Re: Bottlenecks and link upgrades

[William Herrin <bill () herrin us>]

On Wed, Aug 12, 2020 at 12:33 AM Hank Nussbacher <hank () interall co il> wrote:
> At what point do commercial ISPs upgrade links in their backbone as well as peering and transit links that are 
> congested?  At 80% capacity?  90%?  95%?

Hi Hank,

As others have noted, the answer is rarely that simple.

First, what is your consumption? 90th or 95th percentile usually,
after all 100% between 9 and 5 is 100% not 33% but 100% for two
minutes is not 100%. It gets more complicated if any kind of QoS is in
play because capacity-wise QoS essentially gives you not a single
fixed-speed line but many interdependent variable-speed lines.

Next, capacity is not the only question. Here are some of the other factors:

1) A residential customer on the cheapest plan does not merit as clean
a channel as a high-paying business customer you'd like to keep
milking.

2) Upgrades can take months of planning so the capacity now is beside
the point. You'll use your best-guess projection for the capacity at
the time an upgrade can be complete.

3) Some upgrades tend to be significantly more expensive than others.
Lit service to dark fiber, for example. It's pretty ordinary to run
closer to the limit before making an expensive upgrade than a modest
upgrade.

4) A dirty link merits replacement sooner than a clean one. If the
higher-capacity service also clears up packet loss, you'll want to
trigger the decision at a lower consumption threshold.

5) Switching a single path to two paths is more valuable than
switching two paths to three. It has priority at a lower level of
consumption.

Regards,
Bill Herrin

-- 
William Herrin
bill () herrin us
https://bill.herrin.us/