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Reliability

The reliability mechanisms at different layers are complementary, not redundant. Each layer handles a class of failure that the layers below cannot see.

Why COP-1 alone is not sufficient​

COP-1 runs independently on each hop. Consider a three-hop path:

Sat A → Sat B → Sat C → Sat D

COP-1 on the A--B link confirms that Sat B received the frame. But Sat B's router may drop the packet before forwarding it to Sat C — due to queue overflow, a software fault, or a route change. COP-1 on A--B has already reported success. Neither the sender (A) nor the final receiver (D) knows the packet was lost.

Only an end-to-end protocol (SRSPP) between A and D can detect and recover from this. SRSPP's sequence numbers span the entire path, so D knows exactly which packets it has received and can request retransmission from A.

Why SRSPP alone is not sufficient​

Without per-hop reliability, SRSPP must retransmit every packet lost to bit errors. On a lossy RF link this means:

  • Each retransmission traverses the full multi-hop path, consuming bandwidth on every intermediate link.
  • Each retransmission is itself subject to the same per-hop loss rate.
  • Throughput degrades geometrically with hop count: a 1% frame loss rate per hop becomes (1−0.99n)(1 - 0.99^n) end-to-end loss for an nn-hop path.

With COP-1 on each hop, frame losses are recovered locally in one link round-trip time. SRSPP only needs to retransmit when an intermediate router drops a packet — a much rarer event than a bit error.

Recovery summary​

  1. A bit error on the RF link is corrected by the coding layer's FEC. No retransmission occurs.
  2. If FEC cannot correct the damage, the corrupted frame is discarded. COP-1 detects the gap in the frame sequence and retransmits the frame on the same hop.
  3. If a packet survives all hops but is dropped at an intermediate router, SRSPP detects the missing sequence number and retransmits end-to-end.

Each layer handles only the residual failures of the layer below. The result is that SRSPP retransmissions are rare, but they remain necessary for correctness in a multi-hop network.