The network signal processor is the nucleus of the communication systems. It is responsible for processing and routing commands, telemetry and voice between the orbiter and the ground.

Commands and voice uplinked to the orbiter are received by the S-band PM uplink or the Ku-band system's forward link. The NSP accepts this digitized, time-division-multiplexed data stream for further processing. Encrypted data are routed to Comsec, decrypted and returned to the NSP. Comsec interfaces with the NSP to provide communication security during Department of Defense and NASA missions. Digitized air-to-ground voice data are demultiplexed from command data and converted to analog signals before being routed to the ACCU. Command data are routed to the GPCs of the data processing system through the flight forward MDM.

Telemetry and voice are downlinked from the orbiter by the S-band FM, S-band PM and Ku-band systems' return link. The NSP accepts and digitizes analog air-to-ground voice data from the ACCU. The digital voice is multiplexed with telemetry from the PCMMU for real-time transmission to the ground through the S-band PM and Ku-band systems. The multiplexed data also are routed to the operational recorders for later transmission through the S-band FM system or mode 2 of the Ku-band system. Encrypted data are routed to a Comsec encryptor and returned to the NSP before being downlinked.

Instrumentation equipment, except sensors and selected dedicated signal conditioners, is located in the forward and aft avionics bays. Sensors and dedicated signal conditioners are located throughout the orbiter in areas selected on the basis of accessibility, minimum harness requirements and functional requirements. Effective use of remote data acquisition techniques was considered for optimizing equipment location. The factors that were considered in the determination of equipment location were weight, power, physical size, redundancy and wire density, and length to each compartment and interconnect wiring. Abbreviations used to designate the locations of equipment are as follows: OA refers to operational aft, OF to operational forward, OL to operational left, OR to operational right, OM to operational mid.