Four layers, four boundaries. A failure in one does not reach the next.

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Notes

Notes

The shorter pieces.

Everything under this heading

Note · 5 min

Numbering as a boundary

Why numbering is a boundary rather than an inventory.

 Note · 6 min

The shape of fraud

Automated, indiscriminate, and timed against detection latency.

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Layers

The layers these describe

Layer one

Connectivity

The outermost layer. Whether a call can happen at all.

Open Layer two

Control

Where a call goes, and what happens when nobody answers.

Open Layer three

Numbering

What a caller dials, and whether it can ever be unreachable.

Open Layer four

Operation

Change, treated as a layer because it causes more incidents than failure does.

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