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A number of responses from colleagues to the first part of this article "Looking Back..Way Back.. before Unified Communications," included comments on telephone switching technology and its role early in their careers. It got me thinking, reading, and reminiscing about the subject.
Telephone switching began with the Step-by-Step otherwise known as the Strowger switch. The background story of this device is quite interesting. According to legend, Strowger, an undertaker in Kansas City was losing most of his business because the local exchange operators steered callers for funeral services to his primary competitor. Apparently, one of the operators was involved with his rival. So, Mr. Strowger put up some money, hired some engineers and the automatic telephone exchange was developed. It was patented around 1890.
The next significant milestone was nearly 50 years later in the late 1930's with the introduction of the Crossbar switch. There was an incremental improvement to Step-by-Step called the Panel switch developed in the 1920's, however, it was fundamentally similar in architecture to the original Strowger concept. The electromechanical Crossbar switch had significant advantages over predecessors since it would receive all incoming digits, store them, and then process and route the call by doing "translations" of the incoming digits and deciding how and where to send the call. This was the start of central control in a telephone switch. By the way, crossbar switching technology was not a Bell Labs innovation. It was originally licensed by Western Electric from Ericsson of Sweden.
On a personal note, the first U. S. installation of a No. 1 Crossbar was at the Troy Avenue central office in Brooklyn in 1938. This was near where I was born in the year when I born. A later iteration, the No. 5 Crossbar was dominant in the Bell System for many years. It was, among other things, the first switch to handle Dual Tone Multi-Frequency (DTMF, aka “Touch Tone”) dialing in the early 1960's as the rotary dial telephone began to be replaced with now-ubiquitous push buttons. A “tandem” variation (i.e., a switch for interconnecting local offices with the long-distance network) called the No. 4 Crossbar was in use from the 1940’s to the 1980’s.
Applying semiconductor and computer technology to telephony was a very natural progression that Bell Labs and others pursued in the 1950’s. It’s been estimated that the Bell System spent 10 years and $500 million to develop the Number 1 Electronic Switching System (#1ESS), which was first installed in 1965 after extensive field trials. This switch was “analog” since voice within the switch was still handled as an analog audio signal or waveform. In essence the electromechanical logic of the central control portion of the switch was replaced by a computer while the switching matrix or “fabric” continued to be relay-based and crossbar-like albeit updated with newer and smaller technology. As with predecessor systems, each call required a physical connection through the switch (and external network) to be established which remained in place until the call was terminated.
The key breakthrough of #1ESS was the use of a computer and its memory for control, this concept was called Stored Program Control (SPC). This enabled new subscriber features such as call waiting, call forwarding and speed dialing. Other advantages were the ability to modify the programs and data in the switch when needed to change parameters (e.g. routing or translation information) and add more features and functions when they were developed.
In the late 1970’s, the digital telephone switch appeared with the voice signal now encoded within the switch as a digital bit stream with a data rate of 64kbits/sec. By use of time division multiplexing, switching and transmission of a bit stream requires only an assigned time slot for the duration of the call as opposed to dedicated lines, trunks and other facilities in analog voice switching or space division multiplexing. The use of digitized voice led to much simplification and innovation in telephone switching and transmission. It also led to the convergence of voice with digital data with notions like Computer Telephony Integration (CTI) and ISDN. By the mid 1980’s, all new Central Office switches and PBXs were digital, although, it took some time and the threat of Northern Telecom’s DMS family to move the Bell System to the digital #5 ESS (still produced, I believe, by Alcatel-Lucent).
The most recent change in telephone switching occurred when the digital bit stream of encoded voice became a stream of packets in Voice over IP or IP Telephony methods. In effect, packet switching replaces circuit switching for voice communications itself. Interestingly, packet handling and switching have been part of telephony for years; integral to network signaling (Signaling System #7) and an important aspect of ISDN. The transition of users to VoIP is still very much a work-in-progress. Will voice become “just another IP application” in the future? Time will tell.
Finally, the following timeline shows the telephone switching milestones that we’ve been discussing versus well-known developments in IP communications. This historical perspective clearly brings home the point that telephony has a long and storied legacy whereas our IP world, exciting as it may be, is the new kid on the block. Fifty years ago it didn’t exist.
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