Packet switching is a method for transmitting data across digital computer networks and communication systems. Packet switching involves messages being divided into packets (blocks) of data which are received, processed, and forwarded by switch systems (electronic devices). Data packets, like email messages, are comprised of a header and a body (payload): the header contains the routing (address) instructions and the body contains the information used by a software application. Because packet switching networks tend to use specific hardware (switches, router, hubs, gateways) to transport the data (such as the IMP on ARPANET) this network design enables different computer systems to interconnect: because the computer system is only required to translate the data packet information.
The benefits of packet switching are two-fold: it is robust and efficient. Data is streamed on packet switching networks in a chain of packets: this makes the transmission efficient as the channel upon which the packet is sent only needs to be open for each packet, and it is robust because it can cope with a delay in it's transmission. Before packet switching was developed, the circuit switching technology was the primary method of data communication - the telephone system being the prime example of circuit switching - and this method requires a dedicated channel to be open for a specific length of time (telephone call). Therefore, circuit switching communication networks charge users a fee based on the time a channel is used and packet switching communication networks charge users a fee based on the amount of data (packets). The latency - speed by which packets are transmitted from sender to destination - depends on the capacity / load of the network nodes it's bisects. Traffic management techniques, referred to as packet shaping, decide how packets are queued and how traffic flows.
How network messages are divided into packets and formatted depends on the host to host protocols used by the packet switching network. One of the first host-to-host packet switching protocols was the Network Control Program (NCP); an early ARPANET protocol. Later packet switching protocols include: TCP/IP, X.25, IPX and SPX, and Frame Relay. TCP/IP is the protocol suite that is used by the Internet. The IP protocol is described as an unreliable packet switching protocol because it's a connection-less protocol: it does not connect to nodes to discuss the order in which the packets will be sent. The IP protocol relies on a connection protocol (TCP): that protocol connects to the nodes and ensures that the packets are sent in the correct order and ensures a reliable message is sent; the TCP protocol does this by adding it's own header to the packets. Networks that have utilised packet switching include: ARPANET, CSNET, NSFNET, Telenet, JANET, NPL Network (Mark 1 and 2), BITNET, CYCLADES, IBMSNA, SCANNET, and XNSnet.
Packet switching was inspired by the cold war: when a communication system was required that could sustain part of it's infrastructure being destroyed (in a nuclear attack for example) and was still capable of functioning and routing data. In 1964, Paul Baran, while an employee at the RAND Corporation (United States of America), developed the idea of an automatic system that trasnmitted blocks of data - later coined packets by Donald Davies - and could survive 50% of it's infrastructure being destroyed. Donald Davies independently devised the same idea during 1965-1966, while working at the National Physical Laboratory (England). Baran and Davies are usually referred to as the inventors of packet switching, although Leonard Kleinrock also conducted research into the switching of digital messages in the early 1960's.
The first working demonstration of packet switching was displayed at the National Physical Laboratory in 1968. The National Physical Laboratory went on to develop the Mark I computer network; the first working example of a packet switching network. Donald Davies helped to build the Mark I computer network. Baran's and Davies' work also influenced the computer scientists who built the ARPANET packet switching network (late 1969); the first American packet switching network. The Mark I network was later updated to become the Mark II network, this network was eventually disbanded in 1986. ARPANET, on the other hand, would inspire the creation of NSFNET, which eventually transitioned (1990-1995) to become the modern commercial Internet. While the Internet (IP model) is the most popular computer network model to use packet switching, packet switching has been implemented into numerous computer networks and communication systems.