Voice over IP
Kiril Lashchiver
SDBI 2005
Topics
•
•
•
•
What & Why is VoIP
Digital PSTN
Internet, IP/TCP/UDP
VoIP
Voice over IP
sdbi 2005
2
What & Why is VoIP
•
•
•
•
What & Why is VoIP
Digital PSTN
Internet, IP/TCP/UDP
VoIP
Voice over IP
sdbi 2005
3
What is VoIP?
The ability to make telephone calls and send
facsimiles over IP-based data network with a suitable
quality of service and a superior cost/benefit
Voice over IP
sdbi 2005
4
In the beginning…
In 1995 VocalTec Communications Ltd.
introduced the first internet phone
The voice was poor (due to high delay) but it was
free innovative and most important it
demonstrated VoIP capabilities and triggered
new markets and studies
Voice over IP
sdbi 2005
5
Why VoIP?
• Why another ‘telephone’ system?
Voice over IP
sdbi 2005
6
Why VoIP (cont.)
• Cost Reduction
• Advanced Applications
• Standardization
Voice over IP
sdbi 2005
7
Digital PSTN
•
•
•
•
What & Why is VoIP
Digital PSTN
Internet, IP/TCP/UDP
VoIP
Voice over IP
sdbi 2005
8
Digital PSTN
• PSTN: Public Switched Telephone Network
• PBX: Private Branch eXchange
• SSP: Service Switching Point
Voice over IP
sdbi 2005
9
Nyquist theorem
• The Nyquist-Shannon sampling theorem is the
fundamental theorem in the field of information
theorem, in particular telecommunication
• The theorem states that:
When sampling a band-limited signal the
sampling frequency must be greater than twice
the input signal bandwidth in order to be able to
reconstruct the original perfectly from the
sampled version.
Voice over IP
sdbi 2005
10
Nyquist example
Voice over IP
sdbi 2005
11
Voice over PSTN
• The usable voice frequency band ranges
from 300Hz to 3400Hz, or 0Hz to 4000Hz
including guard bands
• According to Nyquist it would take 8000
samples per second to capture
4000(-epsilon)Hz perfectly
• The signal is quantitized to 256 levels, 8 bits
per sample
• 8000 samples * 8 bits = 64K bps
Voice over IP
sdbi 2005
12
255
Voice in PSTN (cont.)
11100000
pitch
11100010
10110000
10100000
01100000
01011111
01000101
00110011
0
0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000 1.125
Voice over IP
sdbi 2005
Time
13
Voice over PSTN (cont.)
• Those 64Kbps streams are sent
together into much larger streams
– 30 voice channels – E1 – 2Mbps
– 120 voice channels – E3 – 8 Mbps
– 24 voice channels – T1 – 1.5 Mbps
– 672 voice channels – T3 – 43 Mbps
…and that is just the beginning
Voice over IP
sdbi 2005
14
Internet, IP/TCP/UDP
•
•
•
•
What & Why is VoIP
Digital PSTN
Internet, IP/TCP/UDP
VoIP
Voice over IP
sdbi 2005
15
Internet
Data
Packet 1
Packet 2
Each data chunk is broken into packets, attached with
header (and a sometimes a trailer)
Voice over IP
sdbi 2005
16
Internet
1
2
3
4
Voice over IP
sdbi 2005
17
IP/TCP/UDP
• As we seen packet 3 didn’t reach it’s
destination and packet 1 arrived last. The
recovery (or lack of recovery) depends on the
transport layer protocol
TCP (Transmission Control Protocol)
UDP (User Datagram Protocol)
TCP will discover packet loss and ask the lost
packet to be re-sent and the order will be
corrected
UDP will not know about packet loss nor the order
change.
