ARPANET (Advanced research project agency network) is the oldest wide area packet switched network build by the DARPA (Defense advanced research project agency). Physically ARPANET consisted of about 50 BBN corp’s C30 and C300 minicomputers called the PSNs (packet switching nodes) that were scattered over US and Europe. Each PSN was a dedicated packet switching node. Point to point data circuits leased from long haul carriers connected the PSNs together to form a network.
ARPANET PSN port and protocol
Each ARPANET PSN port had upto 22 ports that enabled host computers to get connected to the ARPANET via the PSN. Divert connections were made by special interface boards that connected the host computer’s input/output port to the PSN port. Proper programming interfaces allows the host computers to send packets vis the host PSN ports.
The PSN port hardware used a protocol knows as 1822 that permits the host to send a packet across the ARPANET to specified destination PSN and specified port on that PSN. 1822 provides a reliable flow controlled delivery. 1822 limits the number of packets that the host can send and also forces the host to wait for a ready for next message (RFNM) signal from the PSN before transmitting each packet to inside its reception at the destination.
ARPANET doesn’t support message headers and trailers and this the ARPANET messages are not self identifying.