The challenge of increasing broadband capacity.

• Author: Hatfield, Dale N.

1. 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.

2. 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. Many wireless devices today are battery powered, and increasing their transmitted power can significantly decrease the length of time that the device can be operated without recharging the battery. While the base station with which the portable device communicates may be able to operate at a higher transmitter power, the power received at the base station is often limited by practical battery life considerations associated with the portable device. (3)

   1. Understanding Bandwidth

   The term bandwidth originated in the analog world where it is defined as a range (band) of frequencies measured in cycles-per-second or hertz (Hz). As a width, it represents the numerical difference between the upper and lower frequency limits of a channel of communications. In this context, a channel is a path used for the transmission of communications signals between two geographically separate points. For example, an ordinary telephone channel may have an upper frequency limit of 3.5 kHz and a lower frequency limit of 0.3 kHz and, hence, an audio frequency (AF) bandwidth of 3.2 kHz. (4) A high-fidelity audio amplifier, on the other hand, may have an upper frequency limit of 20 kHz and a lower frequency limit of 20 Hz, and thus a bandwidth of 19.98 kHz. In radio frequency (RF) communications, an ordinary television channel in the United States has a bandwidth of 6 MHz. For example, television channel 2 occupies an RF range of 54-60 MHz and channel 3 occupies an RF range of 60-66 MHz. In contrast, a single frequency modulation (FM) radio channel occupies an RF range of just 200 kHz.

   In the analog world, signals and the channels they occupy are typically classified loosely and somewhat fluidly as narrowband, wideband, or broadband. A channel that is classified as broadband because of its greater width (e.g., a television channel) can carry more information per unit of time than a channel classified as narrowband or wideband. A television signal made up of both video and sound (i.e., visual and aural) information contains more content than a simple audio signal associated with a telephone call. Hence, it requires more bandwidth to transmit in a given amount of time. Stated another way, a narrowband channel may be adequate to transmit an ordinary voice call, but totally inadequate to transmit a television signal. In short, the more information one desires to send in a given amount of time, the greater the analog bandwidth required.

   Even though it is, strictly speaking, an analog expression, the term bandwidth has been carried over into the digital world. In the digital world, where information is carried as bits or "ones and zeros," the term bandwidth is also used to indicate how much information a channel can transmit in a given amount of time. However, in the digital world, bandwidth is measured in bits per second (bps). It is important to note that, in a digital network, the bandwidth is expressed as a rate--how many events happen per unit of time. Other examples of rates include a pump that can discharge water at a rate of 10 gallons per minute or a bridge that can carry 1,000 vehicles per hour. Stated again for emphasis, in the digital world, bandwidth refers to a transmission rate expressed in bits per second.

   When digital networks are used to convey analog information, the analog signal is first converted to a digital signal at the originating end through a process known as analog-to-digital conversion, and then, at the terminating end, the digital signal is converted back to an analog signal through a reverse process of digital-to-analog conversion. As is the case in the analog world, in the digital world, the digital signals and the channels they occupy are classified as narrowband, wideband, or broadband.

   After the analog-to-digital conversion process described above, an ordinary voice signal requires a transmission rate on the order of a few tens of kilobits per second (kbps), while the transmission of a high quality still image in a reasonable amount of time may require a transmission rate of several hundred kbps. A high-quality television signal, at the other extreme, may require on the order of several million bps (Mbps) for successful transmission in real time. Transmission rates in the tens of kbps range are typically categorized as narrowband; rates in the hundreds of kbps range are typically categorized as wideband; and rates in the Mbps range are typically classified as broadband. So, to summarize in today's terms, in a digital network, the term broadband is associated with a transmission rate of several Mbps or more. (5)

   As emphasized earlier, in the world of digital communications, bandwidth is associated with a transmission rate, but such rates are sometimes confusingly referred to as speeds. That is, people often speak of high-speed modems or high-speed networks when they really mean high-bit-rate modems or high-rate networks. Speed properly refers to the time it takes for an object--or, in the case of electronic communications, a Signal--to travel from one point to another across an intervening space. In electronic communications, electromagnetic waves (e.g., RF signals) travel through space at the speed of light and via copper wires, coaxial cable, or fiber-optic cables, or other physical media at velocities that approach the speed of light.

   In the digital world, some characteristic (or combination of the characteristics) of the transmitted electromagnetic/RF signal is rapidly changed to reflect whether the bit being sent is a one or a zero. The simplest digital transmission system to envision is one that sends a burst of electromagnetic/RF energy if the bit is a one and does not send a burst when the bit is a zero. In other words, the bursts of energy (or lack thereof) occur at regular intervals representing a sequence of ones and zeros that correspond to the information being sent. In order to send information at a higher rate, the intervals are shortened in time--that is, the ones and zeros are closer together in time and space--such that the transmission rate increases but...