IP-Layer FEC: Beyond Modem Physical-Layer Error Correction

Satellite modems implement physical-layer FEC to correct bit errors on the RF link. Under normal conditions, this works well. However, during rain fade or sudden burst packet loss, the modem's纠错 capacity is exceeded. Uncorrected packets overflow into the IP layer, triggering TCP window collapse and retransmission storms — devastating throughput on a 600ms RTT link.

Sat-Link operates at the IP layer, downstream of the modem. It captures IP packets before they enter the satellite link, applies adaptive FEC encoding, and sends the encoded stream through the modem. On the receiving side, cloud-based decoding reconstructs any packets the modem failed to deliver — filling the gap that physical-layer FEC cannot cover.

This two-layer defense (modem FEC + IP-layer FEC) ensures that even when rain fade overwhelms the modem's correction capacity, your applications continue to receive data without TCP collapse or retransmission delays.

Satellite bandwidth is expensive. A naive FEC implementation would waste capacity by always adding redundancy, even when the link is clean. Sat-Link solves this with adaptive zero-redundancy convergence: the algorithm continuously monitors real-time packet loss rate and RTT jitter, dynamically adjusting FEC overhead.

When link quality is good (loss rate near 0%), FEC redundancy converges to 0% — the system adds zero overhead and does not slow down existing speeds. When rain fade begins and packet loss spikes, redundancy ramps up within seconds to match the loss rate. When conditions improve, it converges back down. This ensures you never pay for bandwidth you don't need.