The Challenge of Increasing Broadband Capacity

I. INTRODUCTION II. UNDERSTANDING CRITICAL RELATIONSHIPS A. Understanding Bandwidth III. OPPORTUNITIES AND CHALLENGES: LOOKING AHEAD A. Twisted-Pair Cable B. Coaxial Cable C. Wireless Links D. Fiber-Optic Cable IV. SUMMARY AND CONCLUSIONS I. INTRODUCTION The recent release of the National Broadband Plan by the FCC has focused the attention of policymakers, industry leaders, academics, and ordinary citizens on the importance of having sufficient bandwidth available anytime and any place to support a growing array of broadband services. Broadband services include both wireline and wireless access to the Internet and the delivery of high-definition and even 3D television. As popular as these two terms--bandwidth and broadband--have become, and as important as they are to our future as a nation, they are not always well understood. The purpose of this Essay is to explain these terms in more technical detail, and relate the explanations to the opportunities and challenges that are associated with increasing fixed and mobile broadband capacity as envisioned in the National Broadband Plan. This Essay is divided into three sections. The first section discusses the critical relationship between the digital transmission capacity of a communications channel (as expressed in binary digits or bits per second-bps) and the amount of bandwidth associated with that channel (as expressed in analog terms). The second section, in turn, builds upon that discussion to explore the opportunities and challenges associated with increasing the capacity of the four primary transmission technologies used in the critical access portion of the network--namely, twisted-pair copper cable, coaxial cable, wireless links, and fiber-optic cable. The third section provides a summary and offers some concluding thoughts. II. UNDERSTANDING CRITICAL RELATIONSHIPS Both analog bandwidth, as traditionally defined, as well as digital "bandwidth" (expressed as a bit rate), determine how much information can be sent over a communications channel in a given amount of time. The two are related to one another by Shannon's law, which is named after Claude Shannon, who is credited with being the founder of information theory-the basis of modern electronic communications. Shannon's law states that the maximum amount of information that a circuit or channel can carry per unit of time (as measured in bits per second) depends upon the (analog) bandwidth and the strength of the desired signal relative to the strength of the accompanying undesired noise and interference as measured at the receiving device. (1) For example, if the bandwidth of the channel is 1 megahertz (MHz), and the received power of the desired signal is fifteen times as strong as the accompanying noise, then the maximum digital capacity of the 1 MHz channel would be 4 megabits per second (Mbps), or 4 bps per hertz of bandwidth (bps/Hz). Shannon's law suggests two fundamental ways of increasing the digital capacity of a channel: increasing the amount of bandwidth devoted to the channel or increasing the received signal level relative to the accompanying noise and interference. (2) Bandwidth increases, however, are often constrained by the technical characteristics of the transmission medium, or, as in the case of wireless communications, by government regulation. Increasing the digital capacity of a channel by increasing the transmitted power suffers from diminishing returns, and from practical constraints. For instance, in the example given above, increasing the received power to thirty-one times as strong as the accompanying noise only increases the capacity to 5 bps/Hz. Moreover such power increases are often impractical in the "real world" because of the increased interference that would be caused to other nearby users of the same radio spectrum (i.e., the same channel) in the case of wireless communications. …