A bank has its head office in the financial district in the middle of a city. It has six branches, three in the north of the city and three in the south all 5 km from the centre and spaced 3 km apart. The bank wishes to link all its branches via private leased lines but must decide between providing an individual line to each branch of via a number of inter-branch links and a smaller number of multiplexed lines to the centre. If a modem costs 100, a multiplexer/demultiplexer costs 200 and leased lines cost 100 per km per year to rent, suggest a design that will yield the most cost effective solution if the installation is to remain in operation for four years.