Assessment of voip service availability
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Assessment of VoIP Service Availability. Wenyu Jiang Henning Schulzrinne IRT Lab, Dept. of Computer Science Columbia University December 2002. Overview. (on-going work, preliminary results, still looking for measurement sites, …) Service availability Measurement setup Measurement results

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Assessment of VoIP Service Availability

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Assessment of voip service availability

Assessment of VoIP Service Availability

Wenyu Jiang

Henning Schulzrinne

IRT Lab, Dept. of Computer Science

Columbia University

December 2002


Overview

Overview

(on-going work, preliminary results, still looking for measurement sites, …)

  • Service availability

  • Measurement setup

  • Measurement results

    • call success probability

    • overall network loss

    • network outages

    • outage induced call abortion probability


Service availability

Service availability

  • Users do not care about QoS

  • at least not about packet loss, jitter, delay

  • rather, it’s service availability how likely is it that I can place a call and not get interrupted?

  • availability = MTBF / (MTBF + MTTR)

    • MTBF = mean time between failures

    • MTTR = mean time to repair

  • availability = successful calls / first call attempts

    • equipment availability: 99.999% (“5 nines”)  5 minutes/year

    • AT&T: 99.98% availability (1997)

    • IP frame relay SLA: 99.9%


Availability pstn metrics

Availability – PSTN metrics

  • PSTN metrics (Worldbank study):

    • fault rate

      • “should be less than 0.2 per main line”

    • fault clearance (~ MTTR)

      • “next business day”

    • call completion rate

      • during network busy hour

      • “varies from about 60% - 75%”

    • dial tone delay


Example pstn statistics

Example PSTN statistics

Source: Worldbank


Measurement setup

Measurement setup


Measurement setup1

Measurement setup

  • Active measurements

  • call duration 3 or 7 minutes

  • UDP packets:

    • 36 bytes alternating with 72 bytes (FEC)

    • 40 ms spacing

  • September 10 to December 6, 2002

  • 13,500 call hours


Call success probability

Call success probability

  • 62,027 calls succeeded, 292 failed  99.53% availability

  • roughly constant across I2, I2+, commercial ISPs


Overall network loss

Overall network loss

  • PSTN: once connected, call usually of good quality

    • exception: mobile phones

  • compute periods of time below loss threshold

    • 5% causes degradation for many codecs

    • others acceptable till 20%


Network outages

Network Outages

  • sustained packet losses

    • arbitrarily defined at 8 packets

    • far beyond any recoverable loss (FEC, interpolation)

  • 23% outages

  • make up significant part of 0.25% unavailability

  • symmetric: AB  BA

  • spatially correlated: AB   AX

  • not correlated across networks (e.g., I2 and commercial)


Network outages1

Network outages


Network outages2

Network outages


Outage induced call abortion proability

Outage-induced call abortion proability

  • Long interruption  user likely to abandon call

  • from E.855 survey: P[holding] = e-t/17.26 (t in seconds)

  •  half the users will abandon call after 12s

  • 2,566 have at least one outage

  • 946 of 2,566 expected to be dropped  1.53% of all calls


Conclusion

Conclusion

  • Availability in space is (mostly) solved  availability in time restricts usability for new applications

  • initial investigation into service availability for VoIP

  • need to define metrics for, say, web access

  • unify packet loss and “no Internet dial tone’’

  • far less than “5 nines”

  • working on identifying fault sources and locations

  • looking for additional measurement sites


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