Packet switching is a method of digital communication in which data is divided into small units called packets before it is sent across a network. Each packet carries a payload, which is the piece of the original data, and a header that includes information such as source and destination addresses, length, and control fields used for forwarding and error checks. Network devices receive a packet, examine the header and then store and forward the packet to the next hop on the path toward its destination. Different packets from the same message can travel over different routes and may arrive in a different order from which they were sent.
In a packet switched network, links are shared by many conversations at the same time. When a link is idle, no capacity is wasted, and when activity increases, packets from multiple users are interleaved on the same wires, fibres or radio channels. This statistical sharing of capacity makes packet switching efficient and scalable for bursty traffic such as web browsing, messaging and most data services. It also improves resilience, since routing equipment can choose alternate paths if a link or node fails.
The Internet Protocol is the best known example of packet switching at the network layer. Routers look only at the destination address in the packet header to make a forwarding decision for the next hop. Because the network provides a best effort service, ordering, loss recovery and flow control are usually handled by transport protocols at the endpoints. For example, a reliable transport will number segments, request retransmission when needed and reassemble data so that applications see a continuous stream.
Packet switching introduces some engineering challenges. Since packets may queue at busy devices, delay and jitter can vary. Networks use quality of service markings and scheduling to give priority to time sensitive traffic such as voice over IP. Packet size is limited by the maximum transmission unit of each link. If a packet is too large, it may be fragmented and must be reassembled at the destination. Monitoring tools track utilisation, loss and delay so operators can plan capacity and tune routing.
Some packet switched systems establish virtual paths to give predictable behaviour. Technologies such as MPLS attach short labels so that routers can forward packets along a chosen route without repeated lookups, which can aid traffic engineering and simplify large networks. Despite these variations, the core idea remains the same. Break data into packets, add enough information for independent forwarding and share the network fairly among many users. This approach underpins local networks, wide area backbones and the global internet.