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The Evolution of Telephony Systems

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Abstract

This paper tries to summarize important steps in development of telephony communications since the invention of telephone, to VoIP telephony systems. Stages in development of traditional, circuit-switched telephony are listed, as well as technologies that integrated voice and data communications (ISDN) or telephony and computer systems in general (Computer Telephony Integration, CTI). Some important differences between packet-switched telephony of today and circuit-switched telephony are outlined.
Abstract—This paper tries to summarize important steps in
developmentof telephony communicationssince the invention of
telephone, to VoIP telephony systems. Stages in development of
traditional, circuit-switched telephony are listed, as well as
technologies that integrated voice and data communications
(ISDN)ortelephonyandcomputersystemsingeneral(Computer
Telephony Integration, CTI). Some important differences
betweenpacket-switchedtelephonyof todayand circuit-switched
telephonyareoutlined.
IndexTerms—telephony,PSTN,ISDN,CTI,VoIP
I.I
NTRODUCTION
sually we say that the era of telephony communications
started in 1876, when Alexander Graham Bell filed an
application with the patent office, and shortly after, U.S
PatentOfficeissuedthepatent[1]whichcovered"themethod
of, and apparatus for, transmitting vocal or other sounds
telegraphically... by causing electrical undulations, similar in
formtothevibrationsoftheairaccompanyingthesaid vocal
orothersound."
Thepotentialofthis inventionwaspromptlyunderstoodby
people,anditenteredeverydayuseofeconomicallydeveloped
societiesverysoon.
Another very important type of personal communications
today are message-based communications, either as e-mail,
SMS/MMSoralreadyalmost forgottenpagersystems.Stillin
significantusearefaxmachines,which can also be classified
in this group. Often cited analogy of po stal and e-mail,very
popularduringthe90s,whenInternetenteredeverydayuseby
laymanpeople, used to label postal mail as "snail mail". The
firstrealuseof Internetthatcaughtthe eyeofpeoplewas the
possibilitytocommunicatefriendsorrelativeswhowereliving
far away, on another continent. And the communication was
veryfastandcheap,comparedtopostalmail.
II.
IMPORTANTSTAGESOFEVOLUTION
A.Theinventionoftelephonedeviceandtelephony
network
The predecessors can be found in XIX century. The first
device used to transmit information "through the wire" was
telegraph, patented [1] in 1837 by SamuelMorse in the US
(developedwithassistanceofAlfredVail)andbySirWilliam
Fothergill Cooke in the UK. The duplex telegraph was
patented by T.A. Edison in 18 92. In 1902, telegraph crossed
thePacific,andthusencircledtheglobe.Thatwasahugeleap,
compared to existing systems for long distance
communications,oftenbasedonhumanmessengers.
The word "telephone", is derived from two Greek words,
"tele"meaningremote,and "phone"meaningsound.Sincethe
time of Bell, to soft phones of today, basic functionality
requirements towards a telephone device has remained the
same-itmustprovide:
-means to enable the calling user to signal the network
thatserviceisdesired
-means to enable the calling user to indicate to the
networktheaddressofthecalleduser
-indicationtothecalleduserthatacallisoffered
-indicationtothenetworkthatconversationhasended
Theservicethateventhefirsttelephonesprovidedwaslater
namedPOTS(PlainOldTelephoneService).
The next step after the invention of telephone was the
establishment of telephone network. First telephones were
wired in pairs, comprising permanent connections. If one
wanted another connection, he had to use another phone
device,orbreakthe existingconnection.Theinfrastructureof
a telephone network consists of telephone exchange devices,
connectedbytrunks.Thefirstcommercialtelephoneexchange
appeared in 1878, Connecticut,US, by George W. Coy [1].
Sometimes Tivadar Puskas is credited for building the first
telephone exchange in 1877, while he was working for T.
Edison. The Coy's exchange could provide two simultaneous
calls.
