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Basic VOIP

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Basic VOIP

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									Introduction to Voice technologies

1

Voice over IP introduction • VoIP = Voice + IP • VOICE
Traditionally, voice was transmitted using a separate dedicated infrastructure and it is still in place i.e. PSTN The first network that was put in place was for voice ONLY.

Based on TDM
2

Voice over IP introduction (contd..)

• VoIP = Voice + IP
• TCP/IP based Data Networks
Most common data network implementations are based on TCP/IP. Internet and most business networks are also based on TCP/IP. The purpose of data networks is to transfer & share computer data between users
3

Voice & Data Network infrastructure

• VOICE
Circuit Switching
Phones/terminals Signaling Routing Transmission facilities

• DATA
Packet Switching
Data Terminals Signaling Routing Transmission facilities
4

What is meant by Data?
• Computer Data

• Voice • Video • What is common in all of them?
They can all be represented as bits i.e. these are all different forms of information As all can be represented as digital data making Voice/Video/Data integration possible
5

Voice technologies

• Voice in PSTN (TDM Based) • Voice over Packet (VoIP, VoFR or VoATM)
6

Voice over IP (contd..)
• Transport voice traffic using IP • Voice over the Internet?
Interconnected networks Applications: e-mail, file transfer, e-com

• The greatest challenges
Voice quality and bandwidth
Control and prioritize the access

• Internet: best-effort transfer
The next generation VoIP != Internet telephony

7

IP (Internet Protocol)
• A packet-based protocol
Routing on a packet-by-packet base

• Packet transfer with no guarantees
May not receive in order May be lost or severely delayed

• TCP/IP
Retransmission Assemble the packets in order Congestion control Useful for file-transfers and e-mail

8

Voice over IP Protocols

Presentation Session Transport Network Link Physical

G.729(A)/G.723(.1)/G.711 H.323/MGCP/SIP RTP/UDP/RSVP IP/WFQ/IP-prec MLPPP/FR/ATM AAL1 –––

9

Why VoIP?
• Why carry voice?
Internet supports instant access to anything “Dot-com” Many new services and applications However, voice services provide more revenues

• Why use IP for voice?
Circuit-switching is not for datacom IP-based Packet switching: Equipment cost, integrated access, less bandwidth, and widespread availability
10

Lower Equipment Cost
– PSTN switch Proprietary – hardware, OS, applications High operation and management cost Training, support and feature development cost – Mainframe computer – The IP world Standard hardware and mass-produced Application software is quite separate – IN does not match the openness and flexibility of IP A few highly successful services
11

Voice/Data Integration
– Click to talk application Personal communication E-commerce CTI – Computer Telephony Integration – Web collaboration Shop on-line with a friend at another location

– Video conferencing
– IP-based PBX – IP-based call centers
12

Enterprise Voice Over IP Applications
• Toll bypass
Most common application

• PBX extension
Saves costs by reducing maintenance costs and overhead

• H.323 interoperability
Supports voice-enabled Web applications
13

Cisco ―Voice over‖ Applications

14

Connection Types

• Local
• On-net • Off-net • PLAR • PBX-to-PBX

• On-net to Off-net
15

Local Connections

555-4001

Between two FXS Stations

555-4002
16

On-net Connections

Site A

Site B

IP Router Gateway Router Gateway

Calls within an enterprise

17

On-net Connections (contd..)

Branch A
192.168.1.1 192.168.1.254 172.16.1.254

Branch B
Soft Phone

Internet

IP Phone

18

Off-net Connections
Dial Access code: 9 Then PSTN number Branch A
192.168.1.1 192.168.1.254 172.16.1.254

Branch B

FR/ATM

PSTN

19

Tie Line Trunks

PBX

PBX

IP,FR ATM

Router Gateway

Router Gateway

20

On to off-net Connections

Branch A
192.168.1.1 192.168.1.254

PSTN

Branch B
172.16.1.254

Internet

21

Toll Bypass Using 3600

PBX
PBX

PSTN

3620

V

QoS WAN
(Intranet)
3640

4 to 12 Analog ports

V

Branch Office

Headquarters
22

Introduction to PSTN
Legacy Voice Infrastructure

23

Addressing in Telephone Systems
• Numbering is never flat, it is always hierarchical • E.163 Standard (replaced by E.164)

• E.164 ITU-T standard for ISDN numbers
• In switching terminology the numbers are termed as DNs or (Directory Numbers)
24

Dialing Types

• Pulse
Each digit is represented as a series of pulses.

