At 5:34 PM 11/27/95, Dave Siegel wrote:

...

It is because of these sorts of questions that I never know what it means when someone says that their network is available 99.9%

Problems with CPE is usually not figured into network uptime.

I've been away from the NOC for a couple of years now, but one way that made sense for reporting a general availability number is to consider the backbone as one component and each leaf node as an identical independent component (leased line, router and CSU/DSU). Then you can define the end to end availability as the concatentation of two independent leaf nodes and the backbone. Highly redundant backbones always have a high availability (>99%). Leased lines are abominable (low 90s or less -- don't ask me about circuits to Italy) and two routers and two CSU/DSUs lead to an overall end to end of 95% or less. In my experience with leased lines, they always dominated this equation and overall availability is strictly limited by actual leased line availability. So keep that CPE in the equation and figure out how to get highly available bit pipes. I think actual networks like frame relay have a chance of being much more available since the network actually understands frames. Leased lines are simply electrical signalling conduits to the phone company -- not a network. I'm skeptical of any end-to-end availability figures over 97%. I don't think they reflect the reality of leased line circuits today, or else they don't include the leaf node circuits and only report backbone availability. For a highly redundant backbone, almost any definition of availability should result in a number like 99.mumble%. Remember 99.9% availability means less than 9 hours outage per year. Routing hiccups take that much. One or two leased lines outages is all you get for 9 hours. The real world is a lot less available than that. But since half the web servers I try to talk to refuse me half the time, I'm not sure that network availability per se (HWB's complaints duly acknowledged) is the tallest pole in the tent. --Kent