Faculty Scholarship 1994 - Present

Modems

Few would disagree that the emergence of tele- and data-communication technologies-specifically, the ability to both transmit and receive voice, video, and data-has had profound impacts on both our personal and professional lives. Although many have contributed to the progression of modern-day communication networks, the works of Samuel Morse and Alexander Graham Bell (i.e., Morse code and the telephone network, respectively) provide the foundation for much of the last mile, or local loop, communication infrastructure in use today. In other words, the same basic physical circuitry between residential homes and a telephone company?s central or end office remains unchanged. Thus, even in today?s technol ogy-driven economy, consumer-based, network-capable technologies (e.g., computers, video games), although digital in nature, continue to utilize, at least in the last mile, both guided and wireless analog-based circuitry (e.g., dialup, cable, wireless). As such, translational protocols and devices are necessary. In this chapter, we discuss one such device: the modem. Modems first emerged as a communication solution to the binary-analog dichotomy that existed between computer data and existing voice-grade circuits. Specifically, computers produce binary or discrete data (e.g., 1?s and 0?s), whereas the existing telephone circuitry and hardware are designed to transmit analog data-that is, data that are continuous in nature and constantly varying. Essentially, to transmit discrete data over voice-grade circuits, digital data must be modulated or converted to analog signals. Similarly, the receiving computer requires that all incoming analog signals be demodulated-that is, converted back to binary data. As such, the term modem emerged as a contraction of these two primary functions-modulation and demodulation- and traditionally has been defined as any physical, networking device that allows computers, or other binary-based technologies, to transmit data over standard telephone or plain old telephone service (POTS) lines. With the proliferation of the Internet and World Wide Web, the definition of modems, like many other networking technologies, has evolved. For example, Horak (2002, p.182) writes, ?Modems modulate and demodulate signals. In other words, they change the characteristics of the signal in some way.? From this definition, the term modem subsumes any device that meaningfully modifies data for the purposes of transmission. In this chapter, the traditional definition is used to refer to dialup modems, while Horak?s definition is used to accommodate broad-band modems (e.g., cable, DSL, and wireless). The chapter begins by providing a brief historical perspective. This is followed by a high-level discussion of how dialup modems work. We then classify modems on several salient dimensions: their architecture, locality, mode of transmission, and synchronization. Following this, modem protocols, or standards, are discussed and presented (e.g., data flow, data rate, compression, and error checking). Subsequently, we present and summarize what we term here as next-generation modems (e.g., wireless, cable, optical). Finally, last mile connectivity technologies and trends are discussed.