Voice over IP

1. Voice over IP B90901033 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 • For data transport • SIP • 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 • 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) • 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 • http://www.skype.com/