X-DSL Architecture

Chapter 1: Broadband Access Network Evolution

Today, we live in a content-driven society. Newsprint, television, and electronic media are engaged daily in what Andy Grove of Intel Corporation once described as the "war for the eyeballs." Not only are we bombarded every day with a combination of voice, text, and video content, we are also expected to react to it. Faxes, pagers, cellular phones, and e-mails have become necessities. Further, this constant interaction with dynamically changing content has an impact on much more than our work. We are also looking to be entertained and educated in new ways and for ways to stay connected no matter where we are physically located. This, then, is the age of networked multimedia-rich, dynamic content that is available to everyone, everytime, everywhere.

From the present, let us take a trip on an imaginary time machine and travel a few years into the future. Picture this scenario: You've just driven home from work. As you enter your home and walk through the living room, you have the following conversation with your home computer, also known as your personal "electronic assistant":

"Do I have any messages?"

Electronic assistant: "Yes, you have three new video messages. Do you want to play them?"

"Yes, I want them on my bedroom screen."

Your electronic assistant displays the video messages on an enormous flat screen on the bedroom wall. After dinner, you would like to help your teenage child with homework on ancient cultures. The assignment is a paper on the Mughal Emperors of India. Although you don't have a clue about the topic, you don't worry. First, you ask your electronic assistant to gather enoughinformation to complete the paper. The electronic assistant searches all available libraries and video images to compile the data. In a few minutes the electronic assistant asks, "Are you ready to view what I found on Mughal Emperors?" Once you are ready, the electronic assistant displays the information on the screen in your child's bedroom.

Now that your child is busy writing the report, you decide to relax and watch a movie. You ask the electronic assistant for a list of the latest hits. It brings up a list of choices, you select one, and ask the electronic assistant to play it. The electronic assistant extracts the movie from the database at a video store nearby and plays it as if it were being played from your home VCR-the only difference being that the VCR is at a remote location.

Suddenly, the electronic assistant interrupts your viewing the movie to announce that a video call is coming in. You ask the electronic assistant to put the movie on pause and answer the video call. When you complete the call, the electronic assistant automatically switches the movie from pause to play so that you can continue to view it from the point at which you stopped before the video call.

This scenario is not several years away. Most of the technological building blocks that can deliver these services are in place today. just as television is delivered to your home via a video network (in the form of cable, satellite, or over-the-air broadcasts), broadband networks will soon be able to deliver a plethora of integrated voice, data, and video services. Technological innovations and competitive factors will make it possible for consumers to enjoy these services at affordable prices.

Rapid technological innovations are occurring from the "last mile" to your home down to the "last inch" in your home. Consider, for example, the Home Phoneline Networking Alliance (HomePNA). The objective of this association of technology companies is to work together to ensure the adoption of a single networking standard that does not require any rewiring in your home. It uses the existing phone wires installed in your residence to deliver a variety of integrated high-speed services, as shown in Figure 1. 1. Other alliances such as HomeRF2 and Bluetooth3 are investigating ways to dispense with wires entirely.

(Figure 1. 1 Vision for the networked home Source: Home Phoneline Networking Alliance)

How is this possible? What are the factors that can enable the vision of "affordable, ubiquitous, high-speed" access to consumers all the way to their homes? To answer these questions, we need to get back in our imaginary time machine, and return to the past. An examination of the history of our communication networks can help us to better understand how we got to where we are today-and where we can go tomorrow.

What Is a Communications Network?

A communications network is a common resource shared by many customers who need to communicate with users at other locations. Not everyone uses the network all the time, so it is logical-often economical-to share this important resource. A communication network consists of the following elements (shown in Figure 1.2):

(Figure 1.2 Elements of Communications network)

Station equipment or customer premises equipment (CPE) sends and receives user information and exchanges control information to gain access to the network and to place calls. Since station or customer premises equipment either begins or terminates the flow of information, i.e., it acts as either information sources or inforination sinks, it is often referred to an endpoint. Information flow within the network is commonly referred to as traffic.

Transmission facilities provide the paths to carry the traffic and establish connectivity between endpoints. In general, transmission facilities consist of a medium such as air, copper wires, coaxial cables, or fiber optic cables. Therefore, transmission facilities are often referred to as links. Physical links are often shared among multiple simultaneous connections between information sources and information sinks. The portion of a physical link dedicated to a single connection is often referred to as a channel. A channel either can be full duplex or half duplex. A full-duplex channel allows an endpoint to transmit and receive simultaneously; by contrast, in a half-duplex channel the endpoint can either be transmitting or receiving, but not both simultaneously.

Switching components interconnect transmission facilities at various locations and route traffic through the network. Skitching makes it possible to share transmission facilities among economically multiple users. Switching components are often referred to as network nodes. The network node into which the source endpoint connects is called an ingress node. The network node from which the sink endpoint is connected is called an egress node.

Architecture of Communication Networks

There are several ways to categorize communication networks: by topology, traffic transport mechanism, functional area, or dominant application. In this section, we briefly discuss categories such as meshed, star, and bus topologies, switching and multiplexing mechanisms, and premises, access, backbone, and service domains. The reader who has knowledge of these topics may skip ahead to later sections in this chapter that discuss the history and architectures of voice, data, and video networks.

Categorization of Communication Networks Based on Topology

There are three basic types of topologies-mesh, star, and bus. In a mesh network, all endpoints are interconnected to each other. In a star network, the endpoints are not directly connected to each other; rather, they are connected directly to a network node. Endpoints can therefore only communicate to each other via the intermediate network node. Finally, in a bus network, all endpoints are connected to each other over a common bus; therefore, any endpoint may communicate to another over the bus....