Voice over IP
sdbi 2005
18
IP/TCP
• TCP is a reliable but slow protocol, suits
for web surfing or file transfer
• Adds big header
• Adds time delay
Therefore not suitable for real time
applications
Voice over IP
sdbi 2005
19
IP/UDP
• UDP is fast but not reliable protocol,
suits for real time applications
• Sometimes used with RTP (Real Time
Protocol) to add timing information to
packets
• Small header
• No time delay
Voice over IP
sdbi 2005
20
Internet vs. PSTN
Size
QoS
Price
PSTN
Worldwide
(static)
Very Good
High
Internet
Worldwide
(increasing)
Unknown
Low
Voice over IP
sdbi 2005
21
VoIP
•
•
•
•
What & Why is VoIP
Digital PSTN
Internet, IP/TCP/UDP
VoIP
– QoS
– Protocols
– Security
– News
Voice over IP
sdbi 2005
22
VoIP QoS
• Providing a level of quality that at least
equals the Digital PSTN
• QoS usually refers to exactness of the
transmitted voice but can also be
applied to network availability,
telephone feature, scalability and
security
Voice over IP
sdbi 2005
23
VoIP Qos (cont.)
• However there are ‘few’ problems on
the way
– Delay
– Packet Loss
– Echo
Voice over IP
sdbi 2005
24
VoIP - Delay
• Algorithmic delay
• IP+UDP+RTP = 40 bytes
This typically added to voice packet that sent
every 20ms (160 bytes)
Makes the total traffic 100kbps
Too much kbps. How we can improve?
Voice over IP
sdbi 2005
25
Voice Coding
• Source waveform
transmission only the delta and the sign
between n and n+1 samples, reduces to 32 –
16 kbps
• Vocoders (voice coders)
attempt to model the vocal tract, very
unnatural sound, low bit rate (e.g. 2.4)
• Hybrid (CELP)
Combines the best of the two.
• Silence detection
Appendix A
Voice over IP
sdbi 2005
26
Voice Coding (cont.)
• Performance of each Voice Coder is
examined by the following:
– Speech quality
– Bit rate
– Algoritmic delay
– Complexity
Voice over IP
sdbi 2005
27
Some examples
•
•
•
•
•
PCM – 64 kbps
ADPCM – 32 kbps
GSM – 13.2 kbps
CELP – 4.5 kbps
OpenLPC – 1.4 kbps
Voice over IP
sdbi 2005
28
VoIP – Delay (cont.)
• Processing delay:
• Caused by the process time of the
decoder and the encoder
• Insignificant in good VoIP software
Voice over IP
sdbi 2005
29
VoIP – Delay (cont.)
• Network Delay:
• Caused by the protocols used to
transmit the voice
• Usually a function of the capacity of the
links in the network.
Voice over IP
sdbi 2005
30
VoIP – Packet Loss
• Internet – Packet switched
• Routers may drop packets
• Packets might get in the wrong order
• Packet loss of 10%+ is not acceptable
Voice over IP
sdbi 2005
31
Packet Loss Compensation
• Skip the missed time and play the next
packet
• Replay last voice packet received
• Send redundant information (e.g. in nth
packet send also (n+1)th voice data
• Hybrid, in nth packet send much lower
quality vocoder of the (n+1)th packet
G.711:
0%:
10%:
Voice over IP
20%:
sdbi 2005
50%:
32
VoIP – Echo Compensation
• Reflection of the speaker’s voice in
speaker’s ear.
• Even in PSTN there is an echo,
however due to a very short delay (less
then 50 ms) it is not noticeable
• In VoIP the delay of the network almost
always greater than 50 ms, thus echo
cancellation techniques must be used
Voice over IP
sdbi 2005
33
VoIP Protocols
• H.323
– NetMeeting, GnomeMeeting and OpenH323
• SIP
– Msn
• Others
– Google talk
– Skype
– Many others…
Voice over IP
sdbi 2005
34
H.323
• In May 1996 ITU released the H.323
specification, which defines how voice, data
and video traffic would be transported over IP
networks
• Written by telephony experts
• Very complicated protocol
• Rarely used in software VoIP phones, much
more popular in hardware
(readable by machines)
Voice over IP
sdbi 2005
35
H.323 (cont.)
• Allows different configurations of audio
video and data
• Uses different protocols to manage
different needs
• H.225 manages registration,
admissions, status and call signaling
Voice over IP
sdbi 2005
36
H.323
Protocol
Voice over IP
sdbi 2005
37
SIP
• Was developed by IETF and in
November 2000 was accepted as
signaling protocol for VoIP applications
Voice over IP
sdbi 2005
38
SIP (cont.)
