Public Data Networks: From Separate PDNs to the ISDN

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At that time, operators were used to manually switch or route a session from one terminal to another. At the beginning of 20th century, the telecommunications industry had already been exposed to a number of important issues, which played different roles, culminating in the emergence of modern wireless networks. Among these important issues were analog versus digital, voice versus data, wireless versus wired, local versus long haul communications, and personal versus group services.

It took 28 years for telegraphy to provide transoceanic services after the first on-land service.

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On the other hand, it took only three years for wireless telegraphy to become transoceanic after a local installation. This reflects the disadvantages of laying cables for wired communications compared with wireless services. To install a wireline in a town, one needs to get wiring permits which take extremely long processing times; conduct expensive, long, and laborious digging to lay the wire; and maintain a service organization for wiring maintenance. The PSTN has emerged as the expert of these details, and as we will later discuss, all networks that emerged use this expertise in one way or another for developing their own network infrastructure.

The telephone service providers have the requisite knowledge for laying wires of three types: first, they know how to connect long-haul networks; second, they know how to provide a twisted pair line to homes and offices; and last, they know how to wire a home or office. Wiring of homes and offices is an expertise shared between telephone companies and electricians.

The telegraph infrastructure evolved as a private network not directly accessible to the public user. Each end terminal of a telegraph network supported a community whereas the telephone end terminals supported a home or an office with at most a few users. As a result, the number and the usage periods of the telephone terminal were orders of a magnitude larger than the telegraph network, resulting in a far more prosperous telephone industry.

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By the s, the PSTN had more than 10 million customers in the United States, and those interested in long-haul communication issues also needed PSTN services to solve their problems. Although end users are still mostly connected to the PSTN with twisted-pair analog lines, to provide flexibility and ease of maintenance and operation of the PSTN, the core network gradually changed to digital switches and digital wired lines connecting switches together.

A hierarchy of digital lines the T-carriers in the United States evolved as trunks to connect switches of different sizes together. Another advancement in the PSTN was the development of private branch exchanges PBXs as privately owned local telephone networks for large offices. A PBX is a voice-oriented local area network owned by the end organization itself, rather than the telephone service provider.

This small switch allows the telephone company to reduce the number of wires that are needed to connect all the lines in an office to the local office of the PSTN. This way, the service provider reduces the number of wires to be laid to a small area where large offices with many subscribers are located. The end user also pays less to the telephone company. The organization thus has an opportunity to enhance services to the end users connected to the PBX.

In this experiment the possibility of using analog telephone lines for transferring transoceanic telegrams was examined. Researchers involved in this project discovered several key issues that included the sampling theorem and effects of phase distortion on digital communications. However, these discoveries did not affect applications until after World War II when Bell Laboratories developed voice band modems for communication among air force computers in air bases that were geographically separated by large distances [PAH98].

These modems soon found their way into commercial airlines and banking industries, resulting in the associated private long-haul data networks. These pioneering computer communications networks consisted of a central computer and a bank of modems operating over four-wire commercial grade leased telephone lines to connect several terminals to the computer.

In late s, the highest data rate for commercial modems was 4, bps. By the early s, with the invention of quadrature amplitude modulation QAM , the data rate of four-wire voice band modems reached 9, bps. In the early s, trellis-coded modulation TCM was invented which increased data rates to In parallel with the commercial four-wire modems used in early long haul computer networks, two-wire modems emerged for distance connection of computer terminals.


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The two-wire modems operate over standard two-wire telephone lines, and they are equipped with dialing procedures to initiate a call and establish a POTS line during the session. These modems started at data rates of bps. By the early s, they reached 1, bps, and by the mid s, they were running at 9, bps. These two-wire voice band modems would allow users in the home and office to have access to a regular telephone to develop a data link connection with a distant modem also having access to the PSTN.

Voice band modems using two-wire telephone connections soon found a large market in residential and small office remote computer access telnet , and the technology soon spread to a number of popular applications such as operating a facsimile machine or credit card verification device. With the popularity of Internet access, a new gold rush for higher speed modems began, which resulted in The 56 kbps modems use dialing procedures and operate within the 4 kHz voice band, but they directly connect to the core pulse code modulated PCM digital network of the PSTN that is similar to digital subscriber lines DSLs.

DSLs use the frequency band between 2. More recently cellular telephone services evolved. To connect a cellular telephone to the PSTN, the cellular operators developed their own infrastructure to support mobility. This infrastructure was connected to the PSTN to allow mobile-to-fixed telephone conversations. The addition of new services to the PSTN demanded increases in the intelligence of the core network to support these services.

As this intelligence advanced, the telephone service provider added value features such as voice mail, autodialing through network operators, call forwarding, and caller identification to the basic POTS service traditionally supported. Figure 1A. Data networks that evolved around voice band modems connected a variety of applications in a semiprivate manner. The core of the network was still the PSTN, but the application was for specific corporate use and was not offered privately to individual users.

Public data networks: from separate PDNs to the ISDN - Josef Pužman, Boris Kubín - Google Books

These networks were private data networks designed for specific applications, and they did not have standard transport protocols to allow them to interconnect with one another. Another irony of this operation was that the digital data was first converted to analog to be transmitted over the telephone network; then within the telephone network, it was again converted to digital format for transmission over long distances using the digital subcarrier system.

To avoid this situation, starting in the mid s, telephone companies started to introduce digital data services DDS which provided a 56 kbps digital service directly delivered to the end user. The idea was great because at that time the maximum data rate for voice band modems was 9, bps. However, like many other good and new ideas in telecommunications, this idea did not become popular.

A large amount of capital was already invested in the existing voice band based data networks. It was not practical to replace them at once, and DDS services were not interoperable with the analog modems. The DDS services later emerged as integrated services digital network ISDN services providing two 64 kbps voice channels and a 16 kbps data channel to individual users. Penetration rates of ISDN services were not as expected, but laid a foundation for digital cellular services. Digital cellular systems can be viewed as a sort of wireless ISDN technology that integrates basic digital voice with a number of data services at the terminal.

The major cost for operation of a computer network over the four-wire lines was the cost of leasing lines from the telephone company. To reduce the operation cost, multiplexers were used to connect several lower speed modems and carry all of them at once over a higher speed modem operating over a long distance line. The next generation of multiplexers consisted of statistical multiplexers that multiplexed flows of data rather than multiplexing individual modem connections. Statistical multiplexer technology later evolved into router technology, which are generalized packet data switches.


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In the early s, the rapid increase in the number of terminals at the offices and manufacturing floors was the force behind the emergence of LANs. LANs provided high-speed connections greater than 1 Mbps among terminals facilitating the sharing of printers or mainframes from different locations.

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LANs provided a local medium specifically designed for data communication that was completely independent from the PSTN. By the mid s, several successful LAN topologies and protocols were standardized, and LANs were installed in most large offices and manufacturing floors connecting their computing facilities.

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