Well, you're preaching to the choir, but I have a couple comments in line
nonetheless. A really technical discussion would require more knowledge of
statistics, queuing theory, etc., than I have. But it can't hurt to discuss
the issues at a high level also.
At 05:44 PM 5/22/01, John Hardman wrote:
>Hi
>
>Keep in mind that many people site the phone as a highly available system,
>but do you know that it is highly available? People get the impression that
>the phone system is very reliable because they are used to hearing a dial
>tone when they pick it up. But most people are not on the phone 24x7 and
>have no real idea if their phone is available or not.
In the phone industry, we can use Erlang and other obscure methods for
calculating the amount of bandwidth needed based on an acceptable frequency
of someone picking up the phone and not getting dial tone. Why can't we do
something similar with networking? I suspect it's because network traffic
is so different from phone traffic. We claim that network traffic is
"bursty," but it's not nearly as bursty as phone traffic. There's very
little quiet time. Even if the user isn't doing something there's still
overhead traffic, keepalives, routing table updates, etc. The consequences
of not being able to send this overhead traffic can result in serious
performance degradation.
>I wish I could find the URL, but there was a study done at one of the
>universities back east that actually checked the availability of the phone
>system there and compared it to peoples belief's as to the availability of
>the system. Most people felt the phone system was up more than 99.9% of the
>time, as they almost never picked up a dead phone, but in fact the phone
>system was only up about 98% of the time.
>
>Now this begs the next question... do people need 99.999% uptime on the
>phone system or on their network? Keep in mind that 99.999% uptime equals to
>apx 1 minute of downtime per 30 days.
On an AVERAGE, 99.999% uptime equals 1 minute of downtime per 30 days. It's
the cases where the amount of downtime deviates a lot from the average that
kill you. What if you go a couple years with no downtime and then it all
hits at once when you are printing pink slips for all the employees you
laid off and are walking out the door. I'm sure we can all come up with
stories like this. (The example probably makes more sense with 99.99%
uptime, but even with 99.999%, it's important to consider the standard
deviation from the average, not just the average.)
>Many network managers want to give the
>99.999% guarantee to their internal/external customer and are willing to
>give SLA's to that effect without ever seeing if there really is a need for
>it.
>
>I am asked a couple of times a month for a 99.999% solution. By the time
>they answer a few questions they figure out that they can easily withstand
>more than 1 minute per month of down time.
>
>With the idea that BGP is growing widely with all of the /24 companies
>joining the table, is a real shame. I would venture to say that many of the
>companies out there could stand to take the down time of a single connection
>or a multiple connection to the same ISP and never really hurt their
>business.
>I can not say if BGP will scale to meet this growing "need", but I
>can tell you that having to get more and more memory and CPU to handle the
>larger and larger routing table is a burden and a pain. Hopefully someone
>much more intelligent than I will find a simple and easy solution.
But as Howard and Geoff would say, we're dealing with the "tragedy of the
commons." Everyone wants to meet their own particular needs and is
unwilling to meet the needs of the overall community. The phrase comes from
something to do with sheep herders sharing a common area in Medieval
Britain, if I recall. ;-)
Priscilla
>BTW, yes some places multihome their phones too... I was at one for awhile.
>
>$0.02
>--
>John Hardman CCNP MCSE
>
>
>""Priscilla Oppenheimer"" wrote in message
>[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
>
> > Aside from Priscilla (not Geoff Huston): What if the phone system had
> > evolved this way? How many companies have redundant trunk lines? Don't we
> > just assume that the "phone company" will always provide service? We
don't
> > multihome to the phone system, (do we?)
> >
>
> > Priscilla
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Priscilla Oppenheimer
http://www.priscilla.com
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