Until recently, most personal computers used a parallel PCI bus as a back plane to interconnect the various components within the computer, but the PCI bus was rarely, if ever, used to connect external devices to the computer. Modern computers often use a serial adaptation of the PCI bus called PCI Express, which is sometimes made available as a port to connect external devices. Identify at least three shortcomings of the original PCI bus that made external use of the bus impractical. Explain how the PCI Express bus overcomes each of these limitations.