Quality of Service in Voice services

Mário Serafim Nunes Instituto Superior Técnico

Agenda 1. Introduction 2. QoS in Voice services 3. R&D in QoS of Voice services

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What is Quality of Serviçe (QoS) in communication networks ITU-T E.800: The collective effect of service performance which determine the degree of satisfaction of a user of the service. IETF RFC 2386: A set of service requirements to be met by the network while transporting a flow.

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Why differentiated QoS in data networks Since there are no resource reservations in classical packet data networks, there is the need to introduce service differentiation. The existence of differentiated QoS allows the users of packet data networks, to define different levels of service for different applications Áudio . Vídeo

Dados

Key concepts: Differenciation of services. Integrated Serviçes Networks (ISDN, ATM, IP with QoS) 4

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QoS Parameters Bitrate or bandwidth




The network must guarantee enough capacity in accordance with the application requirements (bit/s , cell/s, packet/s)

Delay or latency (minimal, mean, maximal)




Time interval between sending an information unit and its arrival to the receiver, through the several network elements.

Delay Variation or jitter




Variation of delay of the information arrival to the receiver

Losses




Bit losses (BER - Bit Error Ratio) – physical layer Frame/packet losses (PER - Packet Error Ratio) – layers 2, 3.

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QoS Requirements of continuous flow services (audio, video, multimedia) Services characterized for generating information in a regular mode along time. Requirements needed for audio and video services:




Guaranteed minimum bitrate Low delay variation (jitter) Low delay (only for interactive services)




Low loss tolerance (< 1%).





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Characteristics of PSTN versus QoS Public Switching Telephone Network (PSTN) is based on circuit switching and connection-oriented. PSTN traffic is constant or slowly variable. PSTN is dimensioned based on well defined traffic engineering rules. Blocking probability due to congestion can be estimated, (PSTN dimensioning is based on Erlang B and C formulas). Grade of Service (Blocking Probability and Delay) is guaranteed. Conclusion: Quality of Service is guaranteed 7

Characteristics of IP networks and QoS Based on the concept of simplicity in core, complexity at the network edge. Only “Best Effort” service available (“classical” IP). IP designed to carry data services, not delay sensitive, not for real-time services. Based on packet switching and connectionless mode. Uses higher layers (TCP) for reliable data delivery, not suitable for real-time services due to high delay. Conclusion: QoS not guaranteed.

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QoS versus Resource Reservation Circuit Switching networks (PSTN, ISDN, GSM): There is resource reservation => QoS is assured (-) Low efficient resource reservation Packet Switching networks (IP): There is no resource reservation => QoS not assured (+) more efficient use of physical connections, through statistical multiplexing. (-) Variable delay, due to congestion, possibility of out-oforder arrival (connectionless). 9

Voice transport in PSTN vs VoIP Parameter

Voice in PSTN (circuit mode)

VoIP (packet mode)

Bandwidth assignment

Reserved

Dynamic

Delay

Low

Medium/high

Variação de Atraso Perdas

Null

High

QoS

Low, isolated (bits) Medium, blocks (packets) High, constant Medium/Low, variable 10

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QoS versus Network Performance User

Terminal

Terminal

Network

User

Network Performance QoS Parameters measured at network edges (L1-L3) User Perception

End-to-end Quality of Service

User Perception

(L1-L7) – application (QoP, QoE)

(ITU-T I.350)

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Delays in VoIP VoIP Gateway

VOIP Backbone

VoIP Gateway

A B

B

E

D

F

E

Fixed delays

C

Variable delays

Fixed Delays A- Compression B- Echo cancelling processing C- Decompression

Variable Delays D – Routing E- Buffers delay (Queuing) F- Transmission

Maximum fixed delay – 100 - 150 ms

Total variable delay -50 - 500ms 12

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2. QoS em serviços de voz








Configuration of IP networks Architectures of Voice over IP (VoIP) Delays in low/medium bitrate accesses MOS model E Model Factor R and its components (Ie, Id) Model P.VTQ

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QoS mechanisms of Access and Core networks Access networks

Access networks

(QoS L2) ADSL ATM (CBR, UBR)

(QoS L2) Wi-Fi IEEE 802.11g/a IEEE 802.11e 8 UP, 4 AC, 5 TSPEC

IAD

Core network FR

CIR, LFI

WiMAX IEEE 802.16a IEEE 802.16e 4 MAC services

DiffServ, IntServ, MPLS (L3)

PLC: HomePlug

HFC: DOCSIS2 6 MAC services

Terminais: RTP, RTCP, DCCP (L4), CODECS (L5/7).

Electrical network

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QoS in different layers and WAN and LAN technologies WAN - Core  

Application Frame Relay ATM

Transport

L4

IP

L3

- Access HFC, ADSL  FWA, UTRAN LAN/ WLAN

L5 (L7)



 

IEEE 802.3 /Ethernet IEEE 802.11/b/g/a

FR, ATM

HFC, ADSL FWA, UTRAN

WAN Core

IEEE 802.x L2/L1

WAN Acesso

LAN/ WLAN

 Huge network heterogeneity (WAN Core, Access, LAN/WLAN)  QoS depend on QoS mechanisms existent on different layers.  Need of coordination of different mechanisms to optimize QoS from user

perspective (QoP, QoE) 15

Protocols with impact on QoS of Voice Application Layer

Transport Layer Network Layer (IPv4/IPv6) Data Link Layer



  

