Transcript Document
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 is Subjective
VQ should be approached from an endto-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.
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 Architecture
HQ
Branch 1
IWF
Server Farm
Telephone
IWF
Packet Network
Branch N
PBX
Telephone
PSTN
IWF
Telephone
Service Requirements
Service Quality
Sound Quality
Conversation Quality
• Offered services—such
as calling card, 1800/900 services, followme, 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
• 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.
Perception of One Aspect Affects Perception
of Overall VQ
Clarity and Delay Are Orthogonal Aspects of
VQ
Users will report unacceptable VQ if only one
aspect of VQ is unacceptable.
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
Normal
Packet
Network
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 errorcorrection 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
Sent
Latency
Received
Data Packet
Time
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
d1
d2
d3
Packet
Network
d4
dm = Delay
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
Variable interpacket timing caused by the
network a packet traverses.
Sent
Jitter
Received
Time
Data Packet
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 realtime 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 QoS
Quality of Service for Voice over IP
IEC
1.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