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 MEGACO 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 Servers 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) 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

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/