Front-end processor (IBM)

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A front-end processor (or FEP) is a computer system peripheral device used on mainframe computers for interfacing with telecommunication devices. In the IBM System 360 architecture, the IBM 3705 FEP ran software downloaded from the mainframe known as Network processor software (or NPS). In effect, the FEP was a mini-computer handling the serial telecommunications functions, freeing up the high-speed mainframe channels from the relatively slow communications speeds.

The patented IBM FEP architecture consisted of one or more channel adapters to one or more mainframes, a processor with memory, and one or more scanners (or multiplexers). Each scanner had one or more line interface units that were tailored for the particular type of communication interface and protocol. Choices included: asynchronous, bisync, SDLC, or other high-speed communication protocols. Cables from the customer's modems were plugged into the interface cards and the modems interacted with the FEP to transfer data to and from remote terminals. Up to 192 circuits could be connected to one FEP, offering service to thousands of terminals almost simultaneously, because communication and typing speeds were so much slower than computing speeds. The fastest async line on the 3705 was 2400 bps and SDLC was 56K. Line sets could be configured for half-duplex or full-duplex.

The scanners would asynchronously interrupt the NPS and the NPS would determine which interfaces had data buffers ready to transfer to the mainframe and would activate the correct interface to make the transfer. The NPS would also generate polling and retry signals on the line interfaces, as necessary. The channel interface would also interrupt the NPS when the mainframe signalled a request to transfer information to a remote terminal, or to the FEP itself. Inside the mainframe, the VTAM software managed the connection between terminals and their respective applications.

Amdahl Corporation also distributed a plug-compatible FEP known as the 4705 for a number of years, offering a machine with performance and reliability improvements over the equivalent IBM FEP. Other companies, including NEC, also offered an FEP for use with the IBM 360 and 370.