1、Carrier-grade vs. Internet VoIP,Henning Schulzrinne (with Wenyu Jiang) Columbia University FCC Technical Advisory Council III Washington, DC October 20, 2003,Overview,Previous talk: interactive communication services signaling & media Now focus on overall architecture: network & service availability
2、 signaling services: SIP, H.323 supporting services: DNS, DHCP, LDAP, network transport network quality-of-service packet loss, delay, jitter,Overview,(on-going work, preliminary results, still looking for measurement sites, ) Service availability Measurement setup Measurement results call success p
3、robability overall network loss network outages outage induced call abortion probability,Service availability,Users do not care about QoS at least not about packet loss, jitter, delay rather, its service availability how likely is it that I can place a call and not get interrupted? availability = MT
4、BF / (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 (2003): Sprint IP frame relay SLA: 99.5%,Availability PSTN metrics,PSTN metrics (Worldbank study): fau
5、lt 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,Source: Worldbank,Measurement setup,Measurement setup,Active measurements call duration 3 or
6、 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,62,027 calls succeeded, 292 failed 99.53% availability roughly constant across I2, I2+, commercial ISPs,Overall network loss,PSTN: once connected
7、, 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,sustained packet losses arbitrarily defined at 8 packets far beyond any recoverable loss (FEC, interpolation) 23% outa
8、ges make up significant part of 0.25% unavailability symmetric: AB BA spatially correlated: AB AX not correlated across networks (e.g., I2 and commercial),Network outages,Network outages,Outage-induced call abortion probability,Long interruption user likely to abandon call from E.855 survey: Pholdin
9、g = 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,Conclusions from measurement,Availability in space is (mostly) solved availability in time restricts usability for new applications initial in
10、vestigation 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,Whats next?,Existing SLAs are mostly useless too
11、 many exceptions wrong time scales: month vs. minutes no guarantees for interconnects Existing measurements similarly dubious Limited ability to learn from mistakes what are the primary causes of service unavailability? what can I do to protect myself multi-homing via same fiber? diverse access mechanisms? Consumers of services have no good ways to compare service availability only some very large customers may get access to carrier-internal data Thus, market failure Need published metrics similar to switch availability reporting,
