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