Early telephone networks contained hierarchically
connected exchange devices. Manual switchboards were
gradually replaced by automated telephone exchanges (in the
US significant automation started during the WWII), and in
thesecond part of XX century withdigitalswitchdevices.In
factthe firstUSpatent foranautomaticswitchingsystemwas
granted to Connolly and McTighe in 1879 [1]. Three main
categoriesofautomaticswitchingmechanismswere:
-step-by-stepselection(byStrowger[2],1887)
-revertiveimpulsivecontrolofselection[3]
-markerregisterbasedcontrolofselection[3]
B.Evolutionofthetelephonynetwork
Foralongtimethemostimportantregulationbodyinthis
area was ITU-T (earlier CCITT). In the last decade though,
with the unification of computer and telecommunications
TheEvolutionofTelephonySystems
IlijaBasicevic
FacultyofTechnicalSciences,UniversityofNoviSad,Serbia
ilibas@uns.ns.ac.yu
U
networks, importance of other regulatory bodies (most
importantlyIETF)emerged.
The evolution of fixed telephony network from the early
switching systems of the end of XIX century until today has
followedfourmajorsteps:
1.Completely analogue network, where each call is
assigned a circuit for the whole duration of the call.
Informationthat is transferred during the call, changes
onlyfromaudiotoelectricaldomain,buttheproperties
of information stay. Frequency division multiplexing
(FDM)isused.
2.Digitalizationofthetransmissionsystem.Atthispoint,
pulsecodemodulation(PCM)hasbeenintroduced.The
inventionofPCMdatesto the 20s and 30s decades of
XX century but the practical use of PCM becameless
expensivewiththe wide commercialuseoftransistors,
in50sdecade. (PCM has been standardized in 1970s.)
At this stage, switching was still analogue, when
enteringtheswitchingpoint,signalwasconvertedfrom
thedigital to analogue form, and againafter switching
convertedtodigital form.Thisprocess deteriorated the
quality of signal so maximum number of conversions
hadtobelimited- in order tokeepacceptablequality.
In this phase time division multiplexing (TDM) is
introduced.
3.Digitalization of the switch. Started during 70s. There
is no need for demodulation/modulation in the transit
anymore. Also at this stage, digital exchange is
controlledbyacentralcomputer.
4.Digitalization of the local loop that connects user
telephonetothecentraloffice.Startedduring80s.Most
importantstepisworkonISDNandDSS1standards.
During 70s it became obvious that the amount of data
communicationsusedbycomputerandtelegraphnetworkswas
largeand growingrapidly.Sothenaturaldecisionwastostart
workingonintegrationofvoiceanddatacommunications.
Development of Integrated Services Digital Network
(ISDN) started in 1980s. However ISDN never took the
envisionedplaceintelecommunicationsbecause oftheadvent
of Internet. ISDN is today sometimes used in the segment
between local exchange and end-user, because it provides
possibilities for simultaneous use of the telephone line for
several applications, and because of its bandwidth
characteristics. ISDN brought the definition of basic and
supplementary services, which is with some changes in use
today. Basic service is defined as circuit mode bearer service
(infactthisisaPOTSservice).Supplementaryservicesarethe
contribution of ISDN, since earlier networks were usually
limitedtoPOTSservice.
Examplesofsupplementaryservicesare:
-directdialingin
-multiplesubscribernumber
-callinglineidentificationpresentation/restriction
-connectedlineidentificationpresentation/restriction
-maliciouscallidentification
-calltransfer
-callforwarding
-calldeflection
-linehunting
-explicitcalltransfer
-callwaiting
-callhold
-completionofcallstobusysubscribers
-callconference
-closedusergroup
Today the circuits between exchanges are mostly digital,
onlythepartbeetwenthelocalexchangeandtheenduserstill
usesanalogtechnology(analogtwo-wirecircuits).Eveninthat
segment digital technology is used more every day (DSL,
ISDN, FTTP, cable modem technology). The basic digital
circuit in the PSTN is 64 kbit per second channel, called
DigitalSignal 0 (DS0). Typically,audiosoundisdigitizedat
8khzsamplerate,using8-bitPCMmodulation.
DS0 is known as timeslot because it is multiplexed using
TDM multiplexing. Several DS0 channels are multiplexed
togetherintoa T1 lineinUS andJapan(24DS0channels)or
E1lineinmostothercountries(32DS0channels).
