The Pentaconta Crossbar and Exchange
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The Pentaconta crossbar dial telephone switching system was developed in 1953 by the International Telephone and Telegraph Company (ITT). The basic switch was patterned after those from Western Electric, the Swedish Nay Autotelefon Betulander and the LM Ericsson companies’ crossbars (developed starting in 1915). For a backgrounder on the first crossbar switches, visit review.
​The first Pentaconta-based exchange was cut over at the end of 1954 at Cento, near Ferrara, Italy. Others were installed during the next four years in France, Chile, Brazil, and Argentina, the largest being at Bergamo, Italy, with 17,000 subscribers' lines. It was popular worldwide, especially with nationalized and independent telephone companies not associated with AT&T. (Ref 1).
In the UK, a Pentaconta-based system was referred to as the TXK3 for local exchanges and TXK4 for toll (transit) exchanges.
A large Pentaconta-based exchange was similar to AT&T’s No. 5 Crossbar exchange. Each type had line subscriber circuits with Line and Cutoff relays, each had dialed digit registers, each had multiple types of common control markers to set up paths through multi-layer switching fabrics, and other common features.
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Switch Internals
Each Pentaconta crossbar switch is equipped with up to 22 vertical bars and up to 14 horizontal bars, allowing for more simultaneous connections than a 200-point crossbar (5 horizontal and 20 vertical rods) from the Western Electric Company.
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Fig 1 shows a single Pentaconta switch with 14 horizontal selector bars and 22 vertical selectors. The largest switch has 52 outputs and may have from 10 to 22 inputs depending on the configuration.​
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Fig 1, 14x22 Pentaconta crossbar switch (Ref 4)
The number of outputs to which each vertical inlet may be connected is twice the number of horizontal bars. Each horizontal bar may be tilted to a high or low position. Of the 14 horizontal bars, one is dedicated as a “doubling bar”.
This specialized bar is used to select one of two contact stacks per intersection. This yields an overall of 13 x 2 x 2 = 52 separate outputs per switch. This capacity has given the switch name Pentaconta which is derived from a Greek word meaning 50. Some versions sport 74 outputs (Ref 5).
An output or input may be 3, 4, 5, 6, 8 or 10 wires depending on the switch configuration. If there are 6, 8 or 10 wires the number of outputs is 26 not 52.
Fig 2, Pentaconta crossbar switch 14x14 in a rack (Ref 3)
Switch Fabrics and the Entraide Routing​
A basic approach to networking individual switches (N_in x N_out) is shown in Fig 3A. This 2-stage fabric could be built with a Pentaconta, Western Electric crossbar switches or other designs.
For Fig 3A (Ref 1), as the number of I/O connections is increased it becomes more likely that a new input cannot be connected to the desired output. The individual switches become fully loaded such that a new desired in-out connection is fully blocked.
In 1953 Bell Labs' engineer Charles Clos devised methods to configure 3-stages of crossbar switches to guarantee non-blocking under certain configurations. See Fig 3B. A Clos proof of non-blocking is provided here.
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Guaranteed non-blocking fabrics use many individual switches, and the cost can be excessive, especially for a telephone fabric that has relatively low utilization (~15% of subs active, max). So, the layout in Fig 4 was invented to decrease the chance of fabric blocking without needing 3-stages.
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Informally, the configuration in Fig 4 could be called a “2.5-stage fabric.” Why? Normally in-to-out signals pass through 2 switches. However, under heavy loading signals could pass through 3 switches. The fabric may still block but within the limits of what traffic engineering decides is acceptable.
Fig 4 Fabric with an Entraide Nework (Ref 1)
The Fig 4 switch wiring has improved non-blocking over Fig 3A but not as good as a Clos designed Fig 3B. Fig 4 offers acceptable tradeoffs to improve call traffic handling.
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In the figure, the French term "entraide" has the meaning of "mutual-aid" and "reciprocal overflow." In effect, the primary switches are partially interconnected to increase the possible paths inside the fabric. Some technical articles on the Pentaconta use the term “mutual-aid” or “interaid” in place of entraide (Ref 2).
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​Some primary switch outputs (12 out of 52, for example) are routed to other primary switch inputs. The function of the entraide arrangement is to provide for those occasions when no free access path is available directly between a primary switch and a secondary switch. The mathematics (chance of blocking) behind the design is daunting and is left to expert fabric designers.
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A typical Pentaconta-based crossbar exchange will have a mix of entraide enabled fabrics along with standalone 1-stage switches and 2-stage fabrics.
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It should be noted that using entraide routing could be done using a WE switch (10x20 or 10x10). However, the Pentconta switch (52x22) has more I/O ports. So, it’s considerably more effective in a fabric with interconnected primary stage switches.
A local office Pentaconta exchange has two large independent crossbar fabrics. These are the Line Unit and Group Unit in similar fashion to the two large independent fabrics (Line Link Frame and Trunk Link Frame) of a No. 5 Crossbar office. Plus, there are many smaller crossbar fabrics for routing control signaling in both exchange types.
Fig 5 is a closeup of some horizontal rods (verticals are partially hidden) in a Pentaconta switch.
Fig 5, Close up of a Pentaconta switch mechanism
Fig 6 shows the final testing of a Pentaconta switch at an ITT manufacturing facility in Puerto Rico in 1962.
Fig 6, Final testing of a large Pentaconta switch
The previous two figures are from an ITT sponsored video. You may enjoy seeing some of the steps required to build a Pentaconta switch, including some relays and telephones. Check check it out below.
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The Pentaconta switch was an exchange workhorse for many years. To increase deployment, ITT licensed its design to other manufacturing companies.