Given
- $Bw = 20 \text { Mbps}$
- $v = 4000 \ km/s$
- $fs = 1000 \ bits$
- $l = 200 \ m$
Transmission delay ($T_t$) $ = \frac {fs}{Bw} = \frac {1000}{20 \times 10^6} = 50 \ \mu s$
Propagation delay ($T_p$) $ = \frac {l}{v} = \frac {200\ m}{4000 \times 10^3 \ m/s} = 50 \ \mu s$
efficiency of CSMA/CD = $\frac {T_t}{T_t \ + \ 6.44 \ T_p}$
$= \frac {50}{50 \ + \ 6.44 \times 50}$
$= \frac {50}{50 \ + \ 322}$
$ = 0.1344$
$= 13.44 \text{ %}$