Networking Chapter 2: Physical Layer and Media

Chapter 2: Physical Layer and Media

• This chapter discusses the relationship between data created by devices and electromagnetic signals transmitted over a medium.
• It covers digital and analog transmission, conversion of data to signals, efficient bandwidth usage, and characteristics of transmission media (guided and unguided).
• Transmission media, though operating under the physical layer, are controlled by it.

Physical Layer

• The physical layer interacts with the transmission medium and connects network components.
• One of its tasks is creating a signal representing a stream of bits.
• The physical layer also manages the transmission medium, including data flow direction and logical channel allocation for data from different sources.

Analog and Digital Data

• Data can be analog or digital.
• Analog data are continuous and take on continuous values.
• Digital data have discrete states and take on discrete values.

Analog and Digital Signals

• Signals can be analog or digital.
• Analog signals have an infinite number of values in a range.
• Digital signals have only a limited number of values.

Periodic and Nonperiodic Signals

• Signals can be periodic or nonperiodic.
• A periodic signal repeats a pattern over time intervals (called a period). Completing the full pattern is a cycle.
• A nonperiodic signal doesn't repeat a pattern over time.

Periodic Analog Signals

• Periodic analog signals can be simple (e.g., a sine wave) or composite (multiple sine waves).
• Sine waves are fundamental and cannot be further decomposed.
• Composite signals are composed of multiple sine waves.
• Topics covered include sine wave, wavelength, time and frequency domain, composite signals, and bandwidth.

Signal Characteristics

• Peak amplitude is the absolute value of the highest intensity, proportional to the energy carried.
• Peak amplitude is typically measured in volts for electrical signals.
• Time period and frequency are inverse of each other.
• Frequency is the rate of change over time (high frequency = short time span, low frequency = long time span)
• Phase is the position of the waveform relative to time 0.

Wavelength

• Wavelength is a property of any signal.
• It's the distance a signal travels in one period.
• Wavelength ties the period or frequency of a sine wave to the propagation speed of the medium.
• In data communications, wavelength is used to describe light transmission in optical fibers. The speed of light and the frequency of the signal determine the wavelength.

Bandwidth and Signal Frequency

• Bandwidth is the difference between the highest and lowest frequencies in a composite signal.
• It represents the range of frequencies used in a signal.

Transmission Impairment

• Signals weaken (attenuate), change shape (distort), or are mixed with unwanted signals (noise) during transmission.

Attenuation

• Attenuation is signal strength loss over distance. It depends on the medium and increases with frequency.
• To overcome attenuation, amplifiers or repeaters are used to increase signal strength.
• Attenuation is measured in decibels (dB).

Distortion

• Distortion is the change in a signal's shape due to different propagation speeds for different frequency components.
• This results in different phase shifts at the receiver.

Noise

• Noise is unwanted signals interfering with the original signal.
• Types of noise include thermal noise, induced noise, crosstalk, and impulse noise.
• Signal-to-noise ratio (SNR) measures the strength of the signal relative to the noise. A higher SNR indicates better quality.

Digital Signals

• Digital signals can convey information using different voltage levels (e.g., 0V, 5V) to represent bits (1 or 0). Digital signals may have more than two levels (e.g., different voltages for 1 and 0)
• Bit rate is the number of bits sent in one second.
• Bit length is the distance one bit travels on a transmission medium.

Data Rate Limits

• Data rate depends on bandwidth, signal levels, and channel quality (noise).

Capacity of a System

• Bit rate, or the rate at which data is sent, depends on the increase in the signal levels to encode a symbol, and this increases the probablity of an error.

Nyquist Theorem

• Nyquist theorem gives the upper bound for the bit rate of a noiseless transmission system, directly related to the number of bits in a symbol, or signal levels, and the bandwidth.

Shannon's Theorem

• Shannon's theorem provides the capacity of a system given the presence of noise.
• The formula relates capacity (C) to bandwidth (B) and signal-to-noise ratio (SNR).