Basic Voip

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

• DATA

Circuit Switching

Packet Switching

Phones/terminals

Data Terminals

Signaling

Signaling

Routing

Routing

Transmission facilities

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

G.729(A)/G.723(.1)/G.711

Session

H.323/MGCP/SIP

Transport

RTP/UDP/RSVP

Network

IP/WFQ/IP-prec

Link Physical

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 B

Site A IP Router Gateway

Router Gateway

Calls within an enterprise

17

On-net Connections (contd..)

Branch A

Branch B Soft Phone

192.168.1.1 192.168.1.254

172.16.1.254

Internet

IP Phone

18

Off-net Connections Dial Access code: 9 Then PSTN number

Branch A

Branch B

192.168.1.1 192.168.1.254

172.16.1.254

FR/ATM

PSTN

19

Tie Line Trunks

PBX

PBX

Router Gateway

IP,FR ATM

Router Gateway

20

On to off-net Connections

Branch A

Branch B

PSTN

192.168.1.1 192.168.1.254

172.16.1.254

Internet

21

Toll Bypass Using 3600

PBX PBX

PSTN

3620

V

4 to 12 Analog ports

QoS WAN (Intranet) 3640

Branch Office

V

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

Telephone Switch DC Current Dial Tone Local Loop

Local Loop

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

DC Current Local Loop

Address to Port Translation

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

Telephone Switch

Voice Energy DC Current Local Loop

Voice Energy DC Current Local Loop

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Common Terms

• Local Loop • Switches • Trunks

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Switch Types • Local Exchange / CO • PBX • Tandem • Transit Switches solve the N² problem

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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

Business Quality

Toll Quality *

PCM (G.711)

(Cellular)

Bandwidth (Kbps)

*

32

ADPCM 32 (G.726)

*

24 16 8 0

ADPCM 24 (G.726)

*

ADPCM 16 (G.726)

*

LPC 4.8

*

LDCELP 16 (G.728)

*

CS-ACELP 8 (G.729)

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

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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

Encoding

Quantizing

Sampling

Filtering

Waveform Coders

Waveform ENCODER

1110010010010110

Waveform DECODER

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PCM Encoder

PCM Decoder

111001001001011

10110010

Parameters

Model Parameters

Sample Frames

Model Parameters

Encoding

Quantizing

Sampling

VocalCords Throat Nose Mouth

Filtering

Vocoders

Human Speech Model

Analysis

Synthesis 54

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

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G.729 an Advanced CODEC Cake

Code Excited Linear Prediction (CELP) Consumes ~ 8 Kbps A/D

Code 16-Bit Linear PCM

• DSP = Digital Signal Processing

DSP

Cake Recipe $0.32 10.1.1.1

Code Look-Up

Ingredients: A-sound K-sound

Packet

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

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Bandwidth Requirements

Voice Band Traffic Encoding/ Compression G.711 PCM A-Law/u-Law G.726 ADPCM

Result Bit Rate 64 kbps (DS0)

16, 24, 32, 40 kbps

G.729 CS-ACELP

8 kbps

G.728 LD-CELP

16 kbps

G.723.1 CELP

6.3/5.3 kbps Variable

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Voice Quality Comparison Anything Above an MOS of 4.0 Is “Toll” Quality Compression Method

MOS Score

Delay (msec)

64K PCM (G.711)

4.4

0.75

32K ADPCM (G.726)

4.2

1

16K LD-CELP (G.728)

4.2

3–5

8K CS-ACELP (G.729)

4.2

15

8K CS-ACELP (G.729a)

3.6

15

65