The digital switch
In this context, a digital switch is a switch that performs time division switching of digitized signals. Digital switches encode live speech in 8000 time slices per second. For each time slice, a digital PCM (pulse-code modulation) representation of the tone is created. The digits associated with these PCM representations are then sent to the receiving end of the line, where the reverse process occurs, to produce the sound for the receiving phone.
One of the most successful digital telephone switches is the AXE system developed by Ericsson. This is a line of circuit switched digital telephone exchange products that was first deployed in 1976. The AXE system has a dual processor system, both of which run together, so that if one processor fails there is no disruption of services. The latest model of the AXE has a capacity of 8 million subscribers and has many GB of memory for storing program code and subscriber data.
For those who are interested, you can learn more about the history of telephone switches by clicking here.
Magnetic-core memory was the predominant form of random-access computer memory for 20 years (circa 1955 to 1975). It uses tiny magnetic toroids (rings) -- the cores -- through which wires are threaded to write and read information.
A 32 x 32 core memory plane storing 1,024 bits of data (physical size = 10.8cm x 10.8cm).
Each core represents one bit of information. The cores can be magnetized in two different ways (clockwise or counterclockwise), and the value of the bit stored in a core is zero or one depending on that core's magnetization direction. The wires are arranged to allow an individual core to be set to either a one or a zero, and for its magnetization to be changed, by sending appropriate current pulses through selected wires.
The process of reading a core causes that core to be reset to a zero, thereby erasing its contents. This is called "destructive readout" and requires that the value read out of the core is subsequently re-written back into the core.
The first commercially available semiconductor memory was the Intel 1103 (1024x1), which was publicly released in October 1970. By 1972, this was the best-selling semi-conductor memory chip in the world, easily defeating its magnetic-core counterparts.
An Intel 1103 DRAM semiconductor memory chip.
The table below shows that memory size has increased 4 million times in 40 years (not shown here is the fact that 4 Gigabit memories became available in 2011).
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