• Touch Tone (DTMF)
Each digit represented as a pair of frequencies
25

Pulse Dialing Scheme

Make = Circuit Closed

Off-Hook

Dialing Inter-Digit Delay Next Digit

Break = Open Circuit 700 ms

Pulse Period (100 ms)

Supported on Cisco routers

26

DTMF Dialing
Supported on Cisco routers
Dual Tone Multifrequency (DTMF) 1209 1336 1477 1633

697

1

2

3

A

770

4

5

6

B

852

7

8

9

C

941

*

0

#

D

27

Types of circuit switched calls

Call on same switch a calling b

Call established through multiple switches c calling d

End-Office Central Office Local Exchange CLASS 5 switch

Tandem for calls within city Transit for calls out of city
28

Introduction to Signaling
The main purpose of Signaling is to setup and tear down a call and providing supervisory functions.
Signaling Classification

Off-hook Dial-tone Ringing Busy Tone Hookflash ISDN Q.931

Subscriber Signaling

Trunk or Inter-switch SS1-6 SS7 Signaling Router-Router R2 (Analog / PCM H.323 / SIP MGCP

29

Types of Signaling
Method of communicating telephony events: Off-hook, busy, on-hook…

Analog
• 2-wire • Loop start • Ground start • • • • E&M 2-wire, 4-wire Five types I-V (Cisco I,II,III,V)

Digital
• Digital subscriber lines: 2-wire, 4-wire • Digital trunks: 4-wire • Channel associated signaling (CAS) • In-band signaling

• Common channel signaling (CCS) • Out-of-band signaling
30

Basic Local Call Flow

31

Subscriber signaling for local calls

32

Basic Call Progress: On-Hook

Telephone Switch

Local Loop

Local Loop

-48 DC Voltage DC Open Circuit No Current Flow

33

Basic Call Progress: Off-Hook
Off-Hook Closed Circuit DC Current Dial Tone Local Loop Local Loop

Telephone Switch

34

Basic Call Progress: Dialing
Off-Hook Closed Circuit Dialed Digits Pulses or Tones

Telephone Switch

DC Current
Local Loop

35

Basic Call Progress: Switching
Off-Hook Closed Circuit Telephone Switch Address to Port Translation

DC Current
Local Loop

Local Loop

36

Basic Call Progress: Ringing
Off-Hook Closed Circuit Ring Back Tone DC Current Local Loop

Telephone Switch

DC Open Cct. Ringing Tone
Local Loop

37

Basic Call Progress: Talking
Off-Hook Closed Circuit Voice Energy DC Current Local Loop

Telephone Switch Voice Energy DC Current Local Loop

38

Common Terms

• Local Loop • Switches • Trunks

39

Switch Types
• Local Exchange / CO • PBX

• Tandem
• Transit

Switches solve the N² problem

40

Trunk Types

• Private Trunks
• CO Trunks • FXO Trunks • FXS Trunks • DID/DOD Trunks

• Inter-office trunks
41

2-to-4 wire conversion

• Done in Telephone Set • Done on Switch side as well

Result: ????

42

Speech-Coding Techniques

43

Introduction
• Codecs / Speech coding schemes
• Subjective impairment analysis: MOS • Digitizing voice • Voice compression
ADPCM CELP Silence Removal Techniques (DSI using VAD)

• Processing Power
A balance between quality and cost

44

Voice Quality Measure
• Bandwidth is easily quantified Voice quality is subjective • MOS, Mean Opinion Score ITU-T Recommendation P.800 Excellent – 5 Good – 4 Fair – 3 Poor – 2 Bad – 1 A minimum of 30 people Listen to voice samples or in conversations
45

ITU-T Voice Quality Standards
P.800 recommendations
The selection of participants The test environment Explanations to listeners Analysis of results

Toll quality
A MOS of 4.0 or higher
46

ITU-T Voice Quality Standards
• Subjective and objective quality-testing techniques

• PSQM – Perceptual Speech Quality Measurement
ITU-T P.861

algorithmic comparison between the output signal and a known input
type of speaker, loudness, delay, active/silence frames, clipping, environmental noise
47

Voice Compression Technologies
Unacceptable
64 (Cellular)

Business Quality

PCM (G.711)

Toll Quality *

Bandwidth
(Kbps)

32 24 16

ADPCM 32 (G.726) ADPCM 24 (G.726) ADPCM 16 (G.726) LPC 4.8

*

*

*

LDCELP 16 (G.728) CS-ACELP 8 (G.729)

*

8
0

*

*

Quality
48

Speech Waveforms & PSD

• Voiced speech

• Power spectrum density
49

Speech Waveforms & PSD (contd..)

