number can be transmitted using DTMF tones. A DTMF decoderprogrammedwiththesamenumberatthereceivingendandconnectedtoareceiverattheremotelocationcanthenbeactivatedandusedtocontroltheequipmentthathasbeenconnectedtoit.Byadding Wildcard characters to the number, information can be embedded in the number thusenablingmulti‐locationcontrolusingasingleDTMFsignal.Forexample,assumethatthelightsinyourhousearecontrolledusingDTMFcodes.IfeachlightismappedtoasingleDTMFsignal,andyouhavefifteenlightsinyourhouse,theninanemergencyifyouwantalllightsswitchedon,youcannotwaitforfifteencodestobetransmitted.Insuchacase,thedecoderwouldbeprogrammedtoallowindividuallightstobecontrolled,whilealsoenablingaMasterSwitchtoturnonalllightssimultaneouslyusingaWildcard.AnotherWildcardmaybeprogrammedtoswitchonalllightsinasingleroom.Thus,groupandsub‐groupcontrolsarepossibleusingDTMFcontrol.

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Similarly, to receive information from a remote site, a transmitter can be added to the two‐wayradio.ThisisknownasatalkbacksystemorSCADAsystemanditcansendinformationbacktoyouviaDTMF,whichcanthenbeusedforstatusindicationorforfeedingintothecomputerforfurtheraction.Thus,DTMFisatrulyinteractivesignalingformat.

5.1 ApplicationsintheCallCenterIndustry

Inthecallcenterindustry,DTMFisprimarilyusedtoprocessuserrequests,toobtainidentificationinformationandtoroutephonecallsusingthekeypadentriesmadebytheuser.DTMFtechnologyisusedinconjunctionwithIVRsoftwareandACDsystemstoofferself‐servicemenuoptionstocallcentercustomers.Ina2005DatamonitorPoolSurveyofover100contactcenterdecisionmakers,to identify the areas and type of calls that they prefer to automate using DTMF, the top threeresponses were call routing, information access/FAQ, and financial transactions/billing. DTMFsignalingisusedinseveralcomputer‐telephonysoftwareapplicationsaswell.

Evenwith theadventofspeechrecognitionbasedapplications,mostcontactcentersoffercallersthechoicetouseeitherDTMForspeechinputtocaptureuserinputs.DTMFinputsareoftenusedinIVRbasedapplicationsformenunavigation,obtainingnumericalinputs(suchasaccountnumber,dateofbirthPINnumberetc.)orevenforrecognizingadigitalpattern.

Whilescripting,oneofthefollowingmethodsistypicallyusedfordeterminingwhentheuserinputiscomplete:

TerminationCharacter:Acallerspecifiestheendofaninputbyenteringaterminationkey,whichistypicallythe#key.

TimeOut: In this case, an input is considered complete, if a specified time has passedwithoutthecallerpressinganykey.

SpecifiedNumberofCharacters:Inthiscase,theinputisconsideredcompleteafterthecallerhas entered a fixed number of tones. This can be used for fixed length inputs such as incaseswheretheaccountnumberissaysixdigitslong.

Mostapplicationswillhavedefaultsettingsforeachoption,whichcanbeoverriddenusingscripts.

MenunavigationusingDTMFisachievedbyaskingtheusertopressakeytoselectamenuoption.Dependingonthemenuchoiceselected,theapplicationcanthenroutethecalltoaspecificgroupofagents,basedontheskillsetrequirements.

DTMFisusedtoobtaininformationfromcustomers,andisemployedinawidevarietyofindustriesranging from financial institutions such as banks and credit card companies to healthcare andinsuranceorganizations.

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6 VoIPandDTMF

There are multiple ways in which DTMF tones are transmitted over a VoIP connection. Theseinclude:

Inband–Inthismethod,theDTMFtonesaretransmittedjustasnormalvoiceaudiousingthesamecodecandwithnospecialcodingtoidentifyitasdifferentfromthevoice.

RFC2833–Inthiscase,DTMFistransmittedbasedonthedefinedstandardsforDTMFtones,faxtonesetc.

SIPsignaling–ThiscanbedoneonlythroughSIPchannelswhereDTMFtonesaretransmittedthroughanSIPmessageafterahandshakewith thereceivingend fornegotiatingpreferencesandestablishingtheparametersforthecall.

