The advantages of reduced cost and bandwidth savings of carrying voice over packet networks are associated with some quality of service issues that are unique to packet networks. In a circuit-switched or TDM environment, bandwidth is dedicated, making QoS—quality of service—implicit, whereas, in a packet-switched environment, all kinds of traffic are mixed in a store-and-forward manner.
So, in a packet-switched environment, there is the need to devise schemes to prioritize real-time traffic. So… in an integrated voice data network, QoS is essential to ensure the same high quality as voice transmissions in the traditional circuit-switched environment.
QoS and Voice Quality
Some of the quality of service issues customers face include the following:
Delay—Delay causes two problems: echo and talker overlap. Echo is cased 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 (ms). Talker overlap becomes significant if the one-way delay becomes greater than 250 ms.
Jitter—Jitter relates to variable inter-packet timing caused by the network that a packet traverses. Removing jitter requires collecting packets and holding them long enough to allow the slowest packets to arrive in time to be played in the correct sequence, which causes additional delay.
Lost packets—Depending on the type of packet network, lost packets can be a severe problem. Because IP networks do not guarantee service, they will usually exhibit a much higher incidence of lost voice packets than ATM networks.
Echo—Echo is present even in a conventional circuit-switched telephone network, but is acceptable because the round-trip delays through the network are smaller than 50 ms and the echo is masked by the normal side tone that every telephone generates. Echo is a problem in voice over packet networks because the round-trip delay through the network is almost always greater than 50 ms. For this reason, echo cancellation techniques must be used.
Solutions to Voice Quality Issues
Quality of service issues for voice may be handled by the H.323, VoIP, VoATM, or VoFR standards, or by an internetworking device. Following are some solutions to quality of service issues:
Delay—Minimize the end-to-end delay budget, including the accumulation delay, processing delay, and network delay.
Jitter—Adjust the jitter buffer size to minimize jitter. On an ATM network, the approach is to measure the variation of packet levels over a period of time and incrementally adapt the buffer size to match the calculated jitter. On an IP network, the approach is to count the number of packets successfully processed and adjust the jitter buffer to target a predetermined allowable late packet ratio.
Lost packets—While dropped packets are not a problem for data (due to retransmission), they cause a significant problem for voice applications. To compensate, voice over packet software can interpolate for lost speech packets by replaying the last packet, or can send redundant information at the expense of bandwidth utilization.
Echo—Echo cancellation techniques are used to compare voice data received from the packet network with voice data being transmitted to the packet network. The echo from the telephone network hybrid is removed by a digital filter on the transmit path into the packet network.
Effect of QoS on Voice Quality
With all of the “marketing hype” around QoS today, many customers have become skeptical of the claims some vendors are making.
Here’s one way to look at the actual effect of Cisco QoS technologies on voice quality.
The blue line represents the total network data load. The green line represents voice quality without QoS. As you can see, the quality of a voice call rises and falls in response to varying levels of background traffic.
The red line represents voice quality with QoS enabled, showing that high voice quality remains constant as background traffic fluctuates.