Notes


Chapter 2

  1. The dictionary defines transducer as a device that converts energy from one form to another. Strictly speaking, a sensor is a transducer, too (it converts from mechanical or optical energy to electrical); however, engineers generally reserve transducer for the conversion from electrical energy. The specific case of a transducer that converts to mechanical energy is an actuator.

  2. The usage of these terms is somewhat variable and inconsistent. For example, data is also commonly applied to information, such as that acquired in a scientific experiment, that has been subject to minimum interpretation.

  3. Information conveyed through touch, taste, smell, and equilibrium may be difficult (although not impossible) to represent digitally. The observation that most information can be represented by data and an understanding of the implications of this are generally attributed to Claude Shannon, the originator of information theory. He developed a powerful mathematical theory on the number of bits required to represent information as well as the capability of communication channels to convey it (Shannon and Weaver 1949). One of the classic results of this theory is that a collection of bits can serve as a representation of information in common forms with an arbitrary accuracy (as more bits are used) and no loss of generality.

  4. In contrast, the form of representation is necessarily much more variable in an analog medium. For example, with magnetic tape as an analog storage medium, the way in which audio and video are represented are necessarily quite different. The video requires special synchronization pulses to distinguish the start and end of individual images, something not required for audio.

  5. This terminology is often applied loosely in practice. For example, in a computer system, replicating a file might be called copying. Digital representations of analog media (like audio or video) might be replicated (making a replica of the digital representation) or copied (converted to analog, copied in analog form, and reconverted to digital). These nuances become extremely important in digital rights management (see chapter 8).

  6. The material world, especially in IT, involves more than just atoms. Nonatomic particles like electrons and photons play an essential role also. A more appropriate metaphor might be particles versus bits.

  7. Occasionally software has been used to denote information content (Shy 2001). For example, the available DVD movies might be called the "software for consumer video equipment." This is confusing and is avoided here. Fortunately it seems to be disappearing from common usage.

  8. In the past IBM has had considerable ambitions in the area of communication but more recently has deemphasized this. Examples include its acquisition of Rolm and Satellite Data Services in the 1970s and its development of its Global Network, which it subsequently divested to AT&T.

  9. The distinction among the three technologies is somewhat less sharp. In reality, storage cannot work without a little communication (the bits need to flow to the storage medium) and communication cannot work without a little storage (the bits cannot be communicated in zero time). Processing cannot work without both a little storage (creation of intermediate results) and a little communication (moving bits from communication or storage to processing units and back).

  10. The details are less important than the overall trend that this model predicts. Nonetheless, we present them here (see Messerschmitt 1999c). Dc is average waiting time for a Poisson arrival of requests and an independent exponentially distributed time to service each request and average service time Ds. For example, u = 0.5 results in a congestion-induced delay equal to the average service time. Of course, this specific model would not always be an accurate reflection of reality in all instances of congestion.

  11. The ability to improve capacity through the addition of hardware ("scaling out") depends on the proper architecture of an application and its underlying infrastructure. This is discussed further in chapter 4.

  12. Up to the mid-1990s communication through fiber was limited by improvements in the electronics to transmit and receive the bits, and thus doubled about every eighteen months. The innovation that circumvented this bottleneck was wavelength-division multiplexing (WDM), which allowed multiple streams of bits to be transmitted simultaneously on different wavelengths. This has increased the rate of improvement dramatically because it bypasses the electronics bottleneck.

  13. This is a fundamental consequence of the observation that the radio spectrum has vastly less bandwidth than the optical spectrum, even as the latter is limited by the material properties of the glass that makes up fiber.

  14. For example, increasing the maximum load capacity of a train likely decreases both fuel economy and maximum velocity.

  15. The advance of magnetic and optical storage focuses on decreasing the area required for storage of each bit, allowing more bits to be stored on a given disk and also, along with advances in electronics, enabling faster transfer of bits in and out. Advances in fiber optics focus on increasing the transfer rate of bits through the fiber, which is equivalent to reducing the length of fiber occupied by each bit, because the bits always travel at the same velocity (a fraction of the speed of light).

  16. The time value of money dictates that a return R occurring n years in the future has present value R/(1 + r)n for prevailing interest rate r. The discount factor (1 + r)n is smaller for earlier returns (smaller n), increasing the present value of the return.

  17. The specific risks to be concerned about are outright failure to achieve the goal, a need to increase the investment to achieve the goal, or a delay in achieving the goal.

  18. These three businesses were once largely captive to large vertically integrated firms. Industry maturity and market forces have favored specialization, so now they are largely separate firms (with some notable exceptions). Many of the largest equipment manufacturers, like IBM, Nortel, Lucent, Siemens, Philips, NEC, Sony, and many others, retain internal integrated circuit manufacturing. However, they purchase many chips from specialized integrated circuit manufacturers like Intel and Texas Instruments. Meanwhile, most of the newer equipment manufacturers (like CISCO) outsource their integrated circuit manufacturing, even chips they design themselves.




Software Ecosystems(c) Understanding an Indispensable Technology and Industry
Software Ecosystem: Understanding an Indispensable Technology and Industry
ISBN: 0262633310
EAN: 2147483647
Year: 2005
Pages: 145

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