For a 1-km cable, the one-way propagation time is 5 µsec, so 2τ = 10 µsec.To make CSMA/CD work, it must be impossible to transmit an entire frame in this interval. At 1 Gbps, all frames shorter than 10,000 bits can be completely transmitted in under 10 µsec, so the minimum frame is 10,000 bits or 1250 bytes