IP Telephony (Voice over IP)
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Instructor
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Ai-Chun Pang, acpang@csie.ntu.edu.tw
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Office Number: 417, New building
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Textbook
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“Carrier Grade Voice over IP,” D. Collins, McGraw-Hill, Second Edition, 2003.
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Requirements
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Homework x 3 30%
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Mid-term exam 25%
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Final exam 25%
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Term project 20%
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TAs (office number: 305, Old building)
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王舜茂 (oncemore@voip.csie.ntu.edu.tw)
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許睿斌 (binbin@voip.csie.ntu.edu.tw)
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詹勝? (kwun@voip.csie.ntu.edu.tw)
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Course Outline
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Introduction
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Transporting Voice by Using IP
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Speech-Coding Techniques (Optional)
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H.323
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Session Initiation Protocol (SIP) and ENUM
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SIP over Network Address Translation (NAT)
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Media Gateway Control and the Softswitch Architecture
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VoIP and SS7
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Quality of Service
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Designing a Voice over IP Network
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From IPv4 to IPv6 Networks
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Mobile All IP Network
n IP Multimedia Subsystem (IMS)
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VoIP over Wireless LAN (WLAN)
Introduction
Chapter 1
Carrier Grade VoIP
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Carrier grade and VoIP
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Mutually exclusive
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A serious alternative for voice communications with enhanced features
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Carrier grade
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The last time when it fails
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99.999% reliability (high reliability)
n Fully redundant, Self-healing
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AT&T carries about 300 million voice calls a day (high capacity).
n Highly scalable
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Short call setup time, high speech quality
n No perceptible echo, noticeable delay and annoying noises on the line
Interoperability
VoIP
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Transport voice traffic using the Internet Protocol (IP)
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One of the greatest challenges to VoIP is voice quality.
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One of the keys to acceptable voice quality is bandwidth.
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Control and prioritize the access
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Internet: best-effort transfer
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VoIP != Internet telephony
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Next generation Telcos
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Access and bandwidth are better managed.
IP
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A packet-based protocol
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Routing on a packet-by-packet base
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Packet transfer with no guarantees
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May not be received in order
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May be lost or severely delayed
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TCP/IP
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Retransmission
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Assemble the packets in order
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Congestion control
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Useful for file-transfers and e-mail
Data and Voice
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Data traffic
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Asynchronous – can be delayed
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Extremely error sensitive
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Voice traffic
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Synchronous – the stringent delay requirements
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More tolerant for errors
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IP is not for voice delivery.
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VoIP must
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Meet all the requirements for traditional telephony
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Offer new and attractive capabilities at a lower cost
Why VoIP?
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Why carry voice?
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Internet supports instant access to anything
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However, voice services provide more revenues.
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Voice is still the killer application.
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Why use IP for voice?
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Traditional telephony carriers use circuit switching for carrying voice traffic.
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Circuit-switching is not suitable for multimedia communications.
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IP: lower equipment cost, lower operating
expense, integration of voice and data applications, potentially lower bandwidth requirements, the
widespread availability of IP
Lower Equipment Cost
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PSTN switch
n Proprietary – hardware, OS, applications
n New software application development for third parties
n High operation and management cost
n Training, support, and feature development
n Mainframe computer
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The IP world
n Standard mass-produced computer equipment
n Application software is quite separate
n A horizontal business model
n More open and competition-friendly
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Intelligent Network (IN)
n does not match the openness and flexibility of IP solutions.
n A few highly successful services
n VoIP networks can interwork with Signaling System 7 (SS7) and take advantage of IN services build on SS7.
Voice/Data Integration
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Click-to-talk application
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Personal communication
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E-commerce
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Web collaboration
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Shop on-line with a friend at another location
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Video conferencing
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Shared whiteboard session
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With IP multicasting
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IP-based PBX
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IP-based call centers
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IP-based voice mail
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Far more feature-rich than the standard 12-
button keypad
Lower Bandwidth Requirements
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PSTN
n G.711 - 64 kbps
n Human speech frequency < 4K Hz
n The Nyquist Theorem: 8000 samples per second to fully capture the signal
n 8K * 8 bits
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Sophisticated coders
n 32kbps, 16kbps, 8kbps, 6.3kbps, 5.3kbps
n GSM – 13kbps
n Save more bandwidth by silence suppression
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Traditional telephony networks can use coders, too.
n But it is more difficult.
