IP Telephony (Voice over IP)
Ai-Chun Pang, email@example.com
Office Number: 417, New building of CSIE
“Carrier Grade Voice over IP,” D. Collins, McGraw-Hill, Second Edition, 2003.
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)
Transporting Voice by Using IP - RTP (Real-Time Transport Protocol)/RTCP (RTP Control Protocol)
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
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Next Generation Networks [1/2]
Internet Telecom & Wireless Communication
Reference: CCL/ITRI Wireless
Next Generation Networks [2/2]
Internet Telecom & Wireless Communication
T-SGW MGW MGW
Server 3rd Parties App.
Carrier Grade VoIP
Carrier grade and VoIP
A serious alternative for voice communications with enhanced features
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).
Short call setup time, high speech quality
No perceptible echo, noticeable delay and annoying noises on the line
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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.
Service-level agreements between providers
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
Assemble the packets in order
Useful for file-transfers and e-mail
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Data and Voice
Asynchronous – can be delayed
Extremely error sensitive
Synchronous – the stringent delay requirements
More tolerant for errors
IP is not for voice delivery.
Meet all the requirements for traditional telephony
Offer new and attractive capabilities at a lower cost
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
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Lower Equipment Cost
Proprietary – hardware, OS, applications
New software application development for third parties
High operation and management cost
Training, support, and feature development
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.
Shop on-line with a friend at another location
Shared whiteboard session
With IP multicasting
IP-based call centers
IP-based voice mail
Far more feature-rich than the standard 12- button keypad
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Lower Bandwidth Requirements
G.711 - 64 kbps
Human speech frequency < 4K Hz
The Nyquist Theorem: 8000 samples per second to fully capture the signal
8K * 8 bits
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
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
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VoIP must offer the same reliability and voice quality as traditional circuit-switched
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
The round-trip delay
Coding/Decoding + Buffering Time + Tx. Time
G.114 < 300 ms
Different routes or queuing times
Adjusting to the jitter is difficult.
Jitter buffers add delay.
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Speech Quality [2/2]
High Delay ===> Echo is Critical
Traditional retransmission cannot meet the real-time requirements
Call Set-up Time
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.
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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
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
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IP-based PBX solutions
A single network
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
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