• Unvoiced speech

• Power spectrum density
50

Type of Speech Coders
• Waveform codecs
Sample and code High-quality and not complex

Large amount of bandwidth

• Source codecs (Vocoders)
Match the incoming signal to a mathematical model Linear-predictive filter model of the vocal tract The information is sent rather than the signal Low bit rates, but sounds synthetic Higher bit rates do not improve much

51

Types of codecs
• Hybrid codecs
Attempt to provide the best of both Perform a degree of waveform matching Utilize the sound production model Quite good quality at low bit rate

52

Waveform Coders

Quantizing

Encoding

Sampling

Filtering

1110010010010110

Waveform ENCODER

Waveform DECODER

53

Vocoders
Quantizing PCM Encoder Encoding
111001001001011

PCM Decoder

Sampling

Filtering

Sample Frames

VocalCords Throat Nose Mouth

Model Parameters

10110010

Parameters

Human Speech Model

Analysis

Synthesis
54

Model Parameters

Voice Digitization

• Analog-to-Digital Conversion
discrete samples of the waveform and represent each sample by some number of bits A signal can be reconstructed if it is sampled at a minimum of twice the maximum freq.

• Human speech
0-4KHz (300-3400 Hz used in telephony) 8000 samples per second
55

Digitizing Voice: PCM Waveform Encoding
• Nyquist Theorem: sample at twice the highest frequency
Voice frequency range: 300-3400 Hz Sampling frequency = 8000/sec (every 125us) Bit rate: (2 x 4 Khz) x 8 bits per sample = 64,000 bits per second (DS-0)

• By far the most commonly used method
CODEC
PCM = DS-0 64 Kbps

56

G.711
• The most common codec
Used in circuit-switched telephone network PCM, Pulse-Code Modulation

• •

Uniform quantization (not done)
12 bits * 8 k/sec = 96 kbps

Non-uniform quantization
64 kbps DS0 rate mu-law North America & Japan A-law Other countries, including Pakistan A MOS (Mean Opinion Score) of about 4.3

57

DPCM

•DPCM, Differential PCM
Only transmit the difference between the predicated value and the actual value Voice changes relatively slowly It is possible to predict the value of a sample based on the values of previous samples

The receiver performs the same prediction
The simplest form • No prediction
58

ADPCM

•

ADPCM, Adaptive DPCM
Predicts sample values based on Past samples Factoring in some knowledge of how speech varies over time The error is quantized and transmitted Fewer bits required G.721 32 kbps G.726 A-law/mu-law PCM -> 16, 24, 32, 40 kbps

An MOS of about 4.0 at 32 kbps

59

CELP
• Code excited linear predictive
Hybrid coding scheme

• Very high voice quality at low bit rates, processor intensive, use of DSPs

• G.728: LD CELP—16 Kbps
Smaller Codebook

• G.729: CS ACELP—8 Kbps
G.729a variant— ―stripped down‖ 8 kbps (with a noticeable quality difference) to reduce processing load, allows two voice channels encoded per DSP

60

G.729 an Advanced CODEC
Cake

Code Excited Linear Prediction (CELP) Consumes ~ 8 Kbps
Cake Recipe $0.32 10.1.1.1

A/D 16-Bit Linear PCM

Code

DSP

Packet

Code Look-Up

• DSP = Digital Signal Processing

Ingredients:
A-sound K-sound

Directions:
Play K, A, and K

Recipe or Code Book
61

G.729x

• G.729.B
VAD, Voice Activity Detection Based on analysis of several parameters of the input The current frames plus two preceding frames DTX, Discontinuous Transmission Send nothing or send an SID frame SID frame contains information to generate comfort noise CNG, Comfort Noise Generation

• G.729, an MOS of about 4.0 • G.729A an MOS of about 3.7
62

Digital Speech Interpolation (DSI)

• Voice Activity Detection (VAD)
• Removal of voice silence • Examines voice for power, change of power • Automatically disabled for fax/modem

63

Bandwidth Requirements

Voice Band Traffic
Encoding/ Compression
G.711 PCM A-Law/u-Law
G.726 ADPCM G.729 CS-ACELP G.728 LD-CELP G.723.1 CELP

Result Bit Rate
64 kbps (DS0)

16, 24, 32, 40 kbps 8 kbps 16 kbps 6.3/5.3 kbps Variable

64

Voice Quality Comparison
Anything Above an MOS of 4.0 Is ―Toll‖ Quality
Compression Method MOS Score
Delay (msec)

64K PCM (G.711) 32K ADPCM (G.726) 16K LD-CELP (G.728)
8K CS-ACELP (G.729) 8K CS-ACELP (G.729a)

4.4 4.2
4.2 4.2 3.6

0.75 1
3–5 15 15

65


								
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