Chapter 3 - Data Transmission

Data is transmitted through some transmission medium (substance that allows the traveling of waves) between the transmitter and the receiver.

Transmitter - the origin of the signal
Receiver - the destination of the signal

Guided media - physical path for data to be transmitted over. The data are guided to its destination. Example - twisted pair or coaxial cable
Unguided media - wireless. The data are not guided. Example - air or water

Absolute bandwidth is the range of frequencies (max freq. - min. freq) of the signal or communication channel(medium). Since absolute bandwidth of many signals tends to be infinite, relative bandwidth is used most of the time. Relative bandwidth or effective bandwidth is the amount of bandwidth required to represent the majority of the energy in the signal. When bandwidth is mentioned, it is usually meant as the relative bandwidth. The unit for bandwidth is Hertz (Hz).

Two forms of transmission: Analog vs Digital Transmission
Analog transmission is a way of transmitting any data, analog or digital, through an analog signal.
Modem - (modulator demodulator) is used to encode digital data into an analog signal. It is used in analog transmission.
Analog signals will loser its power(attenuation) as it travels farther.

Digital transmission is a way of transmitting data through a digital signal.
Codec - (coder decoder) is used to encode analog data into a digital signal or digital data into analog signal. It is used in digital transmission.
Digital signals will usually be in a sequence of zeros and ones. The digital signal will also be affected by attenuation.


Channel Capacity - maximum rate for data transmission over a specified communication channel. It is the ideal value for the data rate and it is unattainable. Data rate is the amount of data that is transmitted over a certain time. It is measured in bits per seconds(bps).

Shannon Capacity - used when theoretical max data rate of channel is desired.

• C = B log_2(1+SNR)
• C - channel capacity/data rate (bps)
  
• B - bandwidth of signal (Hz)
  
• SNR - Signal to noise ratio. Can be found by dividing noise's power by the signal's power
• Signal and noise must be in the units Watts (W) in this formula. Sometimes, they are given in decibels so they will need to be converted.

Nyquist Bandwidth - used when max data rate of channel is desired or when noise is not a factor and signaling levels are.

• R = 2B log_2(M)
• R - data rate/ channel capacity (bps). Sometimes R is replaced with C.
  
• B - bandwidth of signal (Hz).
• M - amount of signaling levels or symbols. Example: binary signaling will have M = 2 since there will be 2 possible signaling levels, 0 and 1.

The information above is based on the textbook Data and Computer Communications by William Stallings.

Computer Architecture: Measuring Computer Performance

Performance can be measured in many different ways depending on what you're looking for. One primary attribute of computer performance is its execution time. Execution time is the total amount of time needed to perform a task. Lower execution time means better performance so performance and execution time are inversely related.

• Performance = 1/Execution time

When comparing performances between two computers, if A is n times faster than B it means

• n = Performance(A)/Performance(B) = Execution Time(B)/Execution Time(A)
  

The execution time takes into account every time factor that a program encounters, like user inputs and outputs. A more focused way to measure performance is CPU time. CPU time is the amount of time the CPU actually works on the task.

• CPU time = Total instructions * (average clock cycles per instruction) * clock cycle time

Every program generates instructions for the computer to carry out. Each instruction takes a certain amount of time to do. In a computer, time is referred to as clock cycles or period. They are based on the clock of the computer. Clock cycle is inversely related to its clock period. In the above equation, clock cycle time is just the clock period.

Amdahl's Law
When you improve only a part of a computer's performance, you need to use a different equation to calculate the CPU time.

• CPU time_after = [(CPU time_before)/amount of improvement + Unchanged CPU time]

Another way to put this equation is:

• 1 / [(1-P) + (P/S)] where P is the proportion that improved(0 < P < 1) and S is the speedup.

The information above is based on the textbook Computer Organization and Design by David A. Patterson and Hennessy.