Business fiber internet sends data as pulses of light through thin strands of glass. DSL and cable connections send electrical signals over copper wire instead. AT&T Business describes fiber as light waves moving along glass fiber lines. Glass resists the electrical interference that degrades copper connections. Actual speed still depends on the plan tier the carrier provisions.
Shared fiber service commonly runs on a passive optical network, or PON. One carrier fiber serves many subscribers through passive optical splitters. GPON, standardized as ITU-T G.984, delivers about 2.5 Gbps down and 1.25 Gbps up. The newer XGS-PON standard, ITU-T G.9807.1, carries 10 Gbps in both directions. Those totals describe the shared line, not the speed each subscriber receives.
Shared fiber tiers from carriers like AT&T offer symmetrical upload and download speeds. Symmetry means upload capacity matches download capacity on the same plan. AT&T Business Fiber lists equal speeds in both directions, from 300 Mbps to 5 GIG. Upload capacity drives video calls, cloud backups, and large file transfers. Cable plans allocate less capacity to upload traffic than to download.