Guaranteed or shared bandwidth: what’s the difference?

Two radically different promises behind the same headline speed:

• Guaranteed bandwidth (CIR, Committed Information Rate): capacity is reserved end-to-end, committed in the contract, with a repair SLA and usually symmetric (as much up as down). This is the promise of FTTO and DIA (dedicated Internet access).
• Shared bandwidth (best effort): capacity is pooled across customers; the advertised speed is a theoretical maximum that drops when everyone uses it. This is the consumer FTTH model — fine for a secondary site, risky for a critical activity.

The decision rule: put guaranteed on what must not stop (HQ, production, video, VoIP) and shared + 4G/5G backup elsewhere. A best-effort “1 Gb/s” and a guaranteed “1 Gb/s” are not comparable — and the price gap (often ×5) reflects it.