example : $1$ Mb$/$s link each user : $100$ Kb$/$s when "active" active $10\%$ of time Circuit$-$switching:$10$ users Packet seitchun:$35$ uses,probability $>10$ active at same time is less than $0\mathrm{.}0004*$ qustion : how did we get value $0\mathrm{.}0004$ and what happens if $>35$ users