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Tech Note: Voice Quality Problems in IP Networks and Root Cause

 

 

Introduction

 

Deploying a VoIP infrastructure introduces a new set of challenges that do not exist in circuit-switched networks like the PSTN. Some of the common network problems encountered by providers deploying VoIP infrastructure include the following:

 

  • Delay/latency
  • Jitter
  • Packet loss
  • Voice Activity Detection (VAD)

 

  • Other issues,

 

1. Physical layer impairments: Noise, interference in the line, loose connectors, badly terminated punch-down block, and so on

 

2. Last-mile connection bandwidth: Low-speed connections, oversubscription of circuits resulting in congestion, and so on

 

3. Network resource overutilization: High CPU and memory utilization on network devices, oversubscription of IP links resulting in congestion, high number of input/output drops under interfaces, lack of QoS for voice, and so on

 

4. VoIP application issues: Poor software implementation on PC-based soft clients, lack of QoS and prioritization of resources for voice, and so on

 

All the above factors to be considered and verified while deploying VOIP infrastructure.

 

 

What are the common Voice Quality Problems in IP Networks?

 

The network problems listed above can seriously impact the quality of voice in an IP network. Some of the common voice quality issues experienced by providers include the following:

 

1. Noise: This is typically any noise on the line introduced by an analog source in addition to the voice signal. Noise will typically leave the conversation intelligible but still far from excellent. Static, hum, crosstalk, and intermittent popping tones are examples where the calling and called parties can understand each other, but with some effort. Some noises are so severe that the voice becomes unintelligible.

 

2. Voice distortion: This is typically any problem that affects the voice (RTP/media stream) itself.

 

3. Echoed voice: Echo voice is where the voice signal is repeated on the line. It can be heard at either end of the call, in varying degrees and with many combinations of delay and loss within the echoed signal.

 

 

Voice Quality Issues and Root Cause

 

 

Voice Quality Issue
Symptom
Root Cause

   

Absolute silence

 

 

This type of silence between speech

can be understood if you have ever

had the experience of not knowing

whether the other person is still there

because there is no sound on the line.

 

A common cause for this problem is

VAD without comfort noise. To experience

this symptom, the background

noise is usually loud enough for the

silence insertion to be noticeable but

soft enough so that VAD will be

engaged.

Clicking

Clicking is an external sound similar to

a knock that is usually inserted at

intervals.

 

A common cause is clock slips or other

digital errors on the line.

Crackling

Crackling is an irregular form of very

light static, similar to the sound a fire

makes.

 

A common cause is poor electrical connections,

in particular poor cable connections.

Other causes are electrical

interference and a defective power

supply on the phone.

Crosstalk

Crosstalk is a familiar concept where

you can hear someone else’s conversation

on the line. Commonly the other

parties cannot hear you. There are also

forms of crosstalk where all parties can

hear each other.

 

Wires in close proximity, where the

signal of one is induced into the other,

is a common cause of this problem.

Hissing

Hissing is more driven and constant

than static. White noise is a term often

associated with strong hissing. Pink

noise is a less constant hissing noise,

and brown noise is even less constant.

 

A common cause of hissing is VAD.

When VAD kicks in, comfort noise

packets are introduced into the audio

stream. The hissing sound is caused by

the introduction of comfort noise into

the conversation.

Static

Severe static is an example of static that

in addition to creating background

noise, affects the dial and ring tones

and the voice itself. Another name for

this symptom might be scratchy or

gravel voice.

 

A common cause is A-law/Mu-law codec

mismatch. A-law is a codec companding

scheme used outside of the United

States, whereas Mu-law is a U.S.-specific

codec companding scheme. This is typically

involved in international calls originating

or terminating in the United

States.

Talker echo

Talker echo is the signal that leaks in

the far-end hybrid and returns to the

sender (talker). The talker hears an

echo of his own voice.

Common causes are

Insufficient loss of the echo signal.

Echo cancellers in the gateway adjacent

to the far-end hybrid not activating.

Acoustic echo caused by the listener’s

phone.

Tunnel voice

Tunnel voice sounds similar to talking

in a tunnel or on a poor-quality

mobile phone car kit.

 

A common cause is tight echo with

some loss. For example, 10-ms delay and

50 percent loss on the echo signal.

Soft voice

 

Soft voice is like a low voice that is

hard to hear.

 

Soft voice is usually caused by too

much attenuation on the signal, possibly

introduced at one of a number of points

in the network such as a voice gateway

when trying to reduce echo.

Version history
Revision #:
2 of 2
Last update:
‎08-29-2017 10:56 AM
 
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