Measurements along the test path indicate that the current call load is causing a reduction in voice quality, due to choice of codec and/or number of concurrent calls.

Recommended action

To improve a degraded MOS:

  • Choose a less demanding codec.
  • If the number of concurrent calls is greater than 1, reduce the number specified.
  • Identify and address issues related to capacity, bottlenecks, and cross-traffic.

Detailed explanation

Each codec differs in terms of size of packets sent, rate of sending, and load associated with a single call. Each may impact the network differently. For example, a network with smaller capacity may require a codec with reduced impact but which provides reduced voice quality.

A network path that includes a bit-bucket mechanism, typical for example on consumer broadband networks, can be overloaded with too many packets. This overloading of the path results in packets being dropped, i.e. "packet loss", as a function of rate limiting. In this case, the probing associated with a sufficiently high VoIP call load can result in loss even though a typical VoIP stream is unlikely to be affected. In this case a secondary message is generated to indicate that a rate-limiting mechanism is interfering with testing.

Possible secondary messages

  • "A less demanding codec may improve MOS ratings"
  • "Reducing the number of concurrent calls may improve MOS ratings"
  • "Rate-limiting mechanism is interfering with testing"
  • "Number of concurrent calls requested exceeds capacity of network path"

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