Network - Simultaneous network traffic handling

How does a network handle traffic that is sent concurrently by two or more hosts? How do switches and routers and other network devices deal with packets arriving exactly simultaneously? Are the processed simultaneously?

Packets that arrive on network devices are always processed sequentially. Refer to the following diagram:


If H1 and H2 each send a packet to H3 simultaneously, how will the switch process these packets? As the packets egress port 3 to reach H3, they will exit the interface sequentially. By default, if no QoS mechanisms are employed, the switch will process packets on a first come first serve basis. If packets arrive simultaneously, there is always some mechanism that will prefer one packet over the other, simply because packets can only be processed sequentially.

If we examine an extreme example, where say 100 hosts send packets to H3 simultaneously, the switch will still find a way to sort out the packets in sequential order. It is likely that in such a scenario, many packets would be queued or buffered, and some may even be lost. But the fundamental mechanism of the switch is to sort out the packets and process them in sequential order.