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Chapter 8
Conventional Public Data Networks (PDNs)

[I]n 1816, Mr. Ronalds (afterwards Sir Francis Ronalds) showed that an electric telegraph was possible, and endeavoured to persuade the Government of the importance of his system. The official reply to his appeal was as follows: “Mr. Barrow presents his compliments to Mr. Ronalds, and acquaints him, with reference to his note of the third instant, that telegraphs of any kind are now wholly unnecessary, and that no other but the one now in use will be adopted. Admiralty Office, Aug. 5, 1816.” The “one in use,” here indicated, was the semaphore, … which, it may be mentioned, was quite useless during the night, or when fog prevented the signals being seen.

—Wonders of the Universe, 1899

Data communications began in 1835 with the invention of the first practical telegraph by Samuel F.B. Morse and with his first long distance message, “What hath God wrought!” sent from Baltimore, Maryland to Washington, D.C. in 1846. This simplex (later diplex and quatraplex) device used start and stop signals of varying lengths over uninsulated iron, and later copper, wire. The first commercial data communications carrier was Western Union, founded in 1856 to provide telegraph services [8-1].

Modern data communications beyond the boundaries of the data processing center involved the physical transfer of punch cards or magnetic tapes via mail or courier. While this approach is still used in some cases as a primary means of data transfer (more than you might think), it is generally a last resort. Sneakernet is slow and expensive. The traditional voice network, despite its severe limitations, became a common means of electronic data transfer for short duration, low-speed communications. As the demand for data networking developed, and as the enabling technology evolved, a number of alternatives emerged. Data communications is growing at an annual rate in excess of 25%, while voice is growing at about 5% to 6%—roughly the rate of population growth.

Evolution of Data Networking

All Wide Area Network (WAN) communications initially were voice, with the exception of the separate telegraph networks, and remained that way for over 100 years. As a result, the networks evolved in a manner that best supported voice communications.

The Voice Model

The voice Public Switched Telecommunications Network (PSTN) was designed to support voice-only communications. As a result, PSTN can be characterized in terms such as analog, two-wire, two-way, narrowband, circuit-switched, and ubiquitous.

Analog

The network is presumed to be analog. The local loop is always analog, unless digital circuits are ordered specifically (e.g., ISDN and T-carrier). The carrier networks largely are digital, for reasons which include increasing traffic capacity, and improving error performance.

Two-Wire

Two-Wire local loops predominate, and are quite satisfactory for narrowband voice and low-speed data. Four-wire local loop circuits are used only to support wideband services such as T1 and emerging broadband services. The internal carrier networks are four-wire, in support of high-capacity, multichannel wideband and broadband communications.

Copper

Copper twisted-pair remains the predominant medium in the local loop, although fiber optic cable is being deployed to the business premise on a selective basis. Fiber and fiber/coax networks are being deployed to the neighborhood and premise on a trial basis. Otherwise, it is unusual for coaxial cable, microwave or fiber to be deployed to the premise. The internal carrier networks of twisted-pair, microwave and coaxial cable are rapidly being replaced with fiber—in many cases that replacement process is virtually complete. Microwave and satellite have continuing application in certain areas due to climate, terrain and population density. They also are used for backup purposes, in the event of a catastrophic failure of the primary facilities, and for access to sparsely populated areas and island nations.

Two-Way

Two-Way or duplex, communications is conversational in nature. Such a mode of communications is absolutely fundamental to the PSTN, as the parties in a voice conversation require the ability to communicate in both directions.

Circuit Switched

Circuit Switched networks are appropriate for voice PBXs and various types of network exchanges, as they provide a continuous talk path for the duration of the conversation. Dedicated, leased-line circuits are provided for very large organizations which communicate intensively between fixed locations (e.g., PBX tie trunks).

Narrowband

Narrowband (low capacity) facilities are quite satisfactory for most voice communications. Wideband (DS-1 and DS-3) access applies to high-capacity local loops for PBX applications. Broadband communications is currently limited for the most part to backbone carrier networks.

Ubiquitous and affordable

Ubiquitous and affordable are key terms in the voice world, at least in developed countries. In fact, universal access to high-quality, affordable service now is viewed as a basic human right in many countries.

Interconnection

Interconnection is fundamental to the PSTN and provides for connection between voice terminals, virtually regardless of location. LECs, IXCs and international carriers all are interconnected.

Hierarchical

Hierarchical structure of PSTNs is traditional, with multiple levels of carriers and switches, including Local Exchange Carriers (LECs), IntereXchange Carriers (IXCs), and International Record Carriers (IRCs). As noted in Chapter 6, hierarchical switch structures dominated carrier network design for many years. Contemporary configurations tend to be flatter, with many switches serving multiple purposes. Increasingly, the network intelligence tends to be centralized, with a small number of databases serving to provide multiple switches with routing instructions, for instance.


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