VoIP

1
VoIP

• Voice over IP

2
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

3
What is VoIP ?
Introduction

• 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

4
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
5
How VoIP works
The Classical Approach Digital Transmission
Usual 256 different values (25628, gt 8
bit) 8000 measures per second gt 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
transmission of 0 and 1 as on and turn-off
processes nearly error free
6
How VoIP works
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)!

7
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 receivers hardware turns the data back into
  speech
• Timestamp guarantees correct time and order

8
Synchronisation through Timestamp
How VoIP works
sampled
Sampling Instants
56
57
58
59
75
76
77
78
79
95
96
97
98
99
packetized
78
98
58
Timestamp
sent
58
78
98
118
138
158
received
58
158
138
118
98
78
replayed
58
78
98
118
138
158
9
Voice over IP Scenarios
The Classical Approach Separate Voice and Data
Networks
10
Voice over IP Scenarios
The Future Approach Voice/Fax over IP - A
Unified Network
11
Voice over IP Scenarios
The Intermediate Approach Voice/Fax over IP - A
Unified Network
12
Voice over IP Scenarios
The Gamers Approach Use of Roger Wilco
PC with Headset Roger Wilco Server
PC with Headset Roger Wilco Client
13
The Pros and Cons

• 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

14
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

15
Real-time Transport Protocol (RTP)
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)

16
RTP and RTCP
Real-time Transport Protocol (RTP)

• 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)
• n1 for RTCP (default 5005)

17
RTCP Paket-Typs and scalability
Real-time Transport Protocol (RTP)

• 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

18
Header Format
Real-time Transport Protocol (RTP)
19
Overhead und Header Compression
Real-time Transport Protocol (RTP)

• 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 - gt high
  processor load
• Slow i-net connection (e.g. VoIP over V.34 modem)

20
VoIP Delay Budget (example G.711)
Real-time Transport Protocol (RTP)
) depends on uses codec

• Coding and framing 20 ms )
• Packetization 20 ms
• Output queuing 0 ... 8 ms

• Access (up) link transmission
• Backbone network transmission t
• Access (down) link transmission

• Input queuing not
  relevant
• Jitter buffer 4 ... 40 ms
• Decoding 1 ms )

ITU-T G.114 Recommended (150 ms)
Total 45 ... 89 t ms
21
H.323 Packet-BasedMultimedia Communications
Systems
H.323 Standard
H.225.0 Layer
Video Codec H.261, H.263
Video I/O Equipment
RTP
Audio Codec G.711, G.722
Audio I/O Equipment
User Data Applications T.120, etc.
System Control

RAS Control
System Control User Interface
Q.931 Call Setup
H.323
H.245 Control
22
MOS - Mean Opinion Score
5 - Excellent 4 - Good 3 - Fair 2 - Poor 1 - Bad
23
Speech Samples
Speech Samples
G. 711 64 kbps
LPC 2.4 kbps
GSM 13 kbps
Single Speaker
Music
Bit Errors 0.1
Bit Errors 1
24
Future Aspects
Future Aspects
25
Thanks for your attention