HISTORICAL BACKGROUND SUMMARY | Network Technology And Methodology

Between around 1960 and 1980, the public switched telephone network underwent rapid and dramatic change from developments in solid-state digital technology. Initially, the diode and transistor were single function devices, but it didn’t take long for them to be packaged into containers and branded integrated circuits. Computers—large, slow data processing machines and systems—were not immune to the same technological turmoil. Consequently, computers and their terminals migrated across the scientific landscape into office territory. Connections between the computers changed significantly as Teletype machine controllers turned into timeshare terminals. Someone figured out a way to convert the digital signal between the timeshare terminal and the computer from digital to analog, and reverse the process at the other end; devices made with modulator and demodulator techniques extended acronym territory with the term MODEM. All of a sudden the analog telephone network could connect timeshare terminals and computers as well as the Teletype network could. Originally, telephone networks were analog. Modems allowed telephone networks to be used to support computer communications.
Add a note hereAs this initial impact from transistors and integrated circuit electronics enabled faster and faster computers, it had a similar effect on network technology. Bell Labs started working on digital transmission technology in the 1960s. The objective was to double voice channel capacity of a single trunk line from 12 simultaneous conversations to 24. This technology had tremendous value in large cities where the potential return was superior compared to digging up the street and burying more conduit.
Add a note hereThroughout the 1970s and 1980s, the long distance switching and transmission network underwent a conversion from analog to digital. Mini-computers replaced many mainframes; mainframes became faster and computer traffic grew. Data communications became full-time jobs for communications-savvy engineers and technicians.
Add a note hereA significant computer standard, developed in the late 1970s, remains in wide use today. The open systems interconnect (OSI) stack defines a hardware section beneath a software section in a total of seven layers, bottom to top. The OSI stack makes a good framework for communications networks, including the Internet. Figure 1 shows the two-section, seven-layer stack with a brief explanation about what it represents and how it is applied. 

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Figure 4-1: OSI Stack: Hardware and Software Sections
Add a note hereWhen the OSI stack was introduced, computers were just beginning to change from stand-alone islands into distributed processing systems connected by data networks. The basic idea behind the stack concept is that each layer interfaces and interacts or communicates with the one immediately above and below, except, of course, the bottom and top layers for obvious reasons. If each layer successfully accomplishes its functions, then the system it’s applied to should operate top to bottom. Attempts to map Internet and Telecom functions or processes to the stack are made from time to time, but in isolated ways such as a reference to layer 2 switching,or layer 3 routing, or even layer 2/3 switching or routing. These references seem to be more of a way to characterize a particular switching or routing function in terms of the OSI stack, rather than applying the OSI stack to communications networks in general. Furthermore, it would seem to be useful in analyzing and structuring or designing networks capable of carrying disparate, converged traffic types on a common access or transport facility.

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