Quality of VoIP
Quality of Service Issues in VoIP,echo is perceived as a significant quality problem,voice-over-packet systems must address the need for echo control in VoIP voice quality.
The advantages of reduced cost and bandwidth savings of carrying voice-over-packet networks are associated with some quality-of-service (QoS) issues unique to packet networks. By definition, QoS is the expectations the user of a system has regarding certain parameters like delay, reliability in terms of loss, duplication or out of order. QoS varies from application to application. You would not have the same expected QoS for email (text-based) and video-conferencing. While the former will lay a lot of emphasis on prevention of loss of packets and much less on delay; the latter will do the contrary. Below, we consider delay, which is the parameter having the most importance in voice applications, especially VoIP.
Talker overlap (or the problem of one talker stepping on the other talker's speech) becomes significant if the one-way delay becomes greater than 250 milliseconds. The end-to-end delay budget is therefore the major constraint and driving requirement for reducing delay through a packet network.
The following are sources of delay in an end-to-end, voice-over-packet call:
Accumulation Delay (Sometimes Called Algorithmic Delay)
This delay is caused by the need to collect a frame of voice samples to be processed by the voice coder. It is related to the type of voice coder used and varies from a single sample time (.125 microseconds) to many milliseconds. A representative list of standard voice coders and their frame times follows:
Network Delay
This delay is caused by the physical medium and protocols used to transmit the voice data and by the buffers used to remove packet jitter on the receive side. Network delay is a function of the capacity of the links in the network and the processing that occurs as the packets transit the network. The jitter buffers add delay, which is used to remove the packet-delay variation to which each packet is subjected as it transits the packet network. This delay can be a significant part of the overall delay, as packet-delay variations can be as high as 70 to 100 milliseconds in some frame-relay and IP networks.
The advantages of reduced cost and bandwidth savings of carrying voice-over-packet networks are associated with some quality-of-service (QoS) issues unique to packet networks. By definition, QoS is the expectations the user of a system has regarding certain parameters like delay, reliability in terms of loss, duplication or out of order. QoS varies from application to application. You would not have the same expected QoS for email (text-based) and video-conferencing. While the former will lay a lot of emphasis on prevention of loss of packets and much less on delay; the latter will do the contrary. Below, we consider delay, which is the parameter having the most importance in voice applications, especially VoIP.
Delay
Delay causes two problems: echo and talker overlap. Echo is caused by the signal reflections of the speaker's voice from the far-end telephone equipment back into the speaker's ear. Echo becomes a significant problem when the round-trip delay becomes greater than 50 milliseconds. As echo is perceived as a significant quality problem, voice-over-packet systems must address the need for echo control and implement some means of echo cancellation.Talker overlap (or the problem of one talker stepping on the other talker's speech) becomes significant if the one-way delay becomes greater than 250 milliseconds. The end-to-end delay budget is therefore the major constraint and driving requirement for reducing delay through a packet network.
The following are sources of delay in an end-to-end, voice-over-packet call:
Accumulation Delay (Sometimes Called Algorithmic Delay)
This delay is caused by the need to collect a frame of voice samples to be processed by the voice coder. It is related to the type of voice coder used and varies from a single sample time (.125 microseconds) to many milliseconds. A representative list of standard voice coders and their frame times follows:
- G.726 adaptive differential pulse-code modulation (ADPCM) (16, 24, 32, 40 kbps)-0.125 microseconds
- G.728 LD-code excited linear prediction (CELP)(16 kbps)-2.5 milliseconds
- G.729 CS-ACELP (8 kbps)-10 milliseconds
- G.723.1 Multirate Coder (5.3, 6.3 kbps)-30 milliseconds
Processing Delay
This delay is caused by the actual process of encoding and collecting the encoded samples into a packet for transmission over the packet network. The encoding delay is a function of both the processor execution time and the type of algorithm used. Often, multiple voice-coder frames will be collected in a single packet to reduce the packet network overhead. For example, three frames of G.729 code words, equaling 30 milliseconds of speech, may be collected and packed into a single packet.Network Delay
This delay is caused by the physical medium and protocols used to transmit the voice data and by the buffers used to remove packet jitter on the receive side. Network delay is a function of the capacity of the links in the network and the processing that occurs as the packets transit the network. The jitter buffers add delay, which is used to remove the packet-delay variation to which each packet is subjected as it transits the packet network. This delay can be a significant part of the overall delay, as packet-delay variations can be as high as 70 to 100 milliseconds in some frame-relay and IP networks.
