What it is
L-band mobile satellite service uses a lower frequency range that supports small, rugged antennas. It is the technology behind Inmarsat FleetBroadband and Iridium services.
Trade-offs
Bandwidth is far lower than Ku- or Ka-band, but the link is highly resistant to weather, ship motion and signal loss, making it ideal for telemetry and safety rather than heavy data.
Its role
In a layered network, L-band is the reliable fallback and safety tier that the rest of the design leans on when faster links are unavailable.