The PSTN Versus the Internet

In one very important fashion, the PSTN and the public Internet are one and the same thing: They both exist on the same physical infrastructure. There would be no Internet without the PSTN. The communications links, or backbones, that ISPs run on are delivered over the PSTN, and the access lines for entry into the Internet are all subscriber lines that are part of the PSTN. But what differentiates the PSTN and the Internet is the equipment attached to each network, its use, and how it formats the information it carries.

PSTN Characteristics

The PSTN basically includes telephones, fax machines, and circuit switches that set up continuous but temporary connections. In the PSTN, a circuit is established between two subscribers and kept open for the duration of the call, including periods of silence. This provides guaranteed QoS and minimal latencies, and it means that the PSTN is optimized for voice and other real-time applications. It also means that the PSTN uses bandwidth inefficiently, making services more expensive. But we are constantly finding ourselves able to provide more bandwidth and derive more channels over that bandwidth.

Internet Characteristics

The Internet basically includes clients, which are the user interface and the input/output device for information; servers, which are the centralized repositories of wisdom that you are seeking; and packet switches, which route the packets of information between the clients and servers. Whereas the PSTN connects two subscribers, the Internet connects networks. As on the PSTN, messages on the Internet are routed to specific end devices. These messages take various forms, such as e-mail, instant messaging, and real-time audio/video communications. Unlike the PSTN, however, the Internet breaks down a message into packets of data, whose routing information guides the packets to their destination. Individual packets may take different routes, but they are reassembled in the proper order at the destination. This system is optimal for the most efficient use of transmission facilities, particularly when you're supporting bursty traffic that involves long periods of silence. In turn, this results in less expensive services. However, the tradeoff is that you get only best-effort QoS. Progress is being made on introducing QoS to the Internet, though, and in the next two or three years, this will change more. However, note that there is greater progress in introducing QoS on private networks than on the public Internet.

What Is Meant by Next Generation Network?

The term next-generation network is used throughout this book with a very specific meaning. The decreasing cost of bandwidth, combined with the availability of low-cost and powerful chip technology, favorably highlights the economies of statistical multiplexing and packet switching, as long as latencies and loss can be controlled. From that standpoint, next-generation networks embody two fundamental concepts. First, a next-generation network is a high-speed packet-based network capable of transporting and routing a multitude of services, including voice, data, video, and multimedia, while supporting QoS. Second, a next-generation network is a common platform for applications and services that the customer can access across the entire network as well as outside the network.

 

Converging Networks: Next-Generation Networks

Networks are evolving so that they can address the growing demand for QoS. The two different infrastructurescircuit switching and packet switchingare not trying to replace each other. Instead, they are converging. This marriage is required between the existing legacy environment (the circuit-switched network) and the new and unique IP marketplace (the packet-switched network). To address this convergence, a number of devices have emerged that have a number of names, including Voice over IP gateways, media gateways, next-generation switches, and softswitches. These elements are discussed in Chapter 9, "IP Services." These new devices in essence allow interoperability to exist seamlessly between the PSTN and packet-switched networks, whether IP or ATM or MPLS.

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