Interface Message Processor

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The Interface Message Processor (or IMP, as they were universally known) was the packet switch which the ARPANET was made out of; the IMPs were connected to each other via high-speed dedicated point-point telephone links, connected to the IMPs via a modem. All ARPANET hosts connected to an IMP; connection of a host to an IMP required a special network interface, an IMP interface, which used the asynchronous bit-serial 1822 interface standard, which was especially created for the ARPANET.

A message from one Host to another was sent via the source Host's IMP, then routed through some number (possibly zero) intermediate IMPs, then through the destination Host's IMP (possibly the same IMP as the first one, in which case the message never went over a telephone line), on to the destination Host. The protocol which controlled both the channel between the host and IMP, and inter-host communication, was the Host-to-IMP Protocol.

The IMPs were Honeywell DDP-516 'ruggedized' minicomputers, with added special high-speed modem interfaces, and 1822 host interfaces. Later, other variant minicomputers were also used; the Honeywell DDP-316 (the non-ruggedized version of the 516), the Pluribus, and the BBN C/30 (which first had microcode which emulated the Honeywell machines, later an updated "native mode" microcode exposing the full 20-bit hardware).

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