•
•
•
•
Supports mobility
Lightweight protocol
Supports any media types (IM, voice, video)
Advanced features: changing setup during
the call, inviting participants during the call
and call holding
• Easy to manage and troubleshoot
Voice over IP
sdbi 2005
39
SIP protocol
Voice over IP
sdbi 2005
40
SIP vs. H.323
• H.323 comes from the ITU (telephony)
• SIP comes from the IETF and borrows many
concepts from Web, DNS and Internet e-mail
• H.323 is a complete, integrated suite of protocols
• SIP addresses only session initiation can be easily
combined with other protocols and services
• H.323 being an umbrella standard, large and
complex
• SIP uses the KISS principle
Keep It Simple, Stupid
Voice over IP
sdbi 2005
41
VoIP Security
• Both parts of VoIP call, the call setup
messages and the voice stream need to be
inspected by the firewall capable of both
network and application level protection.
• Many different protocols
Maybe SIP???
Voice over IP
sdbi 2005
42
Security – signaling path
•
•
•
Problem: Signaling path sometimes contains
vulnerable information about the users
(phone numbers, account codes, id’s etc…)
Call Hijacking
DoS
Partial Solution: authentication and integrity.
Voice over IP
sdbi 2005
43
Security – Media stream
• Problem: Voice stream is vulnerable to
eavesdropping and transport disruption
Partial Solution: The stream can be
encrypted
Voice over IP
sdbi 2005
44
Protocol Examples
NetMeeting
MSN
Google Talk
Skype
Voice over IP
sdbi 2005
45
VoIP News
• Wireless VoIP
• Large (and small) Companies change to
VoIP
• Voice over ATM
• The FBI encountered a new problem in the
war on terror: how to listen to internet phone
calls
Voice over IP
sdbi 2005
46
VoIP and IPv6
• As a real-time application VoIP should
insure short arriving time
• As we seen high delay might be caused
by the network
• VoIP packets are treated as ‘equal’ to
other data packets
• IPv6 has a priority field, so urgent
packets (like VoIP’s) will be forwarded
first
Voice over IP
sdbi 2005
47
IP Phone
Voice over IP
sdbi 2005
48
Software IP phones
Voice over IP
sdbi 2005
49
Anything missing? Let me know.
Voice over IP
sdbi 2005
50
Appendix A
How Vocoders Work?
• Over short intervals, about 2 to 40
milliseconds, human speech cam be modeled
using three parameters
– Sound source
– Pitch
– Recursive linear filter
• This model can be used to synthesize speech
Back
Voice over IP
sdbi 2005
51
How Vocoders Work? (cont.)
• In order to improve the ‘robotic’ sound
generated by the three methods the
vocoder finds the best model to express
the sound and adjusting the
comparison, ignoring the differences
that human ear can not hear
Voice over IP
sdbi 2005
52
Bibliography
•
•
•
•
•
•
•
•
•
•
•
•
•
Voice over IP, by Ayse Yasemin Seydim
Voice over IP, by Bur Goode, IEEE
VoIP and Security, by Greg S. Tucker
Speech synthesis using the CELP algorithm, by Geraldo LIno de
Campos
Secure IP Telephony, by Pingtel and Check Point
Simple VoIP Implementation, by Hong Xiong, Yuan Guo, Fang Zhu
VoIP, by Will Denis
It's Time To Take a Look At SIP, by Peter Morrissey
Computer Networking 3rd edition
RFC 781 IP
RFC 768 UDP
RFC 793 TCP
RFC 3261 SIP
Voice over IP
sdbi 2005
53
Bibliography (cont.)
• http://en.wikipedia.org/wiki/Main_Page
• http://www.nwc.com/1001/1001ws2.html
• http://www.techonline.com/community/ed_resource/feature_artic
le/36563
• http://www.hawksoft.com/hawkvoice/codecs.shtml
• http://www.globalipsound.com/demo/index.php
• http://www.slate.com/id/2095777/
• http://www.networkitweek.co.uk/networkitweek/features/205967
2/rtfm-does-sip-work
• http://www.ee.cityu.edu.hk/~cfchan/demo.html
• http://www.protocols.com/pbook/VoIPFamily.htm
• http://www.cisco.com/warp/public/788/voip/delay-details.html
Voice over IP
sdbi 2005
54