SP-1 (Stored Program 1) was the name of a computerized

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

(a so-called switching office) manufactured by Northern Electric (later Northern Telecom and now Nortel Networks beginning in 1972) in

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

. It was introduced in 1971 (as stated in a Northern Telecom print ad that is included (page 55) in a history of Nortel, written by Peter C. Newman who was commissioned by Nortel to write it. Copyright is 1995. No ISBN.) As indicated by the name "Stored Program", the SP-1 introduced computer control to the telephone switching market. (AT&T's #1ESS preceded the SP-1 by several years.) A central computer controlled the operation of the switch. Switch behavior was determined by the operation of a computer program. With its use of computer hardware and software control, the SP-1 marked an evolutionary step in telephony design. It was an intermediate form between the previous generation of electromechanical systems and the next generation of fully digital systems. Like the previous generation of systems, the SP-1 was an analog switch that used a special form of mechanical relay (Minibar

crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a Matrix (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...

) to provide the voice connections. The voice signal remained in the analog domain throughout the exchange, with the crossbar switches providing a metallic path to connect the end points of a call together. However the SP1 replaced the previous generation's complex relay-based controllers ( markers) with a modern

Harvard architecture The Harvard architecture is a computer architecture with separate computer storage, storage and signal pathways for Machine code, instructions and data. It is often contrasted with the von Neumann architecture, where program instructions and d ...

electronic computer, with separate data and program memories. SSI DTL logic was used throughout. Two types of memory was used - ferrite sheet memory for the CPU and Piggyback Twistor memory for the storage of program and routing information. A number of variations on the base switch were available. A "two wire" version for local service, a four wire version for toll service. Two/four wire versions were also available as was a TOPS variant that was used to provide Operator services. The first four wire switch (four wire-tops, i.e., it provided toll switching and operator services) was placed in service in Thunder Bay circa 1972. The operator service consoles (CRTs) used an Intel 4004 processor. The next generation of systems evolved from the SP1 design to replace the analog switch with a digital technology. SP-1 proved the feasibility of software-controlled systems for telephony. It set the stage for the introduction of fully digital systems with the development of the DMS ( Digital Multiplex Systems) by Nortel in the 1970s. (Nortel's SL-1 (PBX) was Nortel's first fully digital switching system, and it was introduced in service in 1975.)

See also