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/