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 that's 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 a PSTN environment, the circuit is established between two subscribers: The PSTN connects together two subscribers. But 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 release 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; the servers, which are the centralized repositories of wisdom that you are seeking; and the packet switches, which route and relay the packets of information between the clients and servers. Whereas the PSTN connects together two subscribers, the Internet connects together 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 the messages into packets of data, which contain the routing information, which will then lead them to their destination. Individual packets may take different routes, but they'll be reassembled in the proper order at the destination. This system is optimal for the most efficient 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. Significant progress is being made on introducing QoS to the Internet, though, and in the next two or three years, this will change a great deal.

What Is Meant by Next-Generation Network?

At this point, we need to apply a definition to the term next-generation network because throughout this book it is used 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- or cell-based network that's 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: The Next Generation

Networks are evolving so that they can address the growing demand for QoS. The two different infrastructures circuit switching and packet switching are 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 are all elements that are discussed in Chapter 11, "Next-Generation Network Services." These new devices in essence allow interoperability to exist seamlessly between the PSTN and packet-switched networks, whether IP or ATM or MPLS.

For more learning resources, quizzes, and discussion forums on concepts related to this chapter, see www.telecomessentials.com/learningcenter.