A firehose effect occurs in a network when the source (transmitting) computer or terminal sends data too fast for a destination (receiving) computer or terminal to deal with it. The term comes from the analogy between a data stream and the flow of water through the heavy hose used in fighting fire.
In any network, the rate of data transfer (for example, in kilobits or megabits per second) can never be faster than the rate at which the slowest part of the system can function. For example, when a computer has a 56-Kbps modem, it cannot receive data any faster than that, no matter how high the data speed in other parts of the communications circuit.
In parallel-to-serial (P/S) conversion, the firehose effect can result in a problem called buffer overflow. When this takes place, the backlog of data bits increases for as long as the data continues to arrive. Ultimately, some packets of data are lost because the receiving system cannot store a backlog of data bits that increases beyond a certain maximum called the buffer capacity.