PDD (Post Dial Delay)
What is PDD (Post Dial Delay)?
PDD (Post Dial Delay) is the elapsed time between dialing a call and hearing ringback tone or the call connecting at the far end. It's a setup-phase metric, driven by how long SIP signaling takes to cross the carriers between the originating and terminating network, not by anything in the audio path itself. Consistently high PDD — generally above one to two seconds — is a common early symptom of a congested or poorly-routed voice trunk, often showing up before any drop in call audio quality.
More detail
PDD measures the signaling phase, not the media phase: it's the time from the outbound INVITE through provisional responses like 100 Trying and 180 Ringing, across however many carriers and softswitches sit between origination and termination — a route with more intermediate hops typically posts a higher PDD than a direct one.
Voice platforms and wholesale carriers track average PDD per route and per carrier the same way they track jitter, packet loss, and MOS for the media path, and re-route traffic away from a route with elevated PDD even when calls that do connect over it sound clean.
Frequently asked
- Is PDD the same thing as call setup time?
- Yes — PDD specifically measures the interval from dialing to ringback or answer, which is what most voice platforms mean by call setup time. It's distinct from in-call audio metrics like jitter or MOS, which only apply once a call is already connected.
- What causes high PDD on a call?
- High PDD usually traces to the signaling path rather than the audio path — too many carrier hops between origination and termination, an overloaded softswitch, or a terminating carrier slow to return ringback — and is typically resolved by re-routing the call over a more direct or higher-quality trunk.
See also
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