Voice QoS

LANtel Telecommunication Corp.Senior Product Manager

Jeremy Chan

Agenda

Voice Quality is Subjective Voice Quality Defined Clarity Voice Quality Delay Echo Silence Suppression

Voice Quality is Subjective

VQ should be approached from an end-to-end perspective.

Customers need to receive the same quality of voice transmission they receive with basic telephone services.

These end-to-end and subjective characteristics of VQ make measuring it an interesting challenge.

VQ is Subjective

VoIP QoS

VoIP traffic must be guaranteed certain compensating bandwidth, latency, and jitter requirements.

QoS provides better network service: Real-Time Bandwidth Important Gateway Processes Packet Loss Delay Nonlinear Codecs

Voice Quality Defined

VoIP ArchitectureBranch 1

Branch N

Server Farm

PBX

Telephone

Telephone

Telephone

IWF

IWF

IWF

HQ

PSTN

Packet Network

Service Requirements

Service Quality Sound Quality Conversation Quality

• Offered services—such as calling card, 1-800/900 services, follow-me, and voice mail • Availability of users in other countries or regions • Network availability—down time, busy signals • Reliability—such as dropped calls or wrong number • Price

• Loudness • Distortion • Noise • Fading • Crosstalk

• Loudness distortion • Noise • Fading • Crosstalk • Echo • End-to-end delay • Silence suppression performance • Echo canceller performance

Relationship among Clarity, Delay, and Echo with Regard to VQ

• claritya voice signal’s fidelity, clearness, lack of distortion, and intelligibility.

• end-to-end delaythe time it takes a voice signal to travel from talker to listener

• echothe sound of the talker's voice returning to the talker’s ear.

Note: This’s a conceptual model The VQ is influenced by clarity, delay, and echo.

Perception of One Aspect Affects Perception of Overall VQ Users will report unacceptable VQ if only one

aspect of VQ is unacceptable. Clarity and Delay Are Orthogonal Aspects of

VQ Clarity must be reasonably good, delay must be

reasonably short. Echo Depends on Delay and Affects Clarity

Clarity of Voice Quality

Combined PSTN/VoIP Network

PSTN telephone influences clarity through the quality of its loudspeaker and

microphone. PSTN network

uses digital voice transmission for greater efficiency in the backbone

VoIP gateway interconnects the PSTN with the IP network.

Packet Loss

As the network, becomes congested, router buffers fill and start to drop packets.

Route changes as a result of inoperative network links.

Packet experiences a large delay in the network and arrives too late to be used in reconstructing the voice signal.

Packet Loss

PacketNetwork

Normal

Packet Loss X

Avoid Packet Lose

Assure minimum throughput for selected applications.

Prioritization (Classification) Router flow control Dynamic alternative for assigning

resources is the resource reservation protocol (RSVP, RFC 2205)

Speech Codecs

A speech codec transforms analog voice into digital bit streams, and vice versa.

Compression is a balancing act between VQ, local computation power, and the delay and network bandwidth required.

A codec’s effect on VQ is also influenced by packet size, packet loss, and any error-correction mechanisms used by the codec itself.

Other Factors Affecting Clarity

Noise Noise can originate from analog lines or from bit

errors on data transmission lines. Voice Activity Detectors Echo External Environmental Factors

a result of room noise, end-user mood, end-user expectations, and other intangible factors, the audio quality could still be perceived as unacceptable.

Delay

Delay

Delay is the time required for a signal to traverse the network. PSTN Delay IP Network Delay

Time

Received

Sent

Latency

Data Packet

PSTN Delay

PSTN delay is most often the result of transmission delay on long-distance trunks.

Switching delay in network nodes.

IP Network Delay

IP network delay is primarily determined by the buffering, queuing, and switching or routing delay of IP routers. Packet Capture Delay Switching/Routing Delay Queuing Time VoIP Device Delay

End-to-End Latency

PacketNetwork

dm = Delay

d1d2

d3

d4

d5

d6

IP Network Delay (cont.) Packet Capture Delay

Packet capture delay is the time required to receive the entire packet before processing and forwarding it through the router.

This delay is determined by the packet length and transmission speed.

Switching/Routing Delay Switching/routing delay is the time the router

takes to switch the packet. This delay depends on the architecture of the

route engine and the size of the routing table.

IP Network Delay (cont.) Queuing Time

This delay is a function of the traffic load on a packet switch, the length of the packets, and the statistical distribution over the ports.

Designing very large router and link capacities. VoIP Device Delay

Encode the analog voice signal into a digital signal and to decode the digital voice signal back to analog.

Transmit side -- Packetization delay Receive side -- variation in packet interarrival times

(Jitter)

Delay’s Effect on User Experience

<100 ms: users will not notice the delay. 100 ms ~ 300 ms: users will notice a slight

hesitation. >300 ms: the delay is obvious to the users.

Jitter

Data Packet

Time

Received

Sent

Jitter

Variable interpacket timing caused by the network a packet traverses.

Solving Delay Example The default G.729 codec requires packet loss far less than 1

percent to avoid audible errors. Ideally, there should be no packet loss for VoIP.

The ITU G.114 specification recommends less than 150 millisecond (ms) one-way end-to-end delay for high-quality real-time traffic such as voice. (For international calls, one-way delay up to 300 ms is acceptable, especially for satellite transmission. This one-way delay takes propagation delay into consideration—the time required for the signal to travel the distance.)

Jitter buffers (used to compensate for varying delay) further add to the end-to-end delay, and are usually only effective on delay variations less than 100 ms. Jitter must therefore be minimized.

Echo

Echo

Normal Telephony Call

Normal Telephony Call with an Echo

Echo

If the time between the original spoken phrase and the returning echo is short (25 to 30 ms), or if the echo’s level is very low (approximately -25 dB), it probably will not cause any annoyance or disruption to voice conversations.

Echo Issue

Echo is caused by an electrical mismatch between analog telephony devices and transmission media in a portion of the network called the tail circuit. (4 wire to 2 wire)

Acoustic coupling problems between a telephone’s speaker and microphone.

Solving Echo

Echo cancellers Deployed in local VoIP gateway

Using digital circuit T1, E1

Silence Suppression

Silence Suppression

To use bandwidth more efficiently, VoIP networks employ functionality known as silence suppression or voice activity detection.

VAD can realize approximately 50 percent reduction in bandwidth requirements.

VAD Behavior

Silence Suppression (cont.)

While a VAD’s performance does not affect clarity directly.

If it is not operating correctly, it can certainly decrease the intelligibility of voice signals and overall conversation quality.

Comfort noise generator (CNG) -- Complementary to the transmit-side VAD.

Providing Batter QoS

Supporting dedicated bandwidth

Improving loss characteristics

Avoiding and managing network congestion

Shaping network traffic

Setting traffic priorities across the network

Reference

Cisco Voice QoSQuality of Service for Voice over IP

IEC1.Voice Quality (VQ) in Converging Telephony and Internet Protocol (IP) Networks

2. Accelerating the Deployment of Voice over IP (VoIP) and Voice over ATM (VoATM)

Thank You