Voice QoS

1. Voice QoS LANtel Telecommunication Corp. Senior Product Manager Jeremy Chan

2. Agenda • Voice Quality is Subjective • Voice Quality Defined • Clarity Voice Quality • Delay • Echo • Silence Suppression

3. Voice Quality is Subjective

4. VQ 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.

5. 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

6. Voice Quality Defined

7. Packet Network PSTN VoIP Architecture HQ Branch 1 IWF Server Farm Telephone IWF Branch N PBX Telephone IWF Telephone

8. Service Requirements

9. Relationship among Clarity, Delay, and Echo with Regard to VQ • clarity • a voice signal’s fidelity, clearness, lack of distortion, and intelligibility. • end-to-end delay • the time it takes a voice signal to travel from talker to listener • echo • the 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.

10. 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

11. Clarity of Voice Quality

12. 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.

13. 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.

14. Packet Loss Normal Packet Network X Packet Loss

15. 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)

16. 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.

17. 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.

18. Delay

19. Delay • Delay is the time required for a signal to traverse the network. • PSTN Delay • IP Network Delay Sent Latency Received Time Data Packet

20. PSTN Delay • PSTN delay is most often the result of transmission delay on long-distance trunks. • Switching delay in network nodes.

21. 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

22. End-to-End Latency d1 d2 d3 Packet Network d4 d5 dm = Delay d6

23. 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.

24. 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)

25. 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.

26. Jitter Variable interpacket timing caused by the network a packet traverses. Sent Jitter Received Time Data Packet

27. 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.

28. Echo

29. Echo Normal Telephony Call Normal Telephony Call with an Echo

30. 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.

31. 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.

32. Solving Echo • Echo cancellers • Deployed in local VoIP gateway • Using digital circuit • T1, E1

33. Silence Suppression

34. 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.

35. VAD Behavior

36. 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.

37. Providing Batter QoS • Supporting dedicated bandwidth • Improving loss characteristics • Avoiding and managing network congestion • Shaping network traffic • Setting traffic priorities across the network

38. Reference • Cisco Voice QoSQuality of Service for Voice over IP • IEC1.Voice Quality (VQ) in Converging Telephony and Internet Protocol (IP) Networks2. Accelerating the Deployment of Voice over IP (VoIP) and Voice over ATM (VoATM)

39. Thank You