Burst Density — VoIP Quality Metric
RTCP-XR burst density ratio — fraction of VoIP call time in burst loss state, showing whether packet loss is concentrated or evenly spread.
Burst Density
| Property | Value |
|---|---|
| Key | burst_density |
| Unit | Ratio (0–1) |
| Type | Gauge |
| Direction | Receive |
| RFC | RFC 3611 Section 4.1 (RTP Control Protocol Extended Reports) |
What It Measures
Burst Density is an RTCP-XR metric that measures the fraction of time a call spent in the burst loss state — a condition where packets are being lost at a rate above the burst threshold. A value of 0.10 means 10% of the call's duration was spent in active burst loss. A value of 0 means loss was evenly distributed or absent.
The metric is defined in the RFC 3611 VoIP Metrics Report Block, which extends standard RTCP with detailed quality analysis. Endpoints that support RTCP-XR report this value alongside Gap Density, its companion metric that measures the fraction of time in a "good" state.
Why It Matters
Average packet loss statistics hide the most important characteristic of loss: whether it is random or bursty. A 2% average loss rate with high burst density means the call experienced long dropout events separated by clean periods — which is audibly far worse than 2% loss spread evenly across every second.
Carriers using RTCP-XR reports know this metric well. When validating SIP trunks or SBCs, burst density is what separates a network that tolerates VoIP from one that's genuinely optimized for it. A clean network delivers low burst density regardless of total loss; a congested or poorly-QoS-configured network reveals itself through burst density long before average metrics look alarming.
For IVR and load testing scenarios, burst density exposes problems at the infrastructure level that MOS and loss rate alone cannot pinpoint. An FQDN-routed call that suddenly bursts during peak hours? Burst density catches it.
Thresholds
| Level | Value | Meaning |
|---|---|---|
| Good | Below 0.05 | Loss is minimal and non-bursty |
| Warning | 0.05 – 0.20 | Noticeable burst events, investigate path |
| Critical | Above 0.20 | Significant burst loss, users hear dropouts |
How to Fix It
- Compare with Gap Density. High burst density paired with low gap density means the call spent most of its time in a degraded state — not just occasional bursts.
- Check for queue overflow. Burst loss is the signature of router queue overflow. Apply QoS priority queuing and ensure RTP traffic carries DSCP EF markings.
- Look for route instability. Burst events often coincide with BGP reconvergence or OSPF recalculation. Correlate burst density spikes with network change windows.
- Evaluate jitter buffer sizing. Undersized jitter buffers convert bursty delivery into burst loss at the decoder. Review Jitter alongside burst density.
- Review firewall timeout settings. Stateful firewalls with aggressive session timeouts can drop entire streams mid-call, producing extreme burst density values.
Related Metrics
- Gap Density — Companion metric: fraction of time in the good (non-burst) state
- Burst Duration — Average length of each burst event in milliseconds
- Packet Loss Rate — Overall loss percentage; high burst density can accompany low average loss
- Sequence Gaps — Individual burst loss events at the packet level