WIRELESS TECHNOLOGY & MOBILE COMPUTING
As per syllabus of
MCA SEMESTER 5
(For private circulation only)
Q 2) Discuss the various modulation techniques used in the wireless transmissions.
Ans: In wireless networks, the binary bit-stream has to be translated into an analog signal first. The three basic methods for this translation are amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). Apart from the translation of digital data into analog signals, wireless transmission requires an additional modulation, an analog modulation that shifts the center frequency of the baseband signal generated by the digital modulation up to the radio carrier. For example, digital modulation translates a 1 Mbit/s bit-stream into a baseband signal with a bandwidth of 1 MHz. Amplitude shift keying: Figure (1) illustrates amplitude shift keying (ASK), the most simple digital modulation scheme. The two binary values, 1 and 0, are represented by two different amplitudes. In the example, one of the amplitudes is 0 (representing the binary 0). This simple scheme only requires low bandwidth, but is very susceptible to interference. Effects like multi-path propagation, noise, or path loss heavily influence the amplitude. In a wireless environment, a constant amplitude cannot be guaranteed, so ASK is typically not used for wireless radio transmission. However, the wired transmission scheme with the highest performance, namely optical transmission, uses ASK. Here, a light pulse may represent a 1, while the absence of light represents a 0. The carrier frequency in optical systems is some hundred THz. ASK can also be applied to wireless infra red transmission, using a directed beam or diffuse light.
Figure 1. Amplitude shift keying (ASK)
Frequency shift keying:
A modulation scheme often used for wireless transmission is frequency shift keying (FSK). The simplest form of FSK, also called binary FSK (BFSK), assigns one frequency f1 to the binary 1 and another frequency f2 to the binary 0. A very simple way to implement FSK is to switch between two oscillators, one with the frequency f1 and the other with f2, depending on the input. To avoid sudden changes in phase, special frequency modulators with continuous phase
modulation (CPM) can be used. Sudden changes in phase cause high frequencies, which is an undesired side-effect.
A simple way to implement demodulation is by using two band pass filters, one for f1 the other for f2. A comparator can then compare the signal levels of the filter outputs to decide which of them is stronger. FSK needs a larger bandwidth compared to ASK but is much less susceptible to errors.
Figure 2. Frequency shift keying (FSK)
Phase shift keying:
Phase shift keying (PSK) uses shifts in the phase of a signal to represent data. Figure (3) shows a phase shift of 180° or π as the 0 follows the 1 (the same happens as the 1 follows the 0). This simple scheme, shifting the phase by 180° each time the value of data changes, is also called binary PSK (BPSK). A simple implementation of a BPSK modulator could multiply a frequency f with +1 if the binary data is 1 and with –1 if the binary data is 0. To receive the signal correctly, the receiver must synchronize in frequency and phase with the transmitter. This can be done using a phase lock loop (PLL). Compared to FSK, PSK is more resistant to interference, but receiver and transmitter are also more complex.
Figure 3. Phase shift keying (PSK)
Q 3) Explain the various configurations and the profiles supported in J2ME. Ans: Traditional computing devices use fairly standard hardware configurations such as a display, keyboard, mouse, and large amounts of memory and permanent storage. However, the new breed of computing devices lacks hardware configuration continuity among devices. Some devices don’t have a display, permanent storage, keyboard, or mouse. And memory availability is inconsistent among small computing devices.
The lack of uniform hardware configuration among the small computing devices poses a formidable challenge for the Java Community Process Program, which is charged with developing standards for the JVM and the J2ME for small computing devices. J2ME must service many different kinds of small computing devices, including screen phones, digital set-top boxes used for cable television, cell phones, and personal digital assistants. The challenge for the Java Community Process Program is to develop a Java standard that can be implemented on small computing devices that have nonstandard hardware configurations.
The Java Community Process Program has used a twofold approach to addressing the needs of small computing devices. First, they defined the Java run-time environment and core classes that operate on each device. This is referred to as the configuration. A configuration defines the Java Virtual Machine for a particular small computing device. There are two configurations, one for handheld devices and the other for plug-in devices. Next, the Java Community Process Program defined a profile for categories of small computing devices.
A profile consists of classes that enable developers to implement features found on a related group of small computing devices. J2ME Configurations: There are two configurations for J2ME. Connected Limited Device Configuration (CLDC)
The CLDC is designed for 16-bit or 32-bit small computing devices with limited amounts of memory. CLDC devices usually have between 160KB and 512KB of available memory and are battery powered. They also use an inconsistent, small-bandwidth network wireless connection and may not have a user interface. CLDC devices use the KJava Virtual Machine (KVM) implementation, which is a stripped-down version of the JVM. CLDC devices include pagers, personal digital assistants, cell phones, dedicated terminals, and handheld consumer devices with between 128KB and 512KB of memory. Connected Device Configuration (CDC)
CDC devices use a 32-bit architecture, have at least two megabytes of memory available, and implement a complete functional JVM. CDC devices include digital set-top boxes, home appliances, navigation systems, point-of-sale terminals, and smart phones. J2ME Profiles: A profile consists of Java classes that enable implementation of features for either a particular small computing device or for a class of small computing devices. ■Foundation Profile :
The Foundation Profile is used with the CDC configuration and is the core for nearly all other profiles used with the CDC configuration because the Foundation Profile contains core Java classes. ■Game Profile : The Game Profile is also used with the CDC configuration and contains the necessary classes for developing game applications for any small computing device that uses the CDC configuration. ■Mobile Information Device Profile : The Mobile Information Device Profile (MIDP) is used with the CLDC configuration and contains classes that provide local storage, a user interface, and networking capabilities to an application that runs on a mobile computing device such as Palm OS devices. MIDP is used with wireless Java applications. ■PDA Profile : The PDA Profile (PDAP) is used with the CLDC configuration and contains classes that utilize sophisticated resources found on personal digital assistants. These features include better displays and larger memory than similar resources found on MIDP mobile devices. ■Personal Profile : The Personal Profile is used with the CDC configuration and the Foundation Profile and contains classes to implement a complex user interface. The Foundation Profile provides core classes, and the Personal Profiles provide classes to implement a sophisticated user interface, which is a user interface that is capable of displaying multiple windows at a time. ■Personal Basis Profile: The Personal Basis Profile is similar to the Personal Profile in that it is used with the CDC configuration and the Foundation Profile. However, the Personal Basis Profile provides classes to implement a simple user interface, which is a user interface that is capable of displaying one window at a time. ■RMI Profile: The RMI Profile is used with the CDC configuration and the Foundation Profile to provide Remote Method Invocation classes to the core classes contained in the Foundation Profile.
There will likely be many profiles as the proliferation of small computing devices continues. Industry groups within the Java Community Process Program (java.sun.com/about java/community process) define profiles. Each group establishes the standard profile used by small computing devices manufactured by that industry.
A CDC profile is defined by expanding upon core Java classes found in the Foundation Profile with classes specifically targeted to a class of small computing device. These device- specific classes are contained in a new profile that enables developers to create industrial- strength applications for those devices. However, if the Foundation Profile is specific to CDC, not all profiles are expanded upon the core classes found in the Foundation Profile.
Applications can access a small computing device’s software and hardware features only if the necessary classes to do so are contained in
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