Burapha University ก Department of Computer Science 12 Quality of Service (QoS) Quality of Service Best Effort, Integrated Service, Differentiated Service Factors that affect the QoS Ver. 0.1 :, prajaks@buu.ac.th
Why is VoIP QoS important? Quality of Service - Architecture models Voice Quality Definition Primary Factors Measuring Quality of Service Design Recommendation for VoIP Phone Diagnostic Capabilities Conclusion 2 ก 41
VoIP - Quality of Service 3 ก 41
Why is VoIP QoS important? PSTNs have played the first role in voice communications for a long time. But future is going to be different. It seems that VoIP will become the first choice in voice communications. But on one condition: it must be able to provide a QoS as good as PSTNs. 4 ก 41
VoIP QoS Before establishing a call Setup Delay Blocking Probability After establishing a call Voice Quality Delay Echo Etc. 5 ก 41
VoIP QoS PERFORMANCE DIMENSIONS APPLICATION Bandwidth Delay Sensitivity to Jitter Loss IP Telephony Low High High Med Video Conferencing High High High Med Streaming media Low-High Med Low Med Client / Server Transactions Low Med Low High 1 Email (store/forward) Low Low Low High 1 Best Effort Traffic Low-Med Low Low Low 6 ก 41
QoS Architecture Models Best Effort Service Integrated Service (IntServ) Differentiated Service (DiffServ) 7 ก 41
Best Effort Service describes a network service in which the network does not provide any guarantees that data is delivered or that a user is given a guaranteed quality of service level or a certain priority. In a best effort network all users obtain best effort service, meaning that they obtain unspecified variable bit rate and delivery time, depending on the current traffic load. By removing features such as recovery of lost or corrupted data and preallocation of resources, the network operates more efficiently, and the network nodes are inexpensive. 8 ก 41
Best Effort Service What exactly IP does: All packets treated equally Unpredictable bandwidth Unpredictable delay and jitter 9 ก 41
IntServ (RFC1633) The Int-Serv concept reserves just the right amount of bandwidth to support the flow's requirements and protect it from disruptions caused by network congestion. Reservations are negotiated with each network device along a route to a destination. If each device has resources to support the flow, a reserved path is set up. RSVP (Resource Reservation Protocol) is the signaling protocol that sends messages in the forward direction to request reservations, and then sends messages in the reverse direction to set up the reservations if all devices in a route agree to reserve resources. 10 ก 41
IntServ (RFC1633) 11 ก 41
DiffServ (RFC2474/2475) It differentiates traffic by user, service requirements, and other criteria. It marks packets so that network nodes can provide different levels of service via priority queuing or bandwidth allocation, or by choosing dedicated routes for specific traffic flows. A policy management system controls service allocation. Diff-Serv defines a new DS (Differentiated Services) field in the IP header that replaces the older ToS (Type of Service) field. It works on Layer 3. 12 ก 41
DiffServ (RFC2474/2475) 13 ก 41
QoS Architecture Components Classification Coloring Admission Control Traffic Shaping/Policing Congestion Management Congestion Avoidance Signaling 14 ก 41
VoIP traffic VoIP traffic is one of those kinds that must be provided with realtime bandwidth. Thus in order to have good QoS, network manager must guarantee real-time bandwidth for VoIP. 15 ก 41
Voice Quality is Subjective The concept of voice quality is some how subjective. It varies person to person and depends on many factors such as person s mood, person s definition of a good voice quality, etc. 16 ก 41
Voice Quality Definition One of the important factors in VoIP QoS is Voice Quality (VQ). Despite the subjectivity of VQ, it can be quantified according to three parameters: Service quality Sound quality Conversation quality 17 ก 41
Service Quality Service quality depends on services that company provides users with. These services include: Offered Services Availability of users in foreign countries Network availability, down time, Busy time Reliability Price 18 ก 41
Sound Quality By the term sound quality we mean: Loudness Distortion Noise Crosstalk Fading 19 ก 41
Conversation Quality Conversation quality manly depends on: Echo End-to-end delay Loudness distortion noise Fading Crosstalk Silence suppression performance 20 ก 41
Primary Factors Among the parameters mentioned in previous slides, three parameters are more important than others: Clarity End-to-end delay Echo 21 ก 41
Clarity By the term clarity we mean clearness and distortion-less. There are some parameters that affect clarity: Packet loss Speech codecs Noise Echo Voice Activity detectors (VADs) External factors 22 ก 41
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. Solutions for Packet Loss Assure minimum throughput for selected applications. Prioritization (Classification) & Router flow control 23 ก 41
Speech Codecs A speech codec transforms analog voice into digital bit streams, and vice versa. Some codecs perform compression as well. A codec s effect on VQ is influenced by compression, packet size, packet loss, and any error-correction mechanisms used by the codec itself. 24 ก 41
Other Factors Noise Noise can originate from analog lines or from bit errors on data transmission lines. Voice Activity Detectors Echo External Environmental Factors Room noise, end-user mood, end-user expectations, etc. may cause the audio quality be perceived as unacceptable. 25 ก 41
