The crossbar switch
The crossbar telephone switching system was developed by LM Ericsson in the 1930s.
The Crossbar switch had a significant advantage over both its Panel or Step-by-Step counterparts. This was because it was able to use a "store and forward" concept whereby it would take incoming the digits, store them, and then process the call. No extra memory was required to store of the calling number. The crossbar switch was used by all telecom manufacturers for more than 40 years.
A 100-point, 6-wire crossbar switch manufactured by Western Electric circa 1970.
The first electronic switches
The first electronic switching systems were not entirely digital. Instead, they had read relay metallic paths, which were stored-program-controlled. These systems could use the old electromechanical signaling methods inherited from crossbar and step-by-step switches. The Number One Electronic Switching System (1ESS), which was the first large-scale Stored Program Control (SPC) or Electronic Switching System (ESS) in the Bell System, was introduced in Succasunna, N.J., in May 1965.
What kind of memory was used in these early systems? Well, the memory implementation in the 1ESS boasted a 44-bit word length for the "Program Stores." Of these 44 bits, six bits were used for error correction and one bit was employed for an additional parity check. This left 37 bits for the instruction, of which 22 bits were usually used for the address.
The "Program Stores" also contained permanent data that could not be written online; instead, the aluminum memory cards -- each containing 128 words of 44 bits -- had to be removed from the system so their permanent magnets could be written offline by a motorized writer.
An aluminum memory card containing 128 words of 44 bits.
"Call Stores" were the system's read/write memory. These contained the data associated with calls in progress and other temporary data. These boasted a 24-bit word length and operated similar to magnetic core memory, except that the ferrite was in sheets with a hole for each bit and the coincident current address and readout wires passed through that hole. The first "Call Stores" held 8KW (kilowords) in a frame approximately a meter wide and two meters tall.
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