Everything Coming Up Digital.
Western Electric's Lewis Bayers puts this new "digital world" in good perspective when he says: "There is a vision of the future shared by most planners in the telecommunications industry today. It is called IDSN, which stands for Integrated Services Digital Network. ISDN is a vision of a network that can handle voice, data, video: a network that can handle the needs of the Information Age. In Geneva, Switzerland, a United Nations agency is laboriously working towards defining it. Manufacturers and telephone companies around the world are studying it. Large corporations everywhere are taking steps toward planning for it. In the Bell System, we're already building it. And the 5ESS switch, our newest, is not only available for today's needs, but is also another major step toward fulfilling the vision of ISDN.
"Why ISDN? Well, today, the Bell network handles over half a billion calls each day. These are primarily voice calls, that is, people talking to people. But increasingly, the network is being asked to do more. The Information Age sees computers calling computers, special custom calling services, video teleconferencing, and more. The network needs to handle this must be a network of sophisticated equipment providing sophisticated services. It must be able to handle voice and data, simultaneously. It must be able to switch and transport data, facsimile, video, and special services as easily as it handles voice.
"The vision of the future called ISDN is little more than what we in the Bell System have beenm building for years . . . a network of stored program controlled switches, common channel interoffice signaling (CCIS), and digital trunks; a network being built to eventually provide any type of cummunications transport and service. It's been evolving, growing, improving, since the beginning. Now, with new customer needs, and new technologies available, it will continue to grow. As new products come out of our shops, and new designs come out of the labs, the network will increasingly handle even more traffic, provide even more ervices."
The gradual change of telephones from the so-called analog technology, where sound is carried by waves of electricity, to digital technology, where sound is converted into electric pulses, makes such a network feasible. "ISDN is a successive thing," says Joe Shapiro, who heads the digital-transmission planning department at Bell Labs. "Every time we replace a piece of analog equipment with something digital, we're taking one more step toward ISDN."
From the early days of digital communications, a major problem has been to determine the bit rate necessary for voice communications . . . to resolve the tradeoff between speech quality and system costs. Increasing the bit rare increases speech quality, but it also increases system costs. Today's pulse code modulation (PCM) systems use a bit rate of 64 thousand bit per second, a rate considered adequate to provide good quality speech for public telephone services. All digital telecommunications networks are constructed on the basis of this bit rate.
Over the years, communication demands, especially for non-telephone services such as digital data, facsimile and video, have increased in amount and coverage to where they now span continents. To handle telephone and non-telephone services together in a communications system, digital-based networks, such as the integrated services digital network (ISDN), are being developed in various telecommunication organizations, providing the impetus to develop high-capacity, long-distance digital communications systems. They have also made it necessary to dvelop and implement new transmission media, such as optical fibers and satellites. The result has been a widened range for digital communications systems and significantly lower circuit costs.
"The ISDN is the future of the telephone and the great economic hope of most telephone companies," says Paul Reevey, a marketing director for British Telecom, the British telecommunications giant.
The move toward an all-ditital network began in 1962 in metropolitan areas with the introduction of T-carrier lines, which transmitted 24 digitized conversations on two twisted pairs of existing telephone cables at about 1.5 megabits per second for distances up to about 50 miles. The per-circuit cost is low, making T-carrier attractive in large metropolitan areas where additional cable would be expensive to install.
This policy of introducing digital transmission when and where it makes economic sense still guides the evolution of the network. But today telephone companies are motivated by the economic attractiveness of new services, as well as the traditional factors of cost reduction and performance.
In the short-haul network, where transmission distances are generally less than several hundred miles, digital transmission makes economic sense because the cost of terminals dominates the economics of transmission, and digital terminals are less expensive than analog terminals.