End-to-End Delay There are some factors that affect end-to-end delay: PSTN delay IP network delay Packet capture delay Switching/Routing delay Jitter Buffer Delay VoIP devices (gateways, terminals, etc.) delays 26 ก 41
Echo Relationship between echo levels, delay and perception is shown in figure below 27 ก 41
Echo Echo can easily be cancelled by the help of echo canceller blocks or by using digital circuits. Performance of these blocks depend on doubletalk and echo return loss. 28 ก 41
QoS versus QoE Quality of Experience (QoE) is subjective and relates to the actual perceived quality of a service by the user This applies to voice, multimedia, and data Quality of service (QoS) is an optimization tool designed to deliver a certain Quality of Experience (QoE) by ensuring that network elements apply consistent treatment to traffic flows as they traverse the network 29 ก 41
Measuring QoS Two measurement units: Mean Opinion Score (ITU P.800) Subjective call quality measurement perceived by the user E-Model (ITU G.107) Transmission planning tool for estimating user satisfaction Objective measurement E-model output: R value Under 60 is not acceptable Over 94.5 is unattainable in VOIP 30 ก 41
Measuring QoS R-Value User Satisfaction MOS 100 94 90 80 Very Satisfied Satisfied Toll Quality Some Users Dissatisfied 5.0 4.4 4.3 4.0 70 Many Users Dissatisfied 3.6 60 50 0 Nearly All Users Dissatisfied Not Recommended 3.1 2.6 1.0 31 ก 41
Measuring QoS In order to measure QoS, operator must measure certain parameters: Clarity Delay Echo 32 ก 41
Measuring QoS - Measuring Clarity The mostly used method for measuring clarity is to employ human listeners, measure mean opinion score (MOS) and the compare it with standards such as P.800. Another method is Perceptual Speech-Quality Measurement (PSQM) defined by ITU in P.861. The third method is Perceptual Analysis Measurement System (PAMS). 33 ก 41
Measuring QoS - Measuring Delay There are two methods for measuring end-to-end delay: Acoustic Ping MLS Normalized Cross-Correlation (MLS stands for Maximum Length Sequence) 34 ก 41
Measuring QoS - Measuring Echo Measure Echo Delay Measure Echo Return Loss (ERL) According to ITU-T G.168 ERL should be greater than 55dB Measure echo canceller performance Convergence time Cancellation depth Doubletalk robustness Perceived Annoyance Caused by Echo (PACE) 35 ก 41
Example of packetization period (packet size) trade off 36 ก 41
Design Recommendations for VoIP - typical The following slides are typical considerations when designing for VOIP Vendors only supports customers with Layer 2/3 switched networks (no shared media devices, cable-based, hub-based LAN) L2 switch ports must be set to autonegotiate for VoIP devices Goal of Zero Percent Packet Loss for VoIP Use G.711 CODEC when possible Excellent Voice Quality Bandwidth usually available in LAN and MAN Use G.729A or G.729AB to conserve bandwidth Take care to meet customer voice quality requirements Watch out for multiple transcodings (multiple VoIP hops) Be careful with VAD subject to clipping effects Centralised voice mail and music can be a call quality issue 37 ก 41
Traffic engineering process - typical For site pairs, determine voice trunks needed Calculate VoIP bandwidth demands Traffic Bandwidth Calculator / Vivinet Assessor Overlay VoIP traffic patterns onto physical network diagram Vivinet Assessor Size the required primary and alternate converged network links: Evaluate current traffic demand Calculate, add in VoIP traffic demand Evaluate various failure scenarios Factor in desired headroom, unusable bandwidth 38 ก 41
Bandwidth Example Requirement: A company wants to support up to 4 simultaneous voice calls over the IP WAN network (128kbps) between two sites If all 4 calls were simultaneously active, this would require 108.8 kbps (using a G.729 codec, 20 ms voice sample, and PPP overhead/frame) of the available 90 kbps of the 128 kbps link This requirement exceeds the carrying capacity of the link and completely starves that data traffic The solution is to upgrade the WAN connection bandwidth. A 256 kbps link is the minimum speed to provide 109 kbps for four G.729 VoIP calls, 80 kbps for data, and 20% availability for zero-bit stuffing 39 ก 41
Phone Diagnostic Capabilities Ping and Traceroute The administrator can execute the Ping or Traceroute command from a specific endpoint with any arbitrary destination, typically another endpoint or Signaling Server. IP Networking statistics The administrator can view information on the packets sent, packets received, broadcast packets received, multicast packets received, incoming packets discarded, and outgoing packets discarded. Ethernet statistics The administrator can view ethernet statistics (for example, number of collisions, VLAN ID, speed and duplex) for the IP Phone on a particular endpoint. The exact statistics will depend on what is available from the IP Phone for the specific endpoint. UNISTIM statistics (Unified Networks IP Stimulus) The administrator can view RUDP (Reliable User Datagram Protocol) statistics (for example, number of messages sent, received, retries, resets, and uptime) for the IP Phones. Real time Transport Protocol statistics The administrator can view RTP/RTCP QoS metrics (for example, packet loss, jitter, etc.) while a call is in progress. 40 ก 41
Conclusion: Providing Better VoIP QoS Supporting dedicated bandwidth Improving loss characteristics Avoiding and managing network congestion Setting traffic priorities across the network Echo cancellation Minimizing delay 41 ก 41