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Manual (Cordboard) Switches
These switches involved operators who manually established the desired connection at the request of the transmitting party. A unique physical and electrical connection was established, on a plug and jack basis (Figure 2.14), for the duration of the call. When either party disconnected, the operator was alerted and was responsible for manually disconnecting the circuit, which became available for other use. The size of such switches, the complexity of interconnecting long distance calls across multiple switches, and the labor intensity of this approach all contributed to their demise.
Figure 2.14 Manual Switching: Cordboard.
The term tip and ring came from the cordboard plug, which established a connection between both local loop pairs of wires. One set of wires was connected electrically through the tip of the plug, while the other set was connected through the ring, or seat of the plug. The first such switch was installed by the New Haven District Telephone Company in New Haven, Connecticut on January 28, 1878, at a cost of US$28.50. The system connected 21 subscribers who were referred to by name, rather than telephone number. As was typical in those days, the operators were young boys, most of them with experience as messengers and telegraph clerksthey soon proved to be boisterous and unreliable, and were replaced with women [2-6].
Step-by-Step (Sxs) Switches
The first such electro-mechanical switch was invented and patented in 1891 by Almon B. Strowger, a Kansas City undertaker who was frustrated with the behavior of the local telephone company operator. According to legend, the operator was directing Mr. Strowgers calls to her husband, a competing undertaker. Building on earlier Bell system inventions, Strowger invented a system that would serve 99 subscribers. That patent served as the foundation for the company he founded, Automatic Electric Company, which later became the manufacturing subsidiary of General Telephone and Electric (GTE) [2-6].
SxS switches consist of large number of line finders to which groups of individual subscribers are assigned for dial tone. The transmitting party dials a series of numbers, originally with a rotary telephone terminal which causes the making and breaking of an electrical circuit. As noted in figure 2.15, those electrical pulses cause successive mechanical line selectors to click across contacts to set up the conversation path as the complete number is dialed [2-7].
While such switches are still in use (in 1986, approximately 38% of all U.S. switches were based on SxS), they clearly are not desirable, as they are slow, expensive, large, maintenance-intensive and capacity-limited [2-2].
Figure 2.15 Step-by-Step (SxS) Switching. Source: AG Communications Systems Corporation
Crossbar (XBar) Switches
The first common control switches, XBar switches are largely electromagnetic in nature. While the original concept was developed at Bell Telephone Laboratories (now Lucent Technologies), the Swedish Ericsson company accomplished much of the early development work. The first such switch installed in the United States was a central office exchange in Brooklyn, New York (1938) [2-6]. Such switches quickly became predominant.
In a XBar switch (Figure 2.16), a request for dial tone is recognized by a marker, which directs a sender to store the dialed digits. A translator is then directed to route the call, reserving a path through a switching matrix [2-2]. Once the call is connected, these various components are available to serve other calls. Compared to the SxS switch, the XBar had relatively few moving parts. XBar switches offered the advantages of increased intelligence, common control, greater speed of connection, smaller footprint, lower maintenance, and greater capacity.
Figure 2.16 Crossbar (XBar) switching. Source: AG Communications Systems Corporation
Electronic Common Control (ECC) Switches
ECC switches reflect the marriage of computer technology and telephony. While the first ECC switches were analog, contemporary switches are, in fact, fully digital. Voice conversations are digitized and switched over high-speed digital circuits, with all processes accomplished through programmed logic. ECC switches are microprocessor controlled, with the total processing power of such a switch often rivaling that of a general purpose, mainframe computer. The first ECC switch was developed, with the assistance of Western Electric, by AT&T Bell Laboratories, based on its invention of the transistor in 1948 and involving a development effort that began in earnest in the early 1950s. That first Electronic Switching System (ESS) CO was placed into service in Succasunna, New Jersey on May 30, 1965, connecting 200 subscribers. By 1974, there were 475 such offices in service, serving 5.6 million subscribers. The development effort was estimated to involve 4,000 man-years and a total cost of $500 million [2-6].
ECC switches (Figure 2.17), as compared to the previous generations of switching technology, offer the advantages of increased intelligence, greater speed of call set-up and overall call processing, and a still smaller footprint. Additionally, they offer lower maintenance costs and can, in fact, be monitored and managed from a remote location. Many contemporary ECC switches are unmanned in favor of control from a centralized Network Operations Center (NOC). ECC switches offer greater capacity, and on a scaleable basis capacity can be increased through the addition of various system modules, or cabinets. As specialized, software-controlled computer systems, their functionality and feature content often can be upgraded through additional software and/or firmware. Such switches generally possess the ability to switch data and video, as well as voice. Finally, they interface with various application processors to further increase the range of services provided. Such application processors (applications) might include voice processors (e.g., voice mail and language translation) and fax servers (fax mail).
Figure 2.17 Electronic Common Control (ECC) switching. Source: AG Communications Systems Corporation
First deployed in 1971, packet switching grew out of the United States Advanced Research Project Agency (ARPA) network. Commonly referred to as ARPAnet, the network was established to support interactive, asynchronous computer-to-computer communications between the defense and university communities.
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