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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

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 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)
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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