What causes packet delay?

Packet delay is caused by several factors, including propagation delay, transmission delay, queuing delay, and processing delay. Propagation delay occurs as packets travel through the medium, while transmission delay is the time taken to push all packet bits onto the network. Queuing delay happens when packets wait in line at routers or switches due to network congestion, and processing delay is the time taken by devices to process the packet headers and make forwarding decisions. Each of these factors can contribute to the overall delay experienced in data transmission.