Voice over IP B90901033 林與絜. Outline Introduction Some Protocols Example - Skype Conclusion.
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Transcript of Voice over IP B90901033 林與絜. Outline Introduction Some Protocols Example - Skype Conclusion.
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 For data transport
SIP 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 Request or status
Message headers 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/