When does network congestion occur?

Congestion results from applications sending more data than the network devices (e.g., routers and switches) can accommodate, thus causing the buffers on such devices to fill up and possibly overflow. A buffer is a portion of a device's memory that is set aside as a temporary holding place for data that is being sent to or received from another device. This can result in delayed or lost packets, thus causing applications to retransmit the data, thereby adding more traffic and further increasing the congestion.