1. Voice over IP
B 林與絜

2. Outline
Introduction
Some Protocols
Example - Skype
Conclusion

3. Introduction What is VoIP?
The transport of voice traffic using the Internet Protocol (IP)

4. Introduction (cont.) Why VoIP? Lower cost
The widespread availability of IP
Reduced bandwidth
Integration of voice and data applications
New service features

5. Introduction (cont.) VoIP Challenges Speech quality
Managing access and prioritizing traffic
Speech-coding techniques
Network reliability and scalability

6. Introduction (cont.) Speech Quality Delay Jitter Packet Loss
Call Set-up Time

7. Outline
Introduction
Some Protocols
Example - Skype
Conclusion

8. Some Protocols RTP and RTCP SIP MEGACO For data transport
For signaling use
MEGACO
Between signal and data

9. RTP and RTCP RTP (Real-Time Transport Protocol)
A transport protocol for real-time applications
RTCP (RTP Control Protocol)
A companion protocol with RTP

10. 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%

11. RTP and RTCP (cont.) RTP over UDP Sequence number Timestamp
Payload type, marker, etc.
Does not solve the QoS problems; simply provides additional information

12. 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

13. SIP SIP (Session Initiation Protocol) A signaling protocol
Setup, modification, tear-down of mutimedia sessions
A powerful alternative to H.323
More flexible, simpler

14. SIP (cont.) SIP Network Entities User agents Servers User agent client
User agent server
Servers
Proxy server
Location server (Registrar)
Redirect server

15. 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

16. MEGACO MEGACO (Media Gateway Control Protocol) Network Gateway
Signaling conversion
Media conversion

17. MEGACO (cont.) MGC (Media Gateway Controller) MG (Media Gateway)
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

18. MEGACO (cont.)

19. Outline
Introduction
Some Protocols
Example - Skype
Conclusion

20. 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

21. Skype (cont.)

22. Skype (cont.)

23. Skype (cont.)

24. Skype (cont.) Advantages
It can work seamlessly across NATs and firewalls
Better voice quality than the MSN and Yahoo IM applications

25. 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

26. Outline
Introduction
Some Protocols
Example - Skype
Conclusion

27. Conclusion Some Topics QoS Management & Improvement Mobility
Teleconferencing System

28. 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