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.

Chapter 5 - Signal Encoding Techniques Part 2

5.2/ Digital Data, Analog Signals
A modem (modulator-demodulator) is used to modulate, encode, digital data into an analog signal and vice versa.

Basic techniques to convert digital data into analog signals:

1. ASK (Amplitude Shift Keying) - represents digital data as different amplitudes in a carrier signal. For a binary data, zero amplitude can represent a binary 0 and another amplitude will represent a binary 1.
2. FSK (Frequency Shift Keying) - represents digital data as different frequencies in a carrier wave. BFSK is binary FSK.
   
3. PSK (Phase Shift Keying) - represents digital data as different phases in a carrier signal. BPSK is binary PSK.
   

5.3/ Analog Data, Digital Signals
digitization - conversion of analog to digital.
A codec (coder-decoder) is a device used to digitize (encode) or decode data.

Basic techniques to convert analog data into digital signals:

1. PCM (Pulse Code Modulation) - the sampling of analog data at a minimum rate (twice the frequency) and the conversion of the samples to a digital (usually binary) code.
2. DM (Delta Modulation) - the differential of the sampling of analog data is converted to a digital code.
   

5.4/Analog Data, Analog Signals

Basic techniques to convert analog data into analog signals:

1. AM (Amplitude Modulation) - involves modification of the amplitude of the transmitted signal to represent data.
   
2. PM (Phase Modulation) - involves using the phase to represent data.
3. FM (Frequency Modulation) - involves modification of the frequency of the transmitted signal to represent data.

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

Chapter 5 - Signal Encoding Techniques

Modulation - process of encoding source data onto a carrier signal in order to transmit data. Modulation techniques require variation to either the amplitude, frequency, or phase.

Carrier Signal - aka carrier wave or carrier, is a continuous and single-frequency signal that is able to be modulated with another data signal to transmit data.


Analog - Continuous. Ex - sinusoid.
Digital - Discrete and discontinuous. Series or sequence. Uses specific constant values, like 1 and 0. Ex - square wave.

4 Possible Encoding Combinations:

1. Digital data to digital signal
2. Analog data to digital signal
3. Digital data to analog signal
4. Analog data to analog signal

5.1/ Digital Data, Digital Signals
Data is the information, usually a binary one or zero, one wants to transmit. Each data element is a bit. Signal is the way that the data is transmitted. Signal is any quantity that can be changed over time or space. In electronics, voltage or current can be a signal.

A digital signal will be a series of discrete voltage fluctuations or pulses. The pulses are known as signal elements or symbols. A digital symbol is a pulse with constant amplitude. An analog symbol is a pulse with constant frequency, phase, and amplitude. Data can be transmitted by encoding data bits into symbols.

Unipolar - A signal is unipolar when all signal elements are either all positive or all negative.
Polar - Positive and negative values are both used in signal.

Bit - Binary data; zero or one.
Data rate - aka data signaling rate or bit rate, is the amount of bits that are transmitted per second. (Unit is bits/second)
The inverse (1/data rate) of the data rate is the amount of time it takes for one bit to be transmitted.

Modulation rate - the rate at which a carrier signal changes. (Units - baud or symbols/second)

Digital Signal Encoding Formats

1. NRZ-L (Nonreturn to Zero-Level)
   • 0 = high, 1 = low


2. NRZI (Nonreturn to Zero Inverted)
   • 0 = no transition at beginning of bit, 1 = transition at beginning of bit


3. Bipolar AMI (no net DC component)
   • 0 = no signal, 1 = positive or negative, with alternation for successive bits


4. Pseudoternary (opposite of Bipolar AMI)
   • 0 = positive or negative, with alternation for successive bits , 1 = no signal


5. Manchester
   • 0 = high to low transition in middle of bit, 1 = low to high transition in middle of bit


6. Differential Manchester (A transition will always occur in middle of bit)
   • 0 = transition at beginning of bit, 1 = no transition at beginning of bit

Factors that affect the receiving end of the signal:

1. SNR (signal-to-noise ratio) - Inversely related to bit error rate.
   
2. Data Rate - Directly related to BER (bit error rate).
3. Bandwidth - Directly related to data rate.

Bit error rate - aka bit error ratio, is the ratio of incorrect bits to the total number of bits.

Methods to compare encoding techniques:

1. Signal spectrum
   • less high-frequency components
   • no DC (direct-current) components
   • transmitted power mostly in middle of bandwidth instead of ends
     


2. Clocking
   • connection synchronization (determining beginning and end of bits)


3. Error detection
   • speed


4. Signal interference and noise immunity
   • expressed in BER


5. Cost and complexity
   • low cost for high availability

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