Simplex protocol for noisy channel
Assume-if frame damaged, CRC catches it.
SCENARIO (receiver = A, sender = B)
- Network layer on A gives packet to its DLL. B receives packet correctly and is sent to its Network layer. B sends ack frame back to A.
- Ack frame gets lost.
- DLL on A times out. A takes lost ack to mean that frame never made it to B. Sends frame containing packet 1 again.
- B receives packet 1 twice and may process it twice. Protocol fails.