United Tel's Fred Lawrence points out that "digital is different," saying, "From management, operations and planning perspectives, digital technology is a refreshing change. The simplicity and flexibility of digital employment is by and large different than most of today's information networks. Digital is different in terms of costs, revenue potentials, financial manageability, service and finally, planning. It has been called a revolution by some. Within the United Telephone System and among many of our neighbors, it is a planned and well-orchestrated evolution to the networks of tomorrow. Growing sophistication of market demands are driving the industry to robust loop plant design and ubiquitous 64kb capability. I would further postulate that the ISDN is actually an integrated Software Defined Network. Digital modernization is quickly providinb broadband capabilities into the key markets."
General Datacomm's Hugh Goldberg says: "The digital network has been tailored for data. It uses the same subscriber access lines (copper wire), but permits rates up to 56 kb/s, almost four times the speed of the newer breed of very high speed, 14.4-kb/s analog modems. Actually, T1 data at 1.544 Mb/s uses these same wirelines. However, the data is regenerated at 6,000-foot intervals.
"The bulk of dense, high-speed data transmission takes place over dedicated (leased-line) facilities. Dial-up facilities are used when the length of each transmission is relatively short and a number of dispersed remote locations cover a large geographical territory. Timeshare services represent the classic application of the public switched network for data transmission.
"future digital services are targeting dial-up customers with high-speed, low-error-rate networks. The forthcoming Circuit Switch Digital Capability will move full-duplex 56-kb/s data over the two-wire subscriber access pair, providing a switched network counterpart of the current private-line digital offering.
"Much of the world is moving toward new techniques for voice transmission as well as data. The new Integrated Services Digital Network will actually digitize the voice signal at the telephone prior to transmission into a fully integrated digital network. Whereas digital data once required analog technology and media, voice transmission of the future will use digital facilities."
Ed Botwinick, chairman of Timeplex, says: "It is clear that digital communications will become the common denominator for virtually all forms of remote information transfer. Voice, data, facsimile, and video information can all be economically converted to digital form today, with two major benefits. First, integrated digital communications networks are now economically feasible, to replace the separate facilities that have been traditionally required for voice, data, and video. Second, digital information can be readily processed by the low-cost, digital computing elements available from the semiconductor industry. This allows integrated communications switching facilities, and also permits the compaction and compression of many types of digital information in order to reduce transmission bandwidth requirements, thereby lowering operating costs.
"There is little doubt that by the end of the century virtually all electronic communications will be digital, using integrated communications networks. Today, however, we are very far from that objective, and the pathway to its achievement is unclear (Figure 7). Digital transmission services are not yet universally available, and high speed digital service has barely begun. The sheer magnitude of converting or duplicating the entire nationwide . . . not to speak of worldwide . . . telephone system to provide wideband digital terminations to a large number of subscribers, is totally mind boggling. Yet the completion of that task is the key to the future of telecommunications."
A new At&T technology lets computers communicate with each other at high speeds over the same lines that can be used for regular phone calls. circuit Switched Digital Cpability (CSDC), developed by AT&T Bell Laboratories, allows companies to send nearly all business communications . . . voice, graphics, and data . . . over conventional telephone lines. "CSDC is a giant step toward fully integrated voice and data communications networks that will bring new digital services right to the customerhd doorstep and will become the central nervous system of the Information Age," says Mark Mortensen of Bell Laboratories.
When AT&T Chairman Charles Brown keynoted the ITU's "Forum 83" in Geneva, Switzerland last Fall, he said: "In assessing the explosive growth of information technology, I think it's instructive to note that, from the user's point of view, the focus is not on technology but information. The essence of the Information Revolution is that once information of any type . . . spoken, written, recorded or broadcast . . . is converted into digital form, it can be transmitted, stored, retrieved, displayed or processed by anyone who has access to the appropriate information network or system. If universally and uniformly deployed, emerging information technology offers the potential for individuals, institutions, even whole societies, to summon whatever they choose of the world's knowledge . . . in real time, in a sable form and at affordable prices."
As digital communications have proliferated, there has been a corresponding explosion of local and distributed networking schemes, networking protocols, transmission systems and the like.
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|Date:||Sep 1, 1984|
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