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VoIP – Voice over Internet Protocol
VoIP
Voice over IP
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Contents
•
•
•
•
•
•
•
•
Introduction
How VoIP works
Voice over IP Scenarios
The „Pros and Cons“
RTF (Real-time Transport Protocol)
H.323 Standard
Speech Samples
Future Aspects
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Introduction
What is VoIP ?
• VoIP = Voice over Internet Protocol
• Transmission of voice and video over data-networks
(internet, intranet), using the Internet protocol (IP)
• Possible variants using VoIP :
– PC to PC
– PC to Phone and the other way around
– Phone to Phone using the Internet / Intranet
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
How VoIP works
The Classical Approach: Analog Transmission
Signal is transferred
by a wire
microphone
Microphone transforms acoustic waves into "current fluctuations"
Loudspeaker
Loudspeaker transforms
the signal back into
sounds
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
How VoIP works
The Classical Approach : Digital Transmission
Usual: 256 different values (256=2^8, => 8 bit);
8000 measures per second => 64 kBit/s
01110001
11000001
………
Microphone transforms acoustic
waves into "current fluctuations"
In time intervals the amperage is
measured and its value is transferred
Converted back into
current fluctuations and
acoustic waves
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
transmission of 0 and 1
as on and turn-off processes;
nearly error free
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How VoIP works
VoIP – Voice over Internet Protocol
Tomorrow (?): Voice over IP
Transmission of media (audio,video) over the Internet Protocol:
The Internet works package-oriented:
data stream is distributed on
packets, which are sent independently
to the target
This means for the media (speech), like you would:
- record it on a tape
- cut the tape into pieces
- put the pieces in envelopes
- at the destination paste the pieces in the correct
order and play the tape
… and this all in real-time (almost)!
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
How VoIP works
How VoIP works
• General approach in case of submitting speech:
–
–
–
–
–
Recording and digitalisazion of speech
Segmentation of data packets
The packets are transported over the internet to the receiver
The receiver‘s hardware turns the data back into speech
Timestamp guarantees correct time and order
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
How VoIP works
Synchronisation through Timestamp
sampled
56 57 58
59
75 76 77 78 79
58
95
96 97 98 99
78
Sampling
Instants
packetized
98
Timestamp
58
78
58
98
78
58
118
98
78
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
98
138
118
138
118
sent
158
158
138
received
158
replayed
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VoIP – Voice over Internet Protocol
Voice over IP Scenarios
The Classical Approach: Separate Voice and Data Networks
Site A
PC
IP
Router
Site B
Internet
Intranet
Modem
Phone
Voice
Switch
IP
Router
PC
Modem
Private Voice
Network
Voice
Switch
Phone
PSTN
Fax
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
Fax
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VoIP – Voice over Internet Protocol
Voice over IP Scenarios
The Future Approach: Voice/Fax over IP - A Unified Network
Site A
Site B
PC with
Voice/Fax
IP
Router
PC with
Voice/Fax
Internet
Intranet
IP
Router
IP Phone
IP Phone
PSTN
Gateway
PSTN
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
PSTN
Gateway
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VoIP – Voice over Internet Protocol
Voice over IP Scenarios
The Intermediate Approach: Voice/Fax over IP - A Unified Network
Site A
Site B
PC with
Voice/Fax
IP
Router
IP Phone
PSTN
Gateway
Phone
Voice
Switch
PC with
Voice/Fax
Internet
Intranet
PSTN
Fax
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
IP
Router
PSTN
Gateway
IP Phone
Voice
Switch
Phone
Fax
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VoIP – Voice over Internet Protocol
Voice over IP Scenarios
The Gamers Approach: Use of “Roger Wilco”
Site A
Site B
Internet
Intranet
PC with Headset
Roger Wilco Server
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
PC with Headset
Roger Wilco Client
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The „Pros and Cons“
VoIP – Voice over Internet Protocol
• Advantages
–
–
–
–
Cost savings on long distance calls
Less (no) need for private telephone networks
Single RJ-45 connector at the workplace for all services
Enables new multimedia features, e.g. human operator assisted
e-commerce
• Problems / Open Questions
– Control of delay, jitter and packet loss over IP-based networks
– QoS guarantees
– Bandwidth
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Contents
•
•
•
•
•
•
•
•
Introduction
How VoIP works
Voice over IP Scenarios
The „Pros and Cons“
RTF (Real-time Transport Protocol)
H.323 Standard