Approximately, during 80s vendors of telecommunications
equipment tried to enlarge the space for development of new
applications. The idea was that digital switch would provide
basic services, which would be used - invoked from more
complex and sophisticated services implemented as PC
applications.Thatway,foranewservice(inmanycases)there
wouldbe noneedtochangeanythingontheswitchside(such
modificationsareverycumbersomeand expensive). Instead a
new PC software application would be written. This is the
basic premise of computer telephone integration (CTI). The
prerequisite for this is the establishment of a standard for
interfacebetweenswitch and PC.Asit has happened insome
other cases in the history of computing, there were several
vendorgroups,eachproposingitsownstandard.
TheproposedCTIstandardsare[4]:
-CSTA (Computer Supported Telecommunications
Application)
-a group of standards (A.100, C.100, H.100, H.110,
S.100, S.200, S.300, S.900), proposed by Enterprise
ComputerTelephonyForum(ECTF)
-ANSI/VITA6-1994
-SCSA (Signal Computing System Architecture)
proposed by a group of vendors (Dialogic, IBM,
Siemens,etc)
-TAPI(TelephonyAPI)byMicrosoft
-TSAPI - proposed by Novell and AT&T, later
supported by versit (consortium led by Apple
Computer,AT&T,IBMandSiemens)aswell
-SCTP - (Simple Computer Telephony Transfer
Protocol), proposed by a group of vendors led by
Nortel
Inadditiontothe large numberofstandards,another problem
wastheiroverwhelmingcomplexity.TheintroductionofSCTP
was reaction to that problem, a try to introduce a simpler
protocolinthisarea.
Standards published in this area contain models of the
typicalCTIsystem.Asareferentmodelwewilltakeversit[5],
andspeakshortlyaboutit. The modeltriestofocus on theso
called CTI boundary, a boundary between the two domains:
switching (telecommunications) and computing domain. This
boundary has its physical and logical (application
programminginterface)characteristics.Or inotherword,both
the characteristics of physical device connecting the two
domains, and the characteristics of logical protocols used for
communication, have to be specified. One interesting
characteristic of versit standard is that it included a
comprehensive taxonomy of telephony concepts. A thorough
domainanalyseshasbeendone,andreadercanfinddefinitions
offundamentalconceptslikecall,service,deviceetcthere.For
example,eachdevice containsaphysicaland logicalelement.
Aphysicalelementcanbe:
-auditoryapparatus(microphone,speaker)
-hooks witch (for activation/deactivation of audio
apparatus)
-button(usedforactivationofspecificservice)
-lamp
-ringer (used for announcement of a new call, for
example)
-display
We can see that in soft phones of today,there is an interface
that contains the same basic elements. The display is not a
BCD-7 or ASCII display, but a screen that can present
graphicalimagesofhighresolution,oravideostreamingfrom
theotherdeviceparticipatinginthecall.Hookswitchisoneof
theavailablebuttons,notdifferentfromtheothers.Ringerisa
loudspeakerthatcanreproducefinequalityaudiorecords.
A logical element defines if it is a device that is used for
processing of one connection/call (appearance) or several
(groupelement,likeACD,pickgroup,huntgroup).
In the meantime, many of applications which were
envisioned by CTI proponents became available as VoIP-
basedsystems.
C.VoIPTelephonyandApplications
In 90s it became obvious that Internet was a platform of
choice for many communications services, including
telephony.Thisstepfollowedtheadvanceinpacketswitched
networks and statistical multiplexing. ITU-T started the
development of H.323 [6] family of protocols. Lagging a bit
behind ITU-T, IETF started the development of SIP [7]
protocol.Speakingaboutterminology,inIETFdocuments,the
word"call"isusuallyreplacedwith"session".IETFstandards
arepublishedasRFCdocuments.InRFCdocumentsrelatedto
conferencefeatures,though,theterm"callleg"isusuallyused
for the logical connection between the conference participant
andtheconferenceserver.
Generallyspeakingaboutcharacteristicsofthetwoprotocols
they depict much of the tradition of parent organizations.
H.323usesASN.1[8]syntaxwhichiscommonformanyITU-
T protocols, while SIP syntax is ASCII coded and inherits
much from the HTTP [9] protocol. H.323 appeared earlier,
and it seems that its designers had a point of view of
traditional telephony, when speaking about IP telephony.SIP 
appeared abitlater,but itsarchitectureis moreinaccordance
withInternetenvironment.
