Explain why multibus hierarchies are required?

If large number of devices are connected to the single shared bus , performance will suffer. There are following problems

1>Bus length is longer. Therefore propagaton time is more. This propagation dealy can affect performance. When control of the bus passes from one device to another frequently

2>The bus may become bottleneck as aggreagate data transfer demand approaches the capacity of bus. Because data rate generated by attached deviceslike graphics and video controller are growing rapidly

3>Only one master bus can operate at a time, other waits. To overcome this problem most computer system use multiple buses, generally laid out in hierarchy.