$2.($a$)$ The installed communication network in a building has the following parameters:

$($i$)$ the network is serving $118$ terminal devices;

$($ii$).$ the average time engaging a communication channel will be $1\mathrm{.}8$ minutes per call;

$($iii$)$ there are on average $65$ calls per hour.

Calculate the traffic intensity of the communication network for the building. Clearly state all the necessary parameters in your calculations.

$(3$ marks$)$

$($b$)$ Use Erlang's B Formula to illustrate the term "Grade of Service" in a communication network. Find the minimum number of effective channels by circuit switching to serve the above traffic intensity, calculate and comment on the Grade of Service for the circuit$-$switched network.

$(10$ marks$)$