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Practical Fundamentals of Voice over Internet Protocol (VOIP) for Engineers and Technicians


Definition“ Voice Over IP (VOIP) is a converged network application that

allows voice, data, video and fax to be transported over existing IP data networks instead of conventional Public Switched

Telephone Network (PSTN).”


Voice

Data

Packet Voice

IP Data

IP Data

IP Telephony Circuit Switched Networks (TDM)

IP+ATM

IP

Packet Switched (Converged) Networks

Networks


VOIP

• Is a Layer 3 network protocol

• Uses Layer 2 point-to-point or link layer protocols (such as PPP,Frame relay or ATM for its transport)

• Digital Signal Processors (DSPs) segment the voice signal into frames and store them in voice packets.


VoiceSource

VoiceDestination

ADC DAC

ManagedIP CLOUD

AnalogueSignal

AnalogueSignal

DigitalSignal

DigitalSignal

VOIP BASIC DIAGRAM


VOIP

• Voice packets are transported via IP in compliance with voice communication protocols or standards– Such as H.323 ,Media Gateway Control

Protocol(MGCP) or Session Initiation Protocol (SIP)


Relationship of OSI and VOIP

OSI Layer Number

OSI layer Name

VOIP Protocols and Functions

7 Application Net Meeting

6 Presentation CODECS

5 Session H.323/MGCP/SIP

4 Transport RTP/TCP/UDP

3 Network IP

2 Data Link Frame Relay, ATM, Ethernet, PPP


IP is an attractive choice for voice transport because:

• Lower Equipment Cost – – IP based switch architecture is an open and distributed

architecture– Modular in concept

• Integration of Voice and Data application –– IP offers a converged and unified platform for transport

of data as well as voice

IP in VOIP


• Optimum Bandwidth Utilization – – Many bandwidth efficient coding algorithms have been

developed which are bandwidth efficient.

• Universal Acceptance and Availability of IP as a networking protocol

IP in VOIP


The problems in implementing VOIP are – • Speech Quality –

– IP basically designed for transporting data traffic

– Data traffic is asynchronous can tolerate delay

– Any appreciable delay in receipt of voice packet badly impairs quality of speech.

• Delay – – Voice does not tolerate delay

– ITU-T recommendation G.114 states that the round trip delay should be less than 300 ms for telephony applications

Problems in VOIP


• Jitter-Jitter is variation in delay – is caused in IP networks in two ways:

– Packets take different routes from source to destination and experience different delays

– Different packets experience different queuing time (due to sharing of available resources by different users)

Problems in VOIP


Packet loss –

• VOIP is based on packet switched network architecture – Voice is first digitized, packetized and then transported

in the form of packets

• If packets are lost in transit – Any retransmission will introduce delay – Voice quality impaired

Problems in VOIP


• Power Over Ethernet – – Traditional PSTN Telephones are DC powered on the

same conductors– For VOIP on LAN environment

• 802.3af compliant powered Ethernet Switches

• Picker

• Two Methods are –– (Injector) + (POE compatible Device)– (Injector) + (non-POE compatible Device)+(Picker)

Problems in VOIP


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