Chapter 5. The Public Network

In this chapter...

• Switched Services—Local and Long Distance Calling

• Dedicated Services

• Virtual Private Networks—Connectivity for Remote Access, Intranets and Extranets

• “The Last Mile” or Access Networks

• Optical Networking

• Network Intelligence and Signaling

• Convergence—Technical Advances Leading to Improvements in IP Networks

• Examples of Converged Networks

• Document Sharing and Click to Talk

• Summary

The public switched telephone network (PSTN) is made up of switches, cabling and equipment that simultaneously transmits multiple telephone calls over single pairs of fiber cabling. The extraordinary characteristic of public networks is that carriers from all parts of the world have agreed on ways to transmit calls to each other. Enormous efforts have been made to ensure that systems are reliable and dependable and function as much as possible during power blackouts, hurricanes and national emergencies.

Switching is the primary vehicle for carrying voice, facsimile and dialup modem traffic worldwide. Public network switched services are dialup; users dial a telephone number to create a temporary connection to anyone on the public network. Examples of switched services are home telephone, cellular, dialup Internet access and main business lines. Switched services are used for data as well as voice. Switches connect segments of local area networks (LANs) to each other. Asynchronous transfer mode (ATM) and optical switches in public data and voice networks switch vast quantities of traffic between cities.

Dedicated services, also called private lines, are more specialized than switched lines. Organizations use them to save money when they need to transmit large amounts of data or place hours of voice telephone calls to particular sites. Imagine two tin cans and a string between two locations. This is something like a private line. Organizations have the use of the string 24 hours a day, seven days a week. Users pay a flat monthly fee; there are no per-minute charges for private lines with a fixed configuration.

Dedicated lines are expensive and complex to manage. If a firm only has dedicated lines between a few locations, maintenance might not be a problem. However, once private lines connect many locations, they become cumbersome to manage. Moreover, private lines are costly because carriers can't share the dedicated private lines among many customers. For these reasons, many companies are choosing carrier-managed, value-added virtual private networks (VPNs). Virtual private networks are “virtually” private. They have many of the features of private networks; however, network capacity is shared by many customers. Many customers use VPNs for remote access by employees, branch offices and business partners. The VPN providers offer security services to protect corporate files from viruses and hackers.

Network-based computer intelligence has changed the public network from plain old telephone service (POTS) to one capable of delivering advanced features and generating fat profits. Value-added services such as call forwarding, caller ID and voice mail that depend on signaling systems generate large profits. Services dependent on signaling include advanced features associated with 800 and 900 services such as routing by time of day, integrated services digital network (ISDN) service and the capability to keep the same toll free number when changing carriers.

Signaling is the glue that holds the public switched network together. Routing, billing and transferring calls between carrier networks depend on signaling. Network maintenance information also is carried on signaling systems. The way signals are transported impacts network efficiency, costs, reliability and introduction of new services.

Optical technologies have had a major impact on efficiencies in Internet, long distance and local networks. They have lowered the costs significantly of building high-capacity data and voice networks. Recent developments in optical cross connects are adding potential for more capacity in backbone networks and decreasing the expense of providing redundancy. (All major carriers have backup routes to which they transmit traffic in case the primary path goes down.)

Major efforts in development of new optical technology are bringing the benefits of fiber optics closer to homes and small and medium-sized businesses. Passive optical networking and new digital loop carriers (DLCs) will be particularly significant in parts of the world where new infrastructure is being built in metropolitan areas. Applications such as document sharing, email, Web browsing, distance-based learning and remote access to corporate files are affordable to individuals and small and medium-sized businesses. Small and medium-sized businesses need Internet connections to be able to use these applications. Passive optical networks that make fiber links affordable for these types of customers are important.

Convergence, where a single packet network carries all types of traffic, is still in its infancy. However, as these networks and protocols become more suitable for voice, more networks will be built based on data protocols rather than on switched technologies developed for voice. As data continues to grow as a percentage of total traffic carried, new networks will be built to accommodate it. Existing networks that age will be replaced by new networks built using these technologies. This will be a gradual process.