Voip for Beginners 10-02-12

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VoIP IntroductionInternet telephony reers to communications services voice, ax, SMS, and/or voice-messaging applications

that are transported via the internet, rather than the public switched telephone network (PSTN). VoIP uses

internet protocol data packets to transer voice, ax, and other data over the shared network, thereby eliminating

toll charges, which is why they are cheaper than calls over PSTN. Constant technical innovation and upgrades

are being made in this sector to make VoIP quality comparable to the traditional phone system.

This traditional PSTN system works by setting up a dedicated channel between two points or the duration o the

call. This system is based on copper wires carrying analog voice data over the dedicated circuits. VoIP, in contrast

to PSTN, uses what is called packet-switched telephony. Using this system, the voice inormation travels to its

destination in countless individual network packets across the Internet. These packets need to be assembled and

then converted to voice. VoIP systems can be connected to the traditional PSTN system.

In laymans terms VoIP is the transmission o voice over the digital network. The steps involved in originating a

VoIP telephone call are as ollows:

Signalingandmediachannelsetup

Digitizationoftheanalogvoicesignal

Encoding

Packetization

TransmissionasInternetProtocol(IP)packetsoverapacket-switchednetwork

On the receiving end, the above steps take place in the reverse order:

ReceivingtheIPpackets

Decodingofthepackets

Digital-to-analogconversionwhichreproducestheoriginalvoicestream

Codecs

Toenhancethesecurityofthepacketsbeingtransmittedandtopreventanyunauthorizedillegalaccess,they

need to be encoded. Codecs encode and decode both ends o the conversation to allow the conversation to besent and received across the network.

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Ziff Davis | Article | VoIP For Beginners

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Differentcodecshavedifferentbandwidthrequirementsanddifferentcharacteristicsthatcanimpactnetwork

perormance. Some o the popular codecs and their characteristics are defned as ollows:

G.711 Codec - It uses compression schemes and hence requires more bandwidth.

G.729 Codec - It uses lossy compression which reduces the amount o bandwidth required, but can impact the

voice quality.

Once the codec has its payload ready, its up to another protocol, to transer data to its target recipient. So or

applicationslikeaudioandmusicstreaming,aprotocollikeRTP(real-timetransportprotocol)canbeusedwhere

acknowledgement o the protocol is not a key actor.

Protocol

The word protocol comes rom the Greek word protocollon which means a lea o paper glued to a manuscript

volume that describes the contents. In other words, a protocol is a set o rules or procedures that needs to be

ollowed to allow an orderly communication. Webopedia defnes it as An agreed-upon ormat or transmitting

data between two devices. The protocol should determine the ollowing:

Typeoferrorcheckingthatneedstobeused

Datacompressionmethodthatneedstobeused

Howthesendingdevicewillindicatethatithasnishedsendingamessage

Howthereceivingdevicewillindicatethatithasreceivedamessage

There are a variety o protocols that one can choose. The protocol can be either hardware-based or sotware-based. The implementation decision will depend upon a number o actors including the ollowing:

Advantagesanddisadvantagesoftheprotocolbeingused

Complexityinvolvedinitsusage

Speed,quality,andreliability

Defning VOIP Protocols

There are a number o protocols that are employed in order to provide or VoIP communication services. They

can be implemented in using both the proprietary and open protocols and standards. In order to be able to

communicate using a VoIP system, there are two types o protocol that must be used.

1. Signaling protocol

controlsandmanagesthecall

includeselementssuchascallsetup,cleardown,andcallforwarding

examplesofsuchprotocolareH.323,SIP,andskinny


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2. Data exchange protocol

managesthedataexchangefortheVoIPtrafc

handlesbothaudioandvideo(e.g.RTP)

In this report, we will ocus on those important protocols that are widely used and accepted in the industry:

H.323

Packet-basedmultimediacommunicationssystems

Distributedarchitectureforcreatingmultimediaapplications,includingVoIP

Widelydeployedandistheoldestofthecallsetupprotocols Robustandexible

Lotsofhandshakesanddataexchangesforeachfunctionitperforms

Media Gateway Control Protocol (MGCP)

DenedasIETFRFC2705

Centralizedarchitectureforcreatingmultimediaapplications,includingVoIP

UsesUDPonport2427

LetsacallservercontrolaVoIPgatewaytothePSTN.Hence,thebulkofthecallcontrolintelligence

and routing inormation resides in the call server, instead o the gateway because o this call servercontrolling the gateway

Session Initiated Protocol (SIP)

DenedasIETFRFC2543.

Denedasadistributedarchitectureforcreatingmultimediaapplications,includingVoIP

LightweightprotocolthataccomplishesalmosteverythingasH.323withmuchlessoverhead

SIPclientinterfacesareshippedwithMicrosoftWindowsXP

Designedforapplicationsthatneedreal-timeperformancetosenddatainonedirectionwithno

acknowledgments.