DTMFoverVoIPismorepronetoerrors,asthereceivingendoftenfailstorecognizetheDTMForinterpretsit incorrectly.Oneofthepossiblecausesoftroubleiswhenthereisswitchingbetweenin‐band DTMF and RFC2833 modes in the VoIP route. For example, assume a VoIP device istransmitting an in‐band DTMF signal and a switch converts it to RFC2833 DTMF for furthertransmission, which finally terminates in a system that requires a regeneration of the in‐bandaudible tones. Sometimes, the switch fails to completely remove the audible DTMF tones whileconverting toRFC2833, thus resulting in a combined garbledmessage receivedby the voicemailsystem, causing it to report an error. Yet another cause of concern is packet loss,which createsissuesinDTMFoverVoIPtransmissions.

ChangingtheDTMFsettingsisonewayoftroubleshootingsuchproblems,andtheRFC2833DTMFsettingisthemostreliable.Thein‐bandDTMFwheretheG729codecisusedisthemostun‐reliableduetocompressionrelatedissues.

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7 AdvantagesofUsingDTMF

ThereareseveraladvantagesforusingDTMFtechnologyforautomatingsomeofthekeypartsofacall.Someofthesearelistedbelow:

HigherSecurity

UsingDTMFforobtaininguserinputismoresecureandgivescallerstheassurancethatsensitivedata is being protected. DTMF tones cannot be interpreted by the human ear, but can be easilydecoded by a phone system and computer. Thus, sensitive information can be isolated from theagentsaswellas fromcallrecordingsystems. In theabsenceofDTMF,anagentwithamaliciousintentcanstealsensitivecardinformationfromthearchivedcallrecords.

Fromacustomer’sperspectiveaswell,DTMFoffershighersecurity,astheydonothavetospeakoutthecarddetailsoverthephone,andthisprotectsthemfromdatatheftifthecallisbeingmadeinacrowdedenvironment.

ImprovedMetrics

Inaninboundcallcenter,obtaininguserinputsviaDTMFcanimprovekeymetricssuchasAverageHandlingTime(AHT),sincetheagentsneednotrepeatthecarddetailsbacktothecallertoconfirmtheaccuracyoftheinput.

The increasedsecurityofferedbyDTMFalso improvescustomersatisfaction levels,makingthemfeel more comfortable with entering sensitive data using DTMF, and thereby maintaining itsanonymity.

ReducedProbabilityofErrors

Italsoreducesthepossibilityoferrors,asthereisonlyonepointofdataentry.WithoutaDTMF,thecallerwillhavetoreadoutthedataandtheagenttypesitintohisdesktopapplication,givingrisetotwopointsoferror.Ontheotherhand,usingDTMFmeansthecallerisdirectlyenteringthedata,subsequentlyreducingthechancesforerror.

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8 Conclusion

DTMFwasoriginallydesignedtousethefrequenciesinthenormalhumanvoicerange.Asaresult,it can easily pass over normal two‐way radio channels, narrowband, andwide band.DTMF is astraight forwardtechnologythat iseasytounderstand,compatiblewithmostequipmentandcanbe used to provide the most cost effective and flexible features. It does not require expensiveequipmentorspecialchannelsfortransmittingthefrequencies.

Aspulsedialinghasnearlyreachedtheendofitslifespanandvoicerecognitionenginesstillhavealongwaytogo,DTMFsolutionsseemtobetheanswerforcallcentersforatleastthenextcoupleofdecades.Evenwhencall centersmove tovoice recognitionsystems, it is agood idea tooffer thecustomerthechoiceofwhetherhewouldpreferDTMForvoiceinputs.

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9 References

1. http://www.mediacollege.com/audio/tone/dtmf.html

2. http://www.repeater‐builder.com/tech‐info/dtmf/dtmf.html

3. AT&TCompatibilityBulletinNo.105,Issue#1,August8,1975.

4. http://en.wikipedia.org/wiki/Dual‐tone_multi‐frequency_signaling

5. http://www.etsi.org/technologies‐clusters/technologies/fixed‐line‐access/dtmf

6. http://www.cisco.com/en/US/products/sw/custcosw/ps1846/products_programming_reference_guide_chapter09186a00800c49d8.html

7. http://www.cisco.com/en/US/products/sw/custcosw/ps1846/products_programming_reference_guide_chapter09186a00800c49d8.html

Documents

Dual Tone Multi Frequency Signalling - Specialty · PDF fileDual Tone Multi Frequency, or DTMF as it is popularly known, is the technical term for the sound frequencies produced when