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VoIP – two ends of the call to negotiate the coding scheme
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The fundamental architecture of VoIP systems lends itself to more transmission-efficient network designs.
n Distributed (Bearer traffic can be routed more directly from source to destination.)
The Widespread Availability of IP
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IP
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LANs and WANs
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Dial-up Internet access
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IP applications even reside within hand-held computers and various wireless devices.
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The ubiquitous presence
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VoFR or VoATM
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Only for the backbone of the carriers
VoIP Challenges
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VoIP must offer the same reliability and voice quality as traditional circuit-switched
telephony.
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Mean Opinion Score (MOS)
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5 (Excellent), 4 (Good), 3 (Fair), 2 (Poor), 1 (Bad)
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International Telecommunication Union
Telecommunications Standardization Sector (ITU- T) P.800
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Toll quality means a MOS of 4.0 or better.
Speech Quality [1/2]
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Must be as good as PSTN
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Delay
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The round-trip delay
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Coding/Decoding + Buffering Time + Tx. Time
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G.114 < 300 ms
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Jitter
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Delay variation
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Different routes or queuing times
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Adjusting to the jitter is difficult.
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Jitter buffers add delay.
Speech Quality [2/2]
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Echo
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High Delay ===> Echo is Critical
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Packet Loss
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Traditional retransmission cannot meet the real-time requirements
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Call Set-up Time
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Address Translation
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Directory Access
Managing Access and Prioritizing Traffic
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A single network for a wide range of
applications, including data, voice, and video
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Call is admitted if sufficient resources are available
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Different types of traffic are handled in different ways
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If a network becomes heavily loaded, e-mail traffic should feel the effects before synchronous traffic (such as voice).
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QoS has required a huge effort.
Speech-coding Techniques
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In general, coding techniques are such that
speech quality degrades as bandwidth reduces.
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The relationship is not linear.
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G.711 64kbps 4.3
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G.726 32kbps 4.0
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G.723 (celp) 6.3kbps 3.8
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G.728 16kbps 3.9
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G.729 8kbps 4.0
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GSM 13kbps 3.7
Network Reliability and Scalability
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PSTN system fails
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99.999% reliability
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Today’s VoIP solutions
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Redundancy and load sharing
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A balance must be struck between network cost and network quality.
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Finding the right balance is the responsibility of the network architect.
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Scalable – easy to start on a small scale and then
expand as traffic demand increases
VoIP Implementations
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IP-based PBX solutions
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A single network
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Enhanced services
VoIP Implementations
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IP voice mail
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One of the easiest applications
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IP call centers
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Use the caller ID
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Automatic call distribution
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Load the customer’s information on the agent’s desktop
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Click to talk
VoIP Evolution
Overview of the Following Chapters [1/2]
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Chapter 2, “Transporting Voice by Using IP”
n A review of IP networking in general to understand what IP offers, why it is a best-effort protocol, and why carrying real-time traffic over IP has significant challenges
n RTP (Real-Time Transport Protocol)
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Chapter 3, “Voice-coding Techniques”
n Choosing the right coding scheme for a particular network or application is not necessarily a simple matter.
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Chapter 4, “H.323”
n H.323 has been the standard for VoIP for several years.
n It is the most widely deployed VoIP technology.
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Chapter 5, “The Session Initiation Protocol”
n The rising star of VoIP technology
n The simplicity of SIP is one of the greatest advantages
n Also extremely flexible (a range of advanced feature supported)
Overview of the Following Chapters [2/2]
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Chapter 6, “Media Gateway Control and the Softswitch Architecture”
n Interworking with PSTN is a major concern in the deployment of VoIP networks
n The use of gateways
n They enables a widely distributed VoIP network architecture, whereby call control can be centralized.
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Chapter 7, “VoIP and SS7”
n H.323, SIP, MGCP and MEGACO are all signaling systems.
n The state of the art in PSTN signaling is SS7.
n Numerous services are provided by SS7.
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Chapter 8, “QoS”
n A VoIP network must face to meet the stringent performance requirements that define a carrier-grade network.
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Chapter 9, “Designing a Voice over IP Network”
n How to build redundancy and diversity into a VoIP network without losing sight of the trade-off between network quality and network cost (network dimensioning, traffic engineering and traffic routing)?