CODECs

VoIP Signalling:  H.323  SIP  MEGACO

RTCP RTP UDP, TCP, DCCP



IntServ / RSVP DiffServ



MPLS



Core: FR, ATM Access network: ADSL, HFC, FR, PLC LAN: 802.1p/Q CoS WLAN: 802.11e



 

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VoIP Architecture Telephone network PSTN PABX

PABX Gateway VoIP

Gateway VoIP

LAN

IP networks Internet/ VPN-IP

LAN VoIP

VoIP

VoIP Call Manalger

VoIP Call Manager

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Relation between the different factors of Voice QoS Codec Loss of packets in network

Jitter in network

Total loss of packets

Codec performance Quality of voice perceived by user

Reception Buffer (jitter)

Total delay Delay in network QoS factors associated with network

QoS factors associated with application (Terminal)

Quality end-to-end

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Voice Delay in low/medium bitrate connections High Priority queue

Scheduler

Classifier I/F Line

Medium Priority queue Média Prior.

V2 20

V3 40

V4 60

Layers 2/1

Layer 3

100

80

1500

FR, PPP, Série

Low Priority queue

V1 0

20

4 ms 214 ms (56 kbps)

140

120

V5 160

V6 180

V7 200

V8 220

V9 240

V10

260 ms

t

Data (1500 octet) V1

Data (1500 octet)

V2V3V4V5V6 V7V8 V9 V10

High delay and jitter --> degradation of Voice QoS 19

Fragmentation in low bitrate lines (LFI) Low Prior.

Fragmentation

Scheduler

Classifier

Interpolation I/F Line

Medium Prior.

20

128 20 128 64

4 ms 20 ms (56 kbps)

High Prior.

V1 0

V2 20

D1

V1

V3 40

D2

D1

V2

D3

D2

V4 60 D4

V3 V4

80

D5

D3

Layer 3

100

D6

D4

120

D7

D5

Layer 2

140

D8

D6

V5 160 D9

D7

V6 180 D10

Layer1

V7 200

V8 220

V9 240

V10

260 ms

t

D11

V5 V6 D8 V7

D9

V8V9 D10 V10 D11

Delay and jitter controlled --> function of fragment length and line bitrate 20

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Voice QoS models Subjective model Mean Opinion Score (MOS) (ITU-T P.800)

Objective models


Intrusive Perceptual evaluation of speech quality (PESQ ) (ITU-T P.862) Perceptual Analysis / Measurement System (PAMS) (BT)




Non intrusive E Model (ITU-T G.107) Factor R (formula to estimate voice quality) P.VTQ model (ITU-T / IETF) Non intrusive analysis of IP protocol. 21

Mean Opinion Score (MOS) MOS is a subjective method, based on evaluation of voice quality by groups of people in predefined samples (ITU-T P.800)

MOS define scale of 1-5 5 - Excellent 4 – Good 3 - Reasonable 2 - Weak 1 - Bad Reference: Voice in PSTN (G.711/PCM a 64 Kbit/s) -> MOS = 4.3

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Perceptual Evaluation of Speech Quality (PESQ ) Defined in ITU-T P.862. Evolution of Perceptual Speech Quality Measure (PSQM) (P.861) Based on transformation of signals generated at network input e measured at network output by means of model of psycho-physiological representation of audio signals in the auditory organs, that result in quality indicators. Scale 1- 4.5 similar to MOS Objective method, intrusive. Low utilization due to its complexity.

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Perceptual Analysis / Measurement System (PAMS) Intrusive method, insert a voice-like signal at network input and analyses the signal at network output. Levels

Parameters 1

Latency (ms) Packet losses (%) Jitter (ms)

2

3

4

5

< 50

50-75

75-100

100-200

> 200

0

0-1

1-2

2-3

>3

100

Sum of levels 10

Grade Excellent Good Acceptable Weak Innaceptable

MOS equivalent >4 >3 >2 >1 ITU-T P.563 

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Conclusions of Voice QoS





It is possible to specify QoS requirements for the network and monitor them with non-intrusive methods; Still remain problems related with QoS that do not allow widespread use of VoIP, namely in low/ medium bitrate accesses and wireless networks;

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Publications / Research work Thesis “Qualidade de Serviço em Ambiente de Voz sobre IP”, Rui Laginha, LEEC. December 2002. “Estudo do Comportamento do Tráfego de Voz em Redes WiFi em Ambiente de Simulação”, Filipe Santa-Clara, LEEC. October 2005 “Monitorização de Qualidade de Serviço de VoIP em redes sem fios”, Ricardo Aguiar , Luis Rodrigues, LEEC. September 2006. “Qualidade de Serviço Adaptativa em Comunicações de Voz sobre IP”, Nelson Costa. July 2008. “Mecanismos de Qualidade de Serviço em Comunicações de Voz sobre IP", Jorge Cardoso. July 2008. Papers Nelson Costa, Mário Nunes, “Adaptive Quality of Service in Voice over IP Communications”, The Fifth International Conference on Networking and Services (ICNS2009), IARIA / IEEE Computer Society, Valencia, Spain, April 2009, pp. 19-24. (award best paper) doi/10.1109/ICNS.2009.33 Nelson Costa, Mário Nunes, “Dynamic Adaptation of Quality of Service in VoIP Communications”, International Journal on Advances in Network and Services, IARIA, ISSN 1942-2644, Volume 2, Number 2&3, 2009, pp. 155-166. http://www.iariajournals.org/networks_and_services

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