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)
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. |