Speech Samples
Future Aspects
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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Real-time Transport Protocol (RTP)
VoIP – Voice over Internet Protocol
Real-time Transport Protocol (RTP)
•
•
RTP version 2 is specified by RFC 1889
RTP covers functions such as
– Payload type identification (which codec and Framing)
– arranging the packages by sequence numbers synchronisation by time stamps
(playing time of the individual Samples or Frames;
– Synchronisation of several Media Streams
– quality control and statistics
•
•
RTP is defined independent of transportation protocol, sets however
typically on UDP/IP
RTP contains no mechanisms to the safety device of the quality of service
(QoS)
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Real-time Transport Protocol (RTP)
RTP and RTCP
• The concept covers two closely linked protocols
– RTP (real-time transport Protocol): transport the Media Stream
– RTCP (real-time transport control Protocol): informed about the
participants attached at the Media Stream and the quality of service
(QoS)
• For each Media Stream and each direction, received from them, a
separate RTP session opens, you get
– an IP address (multicast address for Conferencing)
– a pair of UDP ports
• n for the Media Stream (default 5004)
• n+1 for RTCP (default 5005)
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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Real-time Transport Protocol (RTP)
VoIP – Voice over Internet Protocol
RTCP: Paket-Typs and scalability
• RTCP reports are generated by all transmitters and
receivers of the session in regularly intervals (statistic
information)
• the interval must be selected in such a way that the total
load remains within limits (recommended: approx.. 5% of
the range of the session)
• very large groups to make possible (with thousands of
participants), the interval computed due to the
– amount of active participant
– the extent of the individual report
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Real-time Transport Protocol (RTP)
Header Format
0
1
2
3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC
|M|
PT
|
sequence number
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
timestamp
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
synchronization source (SSRC) identifier
|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|
contributing source (CSRC) identifiers
|
|
....
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
optional header extension
|
|
....
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Real-time Transport Protocol (RTP)
Overhead und Header Compression
• RTP header Overhead: Example
– compressed language
- 8 kbps
– every 20 ms a RTP package with 20 oktetts Payload
– 40 oktetts header per package - 24 kbps
• RTP header compression:
– reduces Overhead
– router decompression necessarily - > high processor load
– Slow i-net connection (e.g. VoIP over V.34 modem)
IP (20)
UDP (8)
RTP (12)
compressed RTP (2 ... 4) cRTP
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
Payload (20)
24 kbit/s
Payload(20)
~10 kbit/s
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VoIP – Voice over Internet Protocol
Real-time Transport Protocol (RTP)
VoIP Delay Budget (example: G.711)
• Coding and framing
• Packetization
• Output queuing
20 ms *)
20 ms
0 ... 8 ms
• Access (up) link transmission
• Backbone network transmission
• Access (down) link transmission
• Input queuing
• Jitter buffer
• Decoding
Total
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
*) depends on
uses codec
t
not relevant
4 ... 40 ms
1 ms *)
45 ... 89 + t [ms]
ITU-T G.114
Recommended
(150 ms)
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VoIP – Voice over Internet Protocol
H.323 Standard
H.323 : Packet-Based
Multimedia Communications Systems
Video I/O
Equipment
Video Codec
H.261, H.263
Audio I/O
Equipment
Audio Codec
G.711, G.722
User Data
Applications
T.120, etc.
LAN
System Control
RAS Control
System Control
User Interface
RTP
H.225.0
Layer
Q.931 Call Setup
H.245 Control
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
H.323
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VoIP – Voice over Internet Protocol
MOS - Mean Opinion Score
Excellent 5
Speech Quality
5 - Excellent
4 - Good
3 - Fair
2 - Poor
1 - Bad
Good 4
Fair 3
Poor 2
64
32
16
8
4.8
2.4
Bit Rate [kbps]
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Speech Samples
Speech Samples
G. 711
LPC
GSM
64 kbps
2.4 kbps
13 kbps
Single
Speaker
Music
Bit Errors
0.1%
Bit Errors
1%
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Future Aspects
Future Aspects
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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VoIP – Voice over Internet Protocol
Thanks for your attention
Patrick Hügenell, Andreas Vetter – TIM01AGR – 2003
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