IP Telephony (Voice over IP)

IP Telephony (Voice over IP)

„ Instructor

„ Ai-Chun Pang, acpang@csie.ntu.edu.tw

„ Office Number: 417, New building of CSIE

„ Textbook

„ “Carrier Grade Voice over IP,” D. Collins, McGraw-Hill, Second Edition, 2003.

„ Requirements

„ Homework x 2 (Homework I and Homework II) 30%

„ Mid-term exam 15%

„ Final exam 20%

„ Term project (or Homework III) 20%

„ Oral presentation 15%

„ TAs (office number: 442, New building of CSIE)

„ 黃思瑋 (gestalt@voip.csie.ntu.edu.tw)

„ 林俊仁 (jrlin@voip.csie.ntu.edu.tw)

„

Course Outline

„ Introduction

„ Transporting Voice by Using IP - RTP (Real-Time Transport Protocol)/RTCP (RTP Control Protocol)

„ H.323

„ Session Initiation Protocol (SIP) and ENUM

„ VoIP over Network Address Translation (NAT)

„ Skype – Voice over Overlay Networks

„ Media Gateway Control and the Softswitch Architecture

„ VoIP and SS7

„ Quality of Service

„ Designing a Voice over IP Network Mobile All IP Network

4 IP Telephony

Next Generation Networks [1/2]

„ Internet Telecom & Wireless Communication

Reference: CCL/ITRI Wireless

LAN WLAN

FA CA

ProxySIP Server

Internet

MS

CSCF3GPP

3G UMTS

GGSN SGSN

T-SGW

MGW

MGW PSTN

MGCF3GPP

Next Generation Networks [2/2]

„ Internet Telecom & Wireless Communication

IP

MGCF CSCF

T-SGW MGW MGW

WLAN GPRS

CSCF SIP

Server

PSTN

Internet

Wireless ^App.

Server 3rd Parties App.

Introduction

Chapter 1

Carrier Grade VoIP

„ Carrier grade and VoIP

„ Mutually exclusive

„ A serious alternative for voice communications with enhanced features

„ Carrier grade

„ The last time when it fails

„ 99.999% reliability (high reliability)

„ Fully redundant, Self-healing

„ AT&T carries about 300 million voice calls a day (high capacity).

„ Highly scalable

„ Short call setup time, high speech quality

„ No perceptible echo, noticeable delay and annoying noises on the line

Interoperability

8 IP Telephony

VoIP

„ Transport voice traffic using the Internet Protocol (IP)

„ One of the greatest challenges to VoIP is voice quality.

„ One of the keys to acceptable voice quality is bandwidth.

„ Control and prioritize the access

„ Internet: best-effort transfer

„ VoIP != Internet telephony

„ Next generation Telcos

„ Access and bandwidth are better managed.

„ QoS solutions

„ Service-level agreements between providers

IP

„

A packet-based protocol

„ Routing on a packet-by-packet base

„

Packet transfer with no guarantees

„ May not be received in order

„ May be lost or severely delayed

„

TCP/IP

„ Retransmission

„ Assemble the packets in order

„ Congestion control

„ Useful for file-transfers and e-mail

10 IP Telephony

Data and Voice

„

Data traffic

„ Asynchronous – can be delayed

„ Extremely error sensitive

„

Voice traffic

„ Synchronous – the stringent delay requirements

„ More tolerant for errors

„

IP is not for voice delivery.

„

VoIP must

„ Meet all the requirements for traditional telephony

„ Offer new and attractive capabilities at a lower cost

Why VoIP?

„

Why carry voice?

„ Internet supports instant access to anything.

„ However, voice services provide more revenues.

„ Voice is still the killer application.

„

Why use IP for voice?

„ Traditional telephony carriers use circuit switching for carrying voice traffic.

„ Circuit-switching is not suitable for multimedia communications.

„ IP: lower equipment cost, lower operating

expense, integration of voice and data applications, potentially lower bandwidth requirements, the

widespread availability of IP

12 IP Telephony

Lower Equipment Cost

„ PSTN switch

„ Proprietary – hardware, OS, applications

„ New software application development for third parties

„ High operation and management cost

„ Training, support, and feature development

„ Mainframe computer

„ The IP world

„ Standard mass-produced computer equipment

„ Application software is quite separate

„ A horizontal business model

„ More open and competition-friendly

„ Intelligent Network (IN)

„ does not match the openness and flexibility of IP solutions.

