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.

C Programming Part 2: Data Types, Variables, and Constants

How are data stored in C?

• Data are stored as variables or constants.
• Variables are just memory locations that hold specified values/data.
  
• Variables and constants are assigned a data type during creation.
• Data types limit the possible values the variable/constant can have.
• There are data type modifiers which will further limit or increase the range of values.
• The modifiers are short, long, signed, and unsigned. They are placed before the data type.
• Examples: short int, unsigned char, long float.
• Some data type modifiers can also be used as data types. They just act like int.

C Data Types

Type	Description	Bits	Range
char	characters	8	-128 to 127
unsigned char	small unsigned int	8	0 to 255
int	integers	32	-2,147,483,648 to +2,147,483,647
unsigned int	unsigned integers	32	0 to 4,294,467,295
short	small integers	16	-32,768 to 32,767
unsigned short	unsigned small integers	16	0 to 65,535
long	integers	32	-2,147,483,648 to +2,147,483,647
unsigned long	unsigned integers	32	0 to 4,294,467,295
float	floating point #s	32
double	floating point #s (greater precision)	64
long double	floating point #s (even greater precision)	96

Variables

• The format of declaring a variable is: data_type variable_name; (i.e. int one;)
  
• The first character of a variable name MUST be a letter but the rest can be a letters, numbers or underscores(_) .
• Variables are case sensitive.
• The maximum length of the name will depend on the compiler. Although, there is a C standard that states that the initial 63 characters are significant for an internal identifier (name used for variables, types, or labels). The standard for an external identifier is that the first 31 characters are significant.
• It is a good practice to initialize (assign it a base value) a variable when you first declare it. It is known as defining a variable when you do both the initialization and declaration. The main small difference between the two is that defining it means that memory space is taken up. In a declaration, no memory is used and only the data type is mentioned.
  
• Ex. Declaration: int one; | Definition: int one = 1;
• Variables are considered local, global, or external depending on where they are defined and used. This will be discussed more in a later section.
• It is possible to declare all variables on one line (i.e. int one, two, three;). Generally, it is a better practice to declare or define them on separate lines.
  

Constants

• Constants are data that will not be modified.
• Constants are generally defined near the very top of the code and its name is all capitalized.
• They do not take up any variable storage space(memory).
• The format for defining a constant is: #define CONSTANT_NAME constant_value
• Constants can be assigned decimal, octal(put a 0 before the value; i.e. 0742), or hexadecimal(put a 0x before the value; i.e. 0xA2) values.
• To assign a data type(long or float) to a constant, put a L or F after the constant value.
• Ex. #define PI 3.14F

Special Constants

• Another type of constant is an enumeration constant. It is created in this format: enum enumeration_type {constant_name1, constant_name2,...}
• The first constant in a enumeration will be given the value 0 unless otherwise specified. The second constant will have the value of the previous constant plus one. Thus the second constant will be given the value 1 unless it or the constant before was assigned a specific value. This procedure continues on for the rest of the constants in the enumeration.
  
• Ex. enum boolean {FALSE, TRUE, MAYBE=5, NO}; FALSE will have a value of 0 and TRUE will have a value of 1. MAYBE will have a value of 5 and NO will have a value of 6.
  
• Constants can also be a character(using ''), or a string("").
• Some character constants can act as escape sequences(used to represent action or other characters that can't be represented normally).

Escape Sequences

Sequence	Description
\a	Alert
\b	Backspace
\f	Formfeed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\\	Backslash
\?	Question mark
\'	Single quote
\"	Double quote
\0	Null (automatically at end of string)
\ooo	ASCII char. in octal
\xhh	ASCII char. in hex