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Network Characteristics

Each type of network can be described in terms of a number of key characteristics which define its basic nature and application. The Public Switched Telephone Network (PSTN) can be characterized as being designed for voice communications, primarily on a circuit-switched basis, with full interconnection between individual networks. The network is largely analog at the local loop level, digital at the backbone level, and is provisioned using wired facilities, for the most part.

Voice (Primarily)

The original network was designed to carry voice communications only. At the time, the only other form of telecommunications was telegraphy, which was the province of Western Union. The PSTN still exists primarily to support voice communications, although much data traffic also transverses the network. Image information can also be transported, as another form of data. Additionally, video traffic is supported through special videoconferencing equipment and interfaces of various types.

Switched (and Dedicated)

Contemporary voice networks are largely circuit switched, thereby providing great flexibility to the end users and significant economies to the carriers. Large organizations that communicate intensively between well-defined physical locations (headquarters, region and field locations) often employ dedicated circuits in order to mitigate network costs. Such dedicated circuits generally are leased from the carrier(s), making use of existing PSTN transmission facilities. In relatively rare instances, user organizations will deploy their own facilities, usually in the form of private microwave. In very rare instances, large end user organizations will purchase from the carriers a set of fiber optic facilities that can be used to connect the users' locations on a dedicated basis. Such optical fibers would be contained within a larger set of facilities deployed for the carriers own use and sold to the users on an as-available basis.

Analog (and Digital)

The traditional voice network was analog in nature and still is, in large part. The local loop connection terminating at the residence or business premise always is analog, unless arrangements have been made with the carrier for a digital circuit. Large user organizations often have digital connections in the form of T-carrier. ISDN, of course, is entirely digital, from end to end.

The internal telephone company networks and internetworks rapidly are being replaced with digital circuits for reasons which include improved bandwidth and bandwidth utilization, superior error performance and enhanced network management capabilities. This conversion process is largely complete in most major industrial nations and regions, including North America, Australia, New Zealand, Hong Kong, Japan, Singapore, and Western Europe. Developing nations and regions (The Philippines, Thailand, and Eastern/Central Europe) either are converting the PSTN to digital or are abandoning obsolete analog facilities in favor of constructing new digital networks.

Interconnected!

Complete interconnection between all local and long distance, and national and regional networks is fundamental to the PSTN. As discussed earlier, islands of telephony were interconnected many years ago in order that any subscriber might have the ability to connect to any other, subject to availability of transmission and switching capacity.

Wired (and Wireless)

While the traditional networks are largely wired (twisted pair, coax and fiber optic), a great deal of wireless technology is also employed. Wired networks generally perform better in terms of bandwidth, error rate, security and other dimensions, as discussed in Chapter 3. Fiber optic networks are particularly notable in these respects. While wired networks often are more difficult and costly to deploy and reconfigure, they are advantageous in the long term.

As wireless networks are plagued by limited radio spectrum, electromagnetic interference, lack of security, and other considerations, they generally are avoided in the architecture of the PSTNs of developed nations. Microwave, however, is used extensively in many networks, either as a primary means of connection where wired networks are impractical or as backup in the event of network overload or failure. Satellite communications also is used extensively for certain international communications and in order to provide access to island nations and remote areas. Wireless Local Loop (WLL) technology is still largely in the developmental stages, although it holds great promise.

Developing nations, however, make extensive use of satellite and microwave systems, and are deploying WLL aggressively. The advantages of wireless communications, as noted in Chapter 3, include rapid deployment and avoidance of certain issues of difficult terrain. As a result, the nation of India, for instance, is making extensive use of satellite communications to link thousands of remote villages. Similarly, remote towns in the outback of Australia are linked to the PSTN via satellite. Microwave is used extensively to link cities in island nations such as The Philippines and Malaysia.


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