„ A few highly successful services

„ VoIP networks can interwork with Signaling System 7 (SS7) and take advantage of IN services build on SS7.

Voice/Data Integration

„ Click-to-talk application

„ Personal communication

„ E-commerce

„ Web collaboration

„ Shop on-line with a friend at another location

„ Video conferencing

„ Shared whiteboard session

„ With IP multicasting

„ IP-based PBX

„ IP-based call centers

„ IP-based voice mail

„ Far more feature-rich than the standard 12- button keypad

14 IP Telephony

Lower Bandwidth Requirements

„ PSTN

„ G.711 - 64 kbps

„ Human speech frequency < 4K Hz

„ The Nyquist Theorem: 8000 samples per second to fully capture the signal

„ 8K * 8 bits

„ Sophisticated coders

„ 32kbps, 16kbps, 8kbps, 6.3kbps, 5.3kbps

„ GSM – 13kbps

„ Save more bandwidth by silence suppression

„ Traditional telephony networks can use coders, too.

„ But it is more difficult.

„ VoIP – two ends of the call to negotiate the coding scheme

„ The fundamental architecture of VoIP systems lends itself to more transmission-efficient network designs.

„ Distributed (Bearer traffic can be routed more directly from source to destination.)

The Widespread Availability of IP

„

IP

„ LANs and WANs

„ Dial-up Internet access

„ IP applications even reside within hand-held computers and various wireless devices.

„ The ubiquitous presence

„

VoFR or VoATM

„ Only for the backbone of the carriers

16 IP Telephony

VoIP Challenges

„

VoIP must offer the same reliability and voice quality as traditional circuit-switched

telephony.

„

Mean Opinion Score (MOS)

„ 5 (Excellent), 4 (Good), 3 (Fair), 2 (Poor), 1 (Bad)

„ International Telecommunication Union

Telecommunications Standardization Sector (ITU- T) P.800

„ Toll quality means a MOS of 4.0 or better.

Speech Quality [1/2]

„

Must be as good as PSTN

„

Delay

„ The round-trip delay

„ Coding/Decoding + Buffering Time + Tx. Time

„ G.114 < 300 ms

„

Jitter

„ Delay variation

„ Different routes or queuing times

„ Adjusting to the jitter is difficult.

„ Jitter buffers add delay.

18 IP Telephony

Speech Quality [2/2]

„

Echo

„ High Delay ===> Echo is Critical

„

Packet Loss

„ Traditional retransmission cannot meet the real-time requirements

„

Call Set-up Time

„ Address Translation

„ Directory Access

Managing Access and Prioritizing Traffic

„

A single network for a wide range of

applications, including data, voice, and video

„

Call is admitted if sufficient resources are available

„

Different types of traffic are handled in different ways

„ If a network becomes heavily loaded, e-mail traffic should feel the effects before synchronous traffic (such as voice).

„

QoS has required a huge effort.

20 IP Telephony

Speech-coding Techniques

„

In general, coding techniques are such that

speech quality degrades as bandwidth reduces.

„ The relationship is not linear.

„

G.711 64kbps 4.3

„

G.726 32kbps 4.0

„

G.723 (celp) 6.3kbps 3.8

„

G.728 16kbps 3.9

„

G.729 8kbps 4.0

„

GSM 13kbps 3.7

„

MOS values are still subjective in nature.

Network Reliability and Scalability

„

PSTN system fails

„ 99.999% reliability

„

Today’s VoIP solutions

„ Redundancy and load sharing

„ A balance must be struck between network cost and network quality.

„ Finding the right balance is the responsibility of the network architect.

„ Scalable – easy to start on a small scale and then expand as traffic demand increases

22 IP Telephony

VoIP Implementations

„

IP-based PBX solutions

„ A single network

„ Enhanced services

VoIP Implementations

„ IP voice mail

„ One of the easiest applications

„ IP call centers

„ Use the caller ID

„ Automatic call distribution

„ Load the customer’s information on the agent’s desktop

„ Click to talk

24 IP Telephony

VoIP Evolution