Round-robin scheduling (Figure 7.151) allocates each task an equal share of the CPU time. In its simplest form, tasks are in a circular queue and when a task’s allocated CPU time expires, the task is put to the end of the queue and the new task is taken from the front of the queue. Round-robin scheduling is not very satisfactory in many real-time applications where each task can have varying amounts of CPU requirements depending upon the complexity of processing required. One variation of the pure round-robin scheduling is to provide priority-based scheduling, where tasks with the same priority levels receive equal amounts of CPU time. It is also possible to allocate different maximum CPU times to each task. An example project is given later on the use of round-robin scheduling.
Shortest remaining time, also known as shortest remaining time first (SRTF), is a scheduling method that is a preemptive version of shortest job next scheduling. In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute. Since the currently executing process is the one with the shortest amount of time remaining by definition, and since that time should only reduce as execution progresses, processes will always run until they complete or a new process is added that requires a smaller amount of time.
Shortest remaining time is advantageous because short processes are handled very quickly. The system also requires very little overhead since it only makes a decision when a process completes or a new process is added, and when a new process is added the algorithm only needs to compare the currently executing process with the new process, ignoring all other processes currently waiting to execute.
Reference:
 Harchol-Balter, Mor; Schroeder, Bianca; Bansal, Nikhil; Agrawal, Mukesh (2003). “Size-Based Scheduling to Improve Web Performance”. ACM Transactions on Computer Systems. 21 (2): 207–233. CiteSeerX 10.1.1.25.1229. doi:10.1145/762483.762486.(22 February 2019).Wikipedia.Retrieved(2019).
Shortest Process Next (SPN) scheduling, also called Shortest Job First (SJF) scheduling, assigns the process estimated to complete fastest to the CPU as soon as CPU time is available.
This method falls prey to the halting problem. The halting problem is unsolvable, but basically states that given a program and a particular input, can we determine if it will ever finish? With a simple program we could try to run it, and if it completes then we know it will finish, but if it doesn’t complete in a given time, perhaps we just didn’t wait long enough.
There is no way to determine which process is going to run in the least amount of time unless you allow all processes to run and record their execution times or take input from the user.
This method also allows for live lock with many short processes locking a longer process out from the CPU.
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.
Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of {\displaystyle 2^{M}}
possible required time shifts.[1][2] This is repeated every T seconds, such that the transmitted bit rate is {\displaystyle M/T}
 bits per second. It is primarily useful for optical communications systems, which tend to have little or no multipath interference.
Pulse width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. Along with MPPT maximum power point tracking, it is one of the primary methods of reducing the output of solar panels to that which can be utilized by a battery.[1] PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching, because they have inertia to react slow. The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.
The rate (or frequency) at which the power supply must switch can vary greatly depending on load and application. For example, switching has to be done several times a minute in an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz) and tens of kHz for a motor drive; and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies. The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle. PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every single period.
Reference:
 Schreiber, Ulrich (2004). “Pulse-Amplitude-Modulation (PAM) Fluorometry and Saturation Pulse Method: An Overview”. Chlorophyll a Fluorescence. Advances in Photosynthesis and Respiration. 19. Dordrecht: Springer Netherlands. pp. 279–319. doi:10.1007/978-1-4020-3218-9_11. ISBN978-1-4020-3217-2. <2019>.Wikipedia.Retrieved<2015>.
Is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. Along with MPPT maximum power point tracking, it is one of the primary methods of reducing the output of solar panels to that which can be utilized by a battery.[1] PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching, because they have inertia to react slow. The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.
The rate (or frequency) at which the power supply must switch can vary greatly depending on load and application. For example, switching has to be done several times a minute in an electric stove; 120 Hz in a lamp dimmer; between a few kilohertz (kHz) and tens of kHz for a motor drive; and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies. The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle. PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
The Marconi antenna is a vertical wire with a length of 1/4 of the working wavelength, feeded through its lower end. It needs a ground plane with very good conductivity. The Marconi radiator is resonant to any odd multiple of a quarter-wave length.
Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. word “diffraction” .
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