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Voice Over Internet Protocol (VoIP)
Presentation 3 – VoIP: An
Overview
Copyright © 2006 Heathkit Company, Inc. All Rights Reserved
Objectives
At the end of this presentation,
you will be able to:
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 Define
signaling, encoding, transport, and
gateway control as they apply to VoIP.
 Identify
the protocols associated with
signaling, encoding, transport, and gateway
control in VoIP.
 Define
jitter, latency, and packet loss.
Describe techniques designed to overcome
these problems.
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Levels of VoIP
to Computer – One softphone
connects to another directly over the
Internet.
 Computer
 Via
a VoIP Service Provider
System – An organization acts
as its own VoIP Service provider
 Enterprise
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Computer to Computer VoIP
 VoIP over
Instant Messaging
– MSN Messenger and Windows
Messenger
 Microsoft
 AOL – AIM
 Yahoo!
– Yahoo! Messenger
 Google
– Google Talk
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VoIP via a Service Provider
 Great
for homes and small businesses.
 Monthly
fee or per call fee.
 Provide
services which are not
available or cost extra through PSTN.
 Generally
better quality than IM.
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VoIP Providers

AT&T

Comcast Communications

Cablevision

Time Warner

Vonage

Cox Communications

Verizon

SBC Communications

EarthLink

Bell South

Nortel

Quest Communications
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Typical Services Provided

Connect to PSTN

Call Waiting

Select your Area
Code

Call Forwarding

Caller ID

Voice Mail

Online Account
Management

Unlimited long
Distance

Conference Calls
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Vonage
 The
largest VoIP Service provider
 Requires
a broadband connection to
the Internet.
 Signaling
Protocol is SIP.
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Soft-phone
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Analog Telephone
Adapter
IP Phone
Analog Phone
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Enterprise System
Proxy
Server
IP Phones
Internet
IP
PBX
Admin
Server
PSTN
VoIP to PSTN
Gateway
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Major VoIP Issues
 Signaling
 Voice
Coding
 Packet
Delivery
 Gateway
Control
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Signaling
 Find
the called party.
 Determine
the status of the called party
– busy or not.
 Create
and manage calls.
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Voice Coding

Voice is inherently analog.

Microphones produce an analog signal.

Speakers require an analog signal to reproduce
voice.

The Internet requires digital data packets.

The conversion of voice to digital data packets
at the transmitting end and the conversion of the
digital data packets back into voice at the
receiving end.
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Packet delivery

Moving the voice data packets to their
intended recipient while ensuring
acceptable voice quality.

Rearranging the order of the received data
packets back to the order in which they
were transmitted.

Controlling the rhythm of the packets.
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Gateway Control
 A gateway
is a device that converts
from one protocol to another.
 Common
 H.323
gateway conversions include:
to SIP and vice versa
 VoIP to
PSTN
 VoIP to ATM
 VoIP to
ISDN
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VoIP creates two paths within the
IP network.
Path – Carries information
needed to establish and maintain the
media path.
 Signaling
Path – The path used by the
voice packets.
 Media
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VoIP Protocols
Path protocols – SIP, H.323,
MGCP, and Megaco/H.248
 Signaling
Path protocol – Real Time
Protocol (RTP)
 Media
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VoIP Challenges
 Latency
 Jitter
 Packet
loss
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Latency

The delay between the instant a voice
fragment is produced and the later instant at
which the listener hears it.

The amount of time that the packet spends
in the network.

Latency greater than 150 milliseconds
becomes noticeable and may degrade voice
quality.
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Contributors to Latency
 Encoder
delay
 Transport
 Jitter
delay
Buffer delay
 Decoder
delay
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Jitter
 Changes
in latency from one packet to
the next.
 Packets
may take different paths with
different overall delays.
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Packet Jitter
B
Internet
A
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Jitter
Buffer
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Lost Packets

Ignore the problem – “transmit” an instant
of silence.

Repeat the last packet transmitted.

Predict the contents of the lost packet by
examining the contents of previous packets.

Insert an instant of noise.
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Voice Over Internet Protocol (VoIP)
End
Copyright © 2006 Heathkit Company, Inc. All Rights Reserved