ExamplesofcompaniesofferingSIPphonesareCiscoandAvaya

Real-time Transport Protocol (RTP)

UsedtoreportontheperformanceofaparticularRTPtransportsession

Deliversinformationsuchasthenumberofpacketstransmittedandreceived,theround-tripdelay,jitter

delay, etc. that is typically used to measure QoS (Quality o Service) in the IP network


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ThisisanunreliableprotocolbuiltontopoftheUDPprotocolthatdoesnotguaranteedeliveryof

packets, but has little overhead

Widelyusedforstreamingaudioandvideodata.

Designedforapplicationsthatneedreal-timeperformancetosenddatainonedirectionwithno

acknowledgments

Session Description Protocol (SDP)

Intendedfordescribingmultimediacommunicationsessionsforthepurposesofsessionannouncement,

session invitation, and parameter negotiation Doesnotdelivermediaitselfbutisusedfornegotiationbetweenendpointsofmediatype,formatand

all its associated properties

H.248 (Megaco)

AnITURecommendationthatdenesGatewayControlProtocol

ItisalsoknownasTheMediaGatewayControlProtocol(Megaco)whichistheresultofajointeffort

betweentheIETFandtheITU-TStudyGroup16

Skinny Client Control Protocol (SCCP)

Alongwiththesestandardizedcallsetupprotocols,vendorshaveprovidedtheirownproprietary

protocols.E.g.CiscohasprovidedSkinnyClientControlProtocol(SCCP)

Itisasimple,lightweightcallsetupprotocolforCiscodevicesandpassesmessagesusingTCPandport

2000

Who defnes the Protocol

IETF(InternetEngineeringTaskForce)-Thiscommunityofengineersthatstandardizestheprotocols

thatdenehowtheInternetandInternetProtocolswork.Refertothefollowinglinkforfurtherdetails:

http://www.iet.org/

ITU(InternationalTelecommunicationsUnion)-AninternationalorganizationwithintheUnitedNations

System where governments and the private sector coordinate global telecom networks and services.


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Refertothefollowinglinkforfurtherdetails:http://www.itu.int/home/index.html

Service quality

ForVoIPtobearealisticreplacementforstandardpublicswitchedtelephonenetwork(PSTN)telephony

services, customers need to receive the same quality o voice transmission they receive with basic telephone

servicesmeaning consistently high-quality voice transmissions. VoIP is extremely dependent and sensitive to

bandwidthavailabilityanddelaycausedindeliveryofpackets.ThemajorchallengewithVoIPimplementationis

how to guarantee that packet trafc or a voice or other media connection will not be delayed or dropped due

interferencefromotherlowerprioritytrafc.Hencethefollowingthingsmaybeconsideredwhendeningthe

Quality expected rom VoIP:

Latency:delayforpacketdelivery

Jitter:variationsindelayofpacketdelivery

PacketLoss:toomuchtrafcinthenetworkcancausethenetworktodroppackets

Voice Myths

Cisco provides an excellent distinction between the network and what myths we have regarding them in their

Understanding Voice over IP Protocols.

Myths Facts

Networks can only be built one way VoIP allows centralized or distributed

architectures

Networks will only use one protocol H.323, SIP, MGCP and H.248/Megaco will

all be present in VoIP networks

All networks will converge The networks will converge to IP


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Reerenceshttp://en.wikipedia.org/wiki/Voice_over_IP

http://www.packetizer.com/ipmc/papers/understanding_voip/voip_protocols.html

http://www.cisco.com/application/pdf/en/us/guest/tech/tk587/c1506/ccmigration_09186a008012dd36.pdf

http://www.webopedia.com/TERM/P/protocol.html

http://searchnetworking.techtarget.com/defnition/protocol

http://www.cisco.com/application/pdf/en/us/guest/tech/tk587/c1506/ccmigration_09186a008012dd36.pdf

http://www.protocols.com

http://www.nact.com/pds/voip-7_tutorial.pd

http://www.serviceassurancedaily.com/2007/01/voip-protocol-basics-and-why-voip-consumes-more-bandwidth-than-you-might-expect/

About the Author

TheAuthorisaProjectManagerinaleadingmultinationalorganization.Inhiscurrentrole,heleadssoftware

projectsaimedatdrivingdenedbusinessvalue.Amulti-skilledprofessionalwithover12+yearsoftotal

experienceandanexcellenttrackrecordofmanagingcomplexprojectsacrossvarioustechnologies(SAPBI,

MSTR,SharePointand.NET)anddomain.Abletomanagestakeholderexpectationsandwillingtotakefull

responsibilityforthedeliveryofprojectsobjectives.Hasanexcellenttrackrecordofdetermininggoals,time

frames,constraints,risks,stafngrequirementsandproceduresforaccomplishingprojects.Hasplayeddiverseroles and has a passion in teaching, trekking and reading books.

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