Answer - When the I/O is frequent and of very short durations polling is considered to be more efficient than an interrupt driven I/O. Although, a serial port individually can have fairly infrequent number of I/O and hence should ideally use interrupts the case of serial ports in a terminal concentrator is different. A terminal concentrator consists of multiple serial ports and this can lead to the creation of multiple short I/O instances this can create un-necessary load on the system in case of interrupts usage. Instead, if a polling loop is used it can greatly reduce the amount of load on the system by looping through without the requirement of I/O. Due to this reason interrupts are used for single ports as the frequency of I/O on such a port is less and can be managed effectively, whereas we use polling for multiple ports as the frequency of I/O increases and are of short durations which suits polling.