Voice over IP can suffer from a variety of problems that can be challenging to detect and resolve, such as periods of degraded quality because of network congestion, and echo resulting from interconnections to other services. These issues require a new approach to fault and performance management.
A new VoIP management protocol, RTP Control Protocol Extended Reports (RTCP XR), defines a set of metrics that contain information for assessing VoIP call quality and diagnosing problems. The IETF this month published RTCP XR as RFC 3611 .
RTCP XR can be implemented as software integrated into IP phones and gateways inexpensively. RTCP XR messages containing key call-quality-related metrics are exchanged periodically between IP phones and gateways. This lets a probe or analyzer monitor these metrics midstream to support problem resolution, or be retrieved from a gateway using SNMP .
The protocol measures VoIP call quality using these key metrics:
Packet loss and jitter affect call quality. The jitter buffer removes the jitter in the receiving IP phone or gateway, but this process adds delay and causes packets that arrive late to be discarded.
RTCP XR reports the packet loss rate, packet discard rate (because of jitter) and the distribution of lost and discarded packets. The loss/discard distribution describes the call in terms of bursts (periods during which the loss/discard rate is high enough to cause noticeable quality degradation) and gaps (periods during which lost or discarded packets occur infrequently and hence quality is generally good).
Delay can make conversations difficult and make the effects of echo obvious. RTCP XR reports round-trip delay, corresponding to the packet path delay that would be measured using RTCP, and end system delay, which represents the delay that the VoIP endpoint adds (because of encoding, decoding and the jitter buffer).
If a signal is too loud, too quiet or too noisy, call quality suffers. RTCP XR reports the signal and noise level associated with the received signal, making it easier to identify signal- and noise-level problems.
If the IP endpoint contains an echo canceller, RTCP XR can report the residual (uncanceled) echo level, which helps the remote IP endpoint calculate call quality more accurately and make the detection of echo problems easier.
RTCP XR can report call quality directly, in terms of estimated R factor or mean opinion score (MOS). An R factor is a conversational-quality metric in the range of 0 to 100. MOS-LQ is a listening-quality metric and MOS-CQ a conversational-quality metric, both in the range of 1 to 5. These can be generated using embedded software-agent technology.
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