IntheclassificationofVoIPservices,onegroupemergedas
significantly more important than it was in ISDN. That is
because end-user devices grew much more complicated (due
mostlyto characteristics of Internet architectureofdistributed
intelligence and in less extent to the development of
technology),sotheirmanagementis not trivialanymore. We
willmentionheresomeoftheservicesinthisgroup[10]:
-rolebasedsign-in
-softwaredeployment
-informationbrokering
-featureinteractionmanagement
-metering,accounting,andbilling
But from the user point of view, another more important
groupofservicesthatappearedwithVoIParebasedoninstant
messaging and presence. Ubiquitous existence of chat
applications confirms that. By unifyingmessagingfeaturesof
Internet protocols like IRC [11] with IP telephony and
presence, a range of highly commercially successful
applications (ICQ, yahoo messenger, MSN messenger...)
appeared. While some of them are based on open standards
(yahoo messenger uses SIP for VoIP signaling),skype which
was at one moment holding a great majority of market, is
based on a closed, proprietary standard. Compared to IRC
those applications established a new environment which
allowed people to build their profiles (based on a mandatory
permanent account). This was a step towards virtual social
communities.The next step wascreationofwebportalwhich
provided personal home pages, instant messaging, IP
telephonyandsocialnetworkinginoneplace.
III.C
ONCLUSION
So we can summarize the described progress in the
following:
-transition from the completely analogue network to
digitalnetwork
-transition from circuit switched telephony to packet
switchedone
-transitionfromhardwiredlogictosoftwarecontrolled
systems (most important step is introduction of
computercontrolleddigitalexchange).
Inallthelistedstepstechnologicalprogress(forexampleuse
oftransistorsinsteadoftubes, or introduction of optical fiber
technology)hasbeenofcrucialimportance.
Withtheappearanceof IP based communications(not only
telephony)significantprivacy issues havearisen.Thepointis
that in traditional telephony monitoring (in the sense of
wiretapping) of telephone calls was a quite complicated
operationthatincludedhardwaremodificationinthetelephone
switch.Sothenumberofpersonsandorganizationsthatcould
gainaccesstothecontentoftelephonecallsisverylimited.On
theopposite,whencommunicatingovertheInternet, a person
leaves behind a lot of digital fingerprints in the Internet
infrastructure. And since the Internet infrastructure is not
centralized neither centrally controlled, this underlines a
question of privacy. The number of persons that can gain
accesstocontentsofcommunicationsismuchgreater than in
thepreviouscase.Encryptiontechniques are used todecrease
the space for potential attackers, but still there is lot to
consider when speaking about privacy of Internet based
communications.
Themostimportantproblemthatstill staysinthewayofIP
telephonyisqualityofservice(QoS).Basedontheconceptof
the "best effort service", early Internet offered little with
respecttoQoS.Traditionaltelephonynetworkhasaverystrict
demand with respect to availability and QoS in general. T he
variable delay of packet transmission in Internet adversely
affects the voice quality. But there is a significant amountof
researchinthisareaforalreadysometime.
A
CKNOWLEDGMENT
Theauthorthanksto prof. SlavkoSvircevicfordiscussions
oncharacteristicsoftelephonynetworks.
R
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Conference Paper
Full-text available
Voice over IP is on its way to becoming an alternative to the classical telephony system because more and more Voice over IP providers offer their solutions on the Internet and try to increase their clientele. But current voice over IP technology is still in its commercial and technological infancy and has not yet proven to be worldwide accessible, usable and scalable as the classical telephony system has proven in the past 100 years. In this paper, architecture for a next generation Voice over IP model will be outlined and discussed. The main focus lies on interoperability between different Voice over IP providers as well as dependability and robustness.
[3] Introduction to Telecommunications Network Engineering Packet-based multimedia communicationssystems SIP: Session initiation protocol
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  • T Berners-Lee
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SIP: Session initiation protocol. RFC 3261. Internet Engineering Task Force
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Rosenberg, J. Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks R., et al., SIP: Session initiation protocol. RFC 3261. Internet Engineering Task Force, 2002.