Download Voice over IP

Voice over IP
B90901033 林與絜
Outline
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Introduction
Some Protocols
Example - Skype
Conclusion
Introduction
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What is VoIP?
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The transport of
voice traffic using
the Internet Protocol
(IP)
Introduction (cont.)
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Why VoIP?
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Lower cost
The widespread availability of IP
Reduced bandwidth
Integration of voice and data applications
New service features
Introduction (cont.)
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VoIP Challenges
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Speech quality
Managing access and prioritizing traffic
Speech-coding techniques
Network reliability and scalability
Introduction (cont.)
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Speech Quality
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Delay
Jitter
Packet Loss
Call Set-up Time
Outline
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Introduction
Some Protocols
Example - Skype
Conclusion
Some Protocols
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RTP and RTCP
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SIP
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For data transport
For signaling use
MEGACO
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Between signal and data
RTP and RTCP
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RTP (Real-Time Transport Protocol)
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A transport protocol for real-time applications
RTCP (RTP Control Protocol)
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A companion protocol with RTP
RTP and RTCP (cont.)
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Voice over UDP, not TCP
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Data traffic
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Asynchronous
Extremely error sensitive
Voice traffic
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Synchronous – stringent delay requirements
Tolerant of errors – at most 5%
RTP and RTCP (cont.)
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RTP over UDP
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Sequence number
Timestamp
Payload type, marker, etc.
Does not solve the QoS problems; simply
provides additional information
RTP and RTCP (cont.)
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RTCP
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Exchange messages between session users
Quality feedback
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Number of lost packets
Delay
Inter-arrival jitter
Implicitly open when an RTP session is
open
SIP
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SIP (Session Initiation Protocol)
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A signaling protocol
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Setup, modification, tear-down of mutimedia
sessions
A powerful alternative to H.323
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More flexible, simpler
SIP (cont.)
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SIP Network Entities
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User agents
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User agent client
User agent server
Servers
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Proxy server
Location server
(Registrar)
Redirect server
SIP (cont.)
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SIP Messaging
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Text-based
SIP Message:
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Start line
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Message headers
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Request or status
Additional
information of the
request or response
Message Body
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Describe the type
of session
MEGACO
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MEGACO (Media Gateway Control Protocol)
Network Gateway
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Signaling conversion
Media conversion
MEGACO (cont.)
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MGC (Media Gateway Controller)
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Handling call control
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Call-control intelligence
Call-related signaling
MG (Media Gateway)
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Performing the media conversion
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A line or trunk on circuit-switched side
An RTP port on the IP side
MEGACO (cont.)
Outline
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Introduction
Some Protocols
Example - Skype
Conclusion
Skype
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A peer-to-peer VoIP
client developed by
KaZaa in 2003
Allowing its users to
place voice calls and
send text messages
to other users
Skype (cont.)
Skype (cont.)
Skype (cont.)
Skype (cont.)
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Advantages
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It can work seamlessly across NATs and
firewalls
Better voice quality than the MSN and
Yahoo IM applications
Skype (cont.)
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Disadvantages
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The protocol is proprietary
It provides a single service, not an
architecture of new services
It still has centralized elements for login
authentication
Outline
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Introduction
Some Protocols
Example - Skype
Conclusion
Conclusion
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Some Topics
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QoS Management & Improvement
Mobility
Teleconferencing System
Reference
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Daniel Collins, Carrier Grade Voice over IP, McGraw-Hill, 2003
Salman A. Baset and Henning Schulzrinne, An Analysis of the
Skype Peer-to-Peer Internet Telephony Protocol, 2004
http://www.skype.com/