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21: Global Services Mobile Communications (GSM)

The IComparer interface defines a method called Compare( ), which defines the way two objects are compared. It is shown here: int Compare(object x, object y) It must return greater than zero if x is greater than y, less than zero if x is less than y, and zero if the two values are the same. This interface can be used to specify how the elements of a collection should be sorted. IEqualityComparer defines these two methods: bool Equals(object x, object y) int GetHashCode(object obj) Equals( ) must return true if x and y are equal. GetHashCode( ) must return the hash code for obj.

his appendix presents a brief overview of the C I/O system. Even though you will normally use the C++ I/O system, there are several reasons why you may need to understand the fundamentals of C-based I/O. First, if you will be working on C code (especially if you are converting it to C++), then you will need to understand how the C I/O system works. Second, it is common to find both C and C++ I/O within the same program. This is true especially when the program is very large and has been written by multiple programmers over a long period of time. Third, a great number of existing C programs continue to be used and maintained. Finally, many books and periodicals contain programs written in C. To understand these programs, you need to understand the basics of the C I/O system.

Hence, the Laplace transform of f (t) = 1 is {1} = 1 s (13.5)

Ill 15-4

Read the labels on the under- or backside of your AC electric appliances, tools, and entertainment devices. Which is correct: 110, 112, 115, 117, 120, or 125 volts There is no universal agreement as to standard AC voltage. Different utility companies aim at different nominal voltages. In addition, the voltage delivered to your home is likely to vary by a few volts, depending on the other loads in your neighborhood and the total load on the grid. In this book, we will adopt a nominal standard of 120 volts. The rst electric service supplied by utilities was in the form of 120 volts DC. The change to AC was made because AC power is easily transformed (by transformers) to higher voltage and correspondingly lower current. Since voltage drop in a wire is proportional to current and not voltage, greater voltage results in reduced loss in transmission. To take advantage of this fact, the power company distributes power over the grid at extremely high voltages (several hundred thousand), then transforms it down in progressive steps to the 120 volts AC that enters your home. Electric lights, the rst electric devices used in the home, work equally well on AC and DC, provided the power dissipated in the lament is the same. As we learned in 1, electrical power is the product of voltage and current, P =V I where: P = power consumption in watts V = volts across the load I = amps through the load Using Ohm s Law, which works equally well for DC and AC with purely resistive loads, we can also express power as P = V2/R. Since the resistance, R, of a lamp lament is purely resistive, equal AC and DC power dissipation reduces to: (VDC2)ave = (VAC2)ave In other words the average value of the squared AC voltage must equal the average value of the squared DC voltage. It is easier to think in terms of equivalent DC voltage, so we say, Equivalent VDC = (VAC2ave)1/2 The equivalent DC voltage is computed as the square root of the mean value of the squared AC voltage (the root mean square [RMS] value of the AC voltage).

Part III: