Chapter 2 part 1(lecture)

Concise Version

Physical Layer Fundamentals

Introduction

• The physical layer is the foundation upon which all other network layers are built.

• It deals with the physical connections and the transmission of data over various media like wires, fiber optics, or wireless channels.

• The primary goal is to send digital bits (0s and 1s) using analog signals.

• Digital signals are actually abstractions of analog signals.

Fourier Analysis

• Fourier analysis is a core concept in electrical engineering used for analyzing signals.

• Any signal can be represented as a summation of sinusoids with different frequencies and amplitudes.

• This is a transform that converts a signal from the time domain to the frequency domain.

• $f$ represents the frequency of a sinusoid.

• The x-axis in the frequency domain represents frequency (f) instead of time.

• Digital signals are represented in the frequency domain as multiples of a fundamental frequency (harmonics).

• Information is maintained during the transformation between the time and frequency domains (linear transform).

• Lower frequencies correspond to slow-changing signals, while higher frequencies represent sharp edges or rapid changes in the signal.

• By removing higher-frequency harmonics, the signal becomes smoother.

Bandwidth Limited Signals

Nyquist Theorem

• The Nyquist Theorem provides a limit on the maximum data rate of a channel.

• $Maximum Data Rate = 2 \cdot Bandwidth \cdot log_2(V)$

• Where:

  • $Bandwidth$ is the range of frequencies available for the signal.

  • $V$ is the number of signal levels (how many bits it takes to represent the data).

• Bandwidth is measured in the Fourier domain and represents the range of frequencies the signal occupies.

Shannon's Theorem

• Shannon's Theorem offers another perspective on the maximum data rate, considering the signal-to-noise ratio.

• $Maximum Data Rate = Bandwidth \cdot log_2(1 + \frac{Signal}{Noise})$

• Where:

  • $Bandwidth$ is the bandwidth of the signal.

  • $\frac{Signal}{Noise}$ is the signal-to-noise ratio (the power of the signal divided by the power of the noise).

• A higher signal-to-noise ratio means it is easier to distinguish between 0s and 1s.

• If both the number of signal levels and the signal-to-noise ratio are known, calculate the maximum data rate using both Nyquist's and Shannon's theorems; the smaller of the two values is the limiting data rate.

Physical Media for Transmission

Sending data via mail

• Sending data on a DVD or hard drive through the mail can provide a high bandwidth link, however, the large message delay makes this impractical for most real-time communications.

• Example: Sending a 6.4 terabyte hard drive via mail with a 24-hour delivery time equates to a data rate of approximately 70 gigabits per second. $\frac{6.4 \cdot 10^{12} bits}{24 \cdot 60 \cdot 60 seconds} \approx 70 \cdot 10^9 bits/second$

• The limiting factor is message delay.

Wired Communication

• Twisted Pair Wires:

  • Common in Ethernet and telephone lines.

  • Consist of twisted pairs of wires to reduce crosstalk.

  • Twisting cancels out electromagnetic fields generated by the current flow, minimizing interference.

  • Full-duplex links allow simultaneous transmission in both directions using separate twisted pairs for each direction.

  • Half-duplex links allow communication in only one direction at a time, sharing the same wire for transmission and receiving.

  • Simplex links provide fixed communication in one direction only.

• Coaxial Cable:

  • Used for cable Internet and cable TV.

  • Features a copper core, insulation, and a braided conductor.

  • The braided conductor is grounded, creating a Faraday cage that blocks electromagnetic noise from the outside.

  • Provides higher bandwidth due to the shielding.

• Power Lines:

  • It is possible to send data signals along power lines, however, there is a lot of noise on these systems, which results in low data rates.

Fiber Optics

• Uses light signals (photons) to transmit data.

• A laser or LED source sends light pulses through a thin glass fiber to a photoreceptor on the other end.

• Less signal degradation over long distances compared to wired approaches.

• Typically used by ISPs and increasingly for home internet connections.

• Certain wavelengths (colors) of light are used, depending on the attenuation factors of the glass.

• The limiting factor is often the electrical signals on either end of the fiber optic link.

  • Single Mode Fiber:

    • Narrow core (approximately 10 micrometers).

    • Uses lasers for coherent light transmission.

    • Enables long-distance communication (e.g., 100 kilometers without a repeater).

    • The structure resembles coaxial cable, providing strength and preventing outside interference.

  • Multicore Fiber:

    • Cheaper than single mode fiber.

    • Larger diameter (approximately 50 micrometers).

    • Light bounces around more, resulting in slower transmission speeds.

    • Can use LEDs, which are more affordable than lasers.

• Comparison: Wires vs. Fiber Optics:

  • Distance: Wires are short (hundreds of meters to a kilometer without repeaters), while fiber optics can span tens to a hundred kilometers with repeaters.

  • Bandwidth: Fiber optics offer much higher bandwidth.

  • Cost: Wires are less expensive than fiber optics.

  • Convenience: Wires are more convenient and forgiving to set up and take down.

  • Security: Wires are easier to tap into, while fiber optics are harder to intercept without detection.

Wireless Communication

• Utilizes the electromagnetic spectrum for data transmission.

• Electromagnetic Spectrum:

  • Various frequencies are used, including radio (AM, FM), LAN, 3G, 4G, and 5G networks.

  • Frequencies are categorized as VHF (very high frequency), UHF (ultra-high frequency).

  • Fiber optics operate in the UV range of light.

  • Traditional communications use microwave frequencies.

  • Maritime radio uses very low frequencies for better water penetration.

• US Frequency Allocations:

  • Governed and maintained by the government.

  • ISM (Industrial, Scientific, and Medical) bands are free for public use, such as WiFi and Bluetooth.

  • The FCC regulates power limits in ISM bands to prevent interference.

  • Companies buy or rent portions of the electromagnetic spectrum for specific uses (e.g., cell phone companies).

  • Bandwidth is the most limiting and expensive factor in wireless communication.

• WiFi:

  • Operates in ISM bands.

  • Standards include 802.11b, g, n, ac, and ax, using different frequency ranges (2.4 GHz and 5 GHz).

• Radio Transmission:

  • Direct: Line-of-sight communication over short distances (used in cell phone towers, AM/FM radio).

  • Ionospheric Bounce:

    • High-frequency signals bounce off the ionosphere, enabling communication around the Earth.

    • This method proves the Earth is not flat through long distance radio communication.

• Microwave Transmission:

  • Used for Wi-Fi, 3G, and satellites.

• Multipath Model:

  • Wireless signals bounce off reflectors in the environment, leading to multiple paths and signal fading.

  • This must be accounted for to ensure reliable communication.

• Light Transmission:

  • A laser and photo detector are aligned for direct line-of-sight communication without wires.

  • Can be affected by atmospheric conditions, like heated roofs causing refraction.

Detailed Version

Physical Layer Fundamentals

Introduction

• The physical layer is the foundation upon which all other network layers are built, providing the means to transmit raw data bits over a communication channel.

• It deals with the physical connections, voltage levels, timing, data rates, and the transmission of data over various media such as wires, fiber optics, or wireless channels.

• The primary goal is to reliably send digital bits (0s and 1s) using analog signals across the communication channel.

• Digital signals are actually abstractions of analog signals represented by discrete values, but these are physically transmitted using analog waveforms.

Fourier Analysis

• Fourier analysis is a core concept in electrical engineering and signal processing used for analyzing signals by decomposing them into their constituent frequencies.

• Any complex signal can be represented as a summation of simpler sinusoidal signals with different frequencies, amplitudes, and phases.

• This is a transform that converts a signal from the time domain, where the signal's amplitude is known for all times, to the frequency domain, where the signal's frequency components are identified.

• $f$ represents the frequency of a sinusoid, indicating how many cycles occur per unit of time (usually seconds), measured in Hertz (Hz).

• The x-axis in the frequency domain represents frequency (f) instead of time, showing the amplitude of each frequency component present in the signal.

• Digital signals are represented in the frequency domain as a series of discrete frequencies, which are multiples of a fundamental frequency (harmonics).

• Information is maintained during the transformation between the time and frequency domains, indicating that this is a linear transform and no information is lost.

• Lower frequencies correspond to slow-changing signals, representing gradual variations in amplitude over time, while higher frequencies represent sharp edges or rapid changes in the signal.

• By removing higher-frequency harmonics, the signal becomes smoother, as these high-frequency components often contribute to the sharpness or abruptness of the signal.

Bandwidth Limited Signals

Nyquist Theorem

• The Nyquist Theorem, formally the Nyquist-Shannon sampling theorem, provides a limit on the maximum data rate of a channel based on its bandwidth and the number of discrete signal levels.

• $MaximumDataRate=2\cdot Bandwidth\cdot log_2(V)$

• Where:

  • $Bandwidth$ is the range of frequencies available for the signal, measured in Hertz (Hz), which limits the amount of data that can be transmitted.

  • $V$ is the number of signal levels (how many distinct voltage or amplitude levels are used to represent the data), which determines how many bits can be encoded per signal.

• Bandwidth is measured in the Fourier domain and represents the range of frequencies the signal occupies, defining the channel's capacity to carry information.

Shannon's Theorem

• Shannon's Theorem, also known as the Shannon-Hartley theorem, offers another perspective on the maximum data rate, considering the signal-to-noise ratio, which affects the reliability of the communication.

• $MaximumDataRate=Bandwidth\cdot log_2(1+\frac{Signal}{Noise})$

• Where:

  • $Bandwidth$ is the bandwidth of the signal, measured in Hertz (Hz), representing the range of frequencies available for transmission.

  • $\frac{Signal}{Noise}$ is the signal-to-noise ratio (the power of the signal divided by the power of the noise), indicating the quality of the communication channel.

• A higher signal-to-noise ratio means it is easier to distinguish between 0s and 1s, as the signal is much stronger relative to the background noise.

• If both the number of signal levels and the signal-to-noise ratio are known, calculate the maximum data rate using both Nyquist's and Shannon's theorems; the smaller of the two values is the limiting data rate, as both factors constrain the channel's capacity.

Physical Media for Transmission

Sending data via mail

• Sending data on a DVD or hard drive through the mail can provide a high bandwidth link, however, the large message delay makes this impractical for most real-time communications due to the latency involved.

• Example: Sending a 6.4 terabyte hard drive via mail with a 24-hour delivery time equates to a data rate of approximately 70 gigabits per second. $\frac{6.4 \cdot 10^{12} bits}{24 \cdot 60 \cdot 60 seconds} \approx 70 \cdot 10^9 bits/second$

• The limiting factor is message delay caused by the physical transportation time, making it unsuitable for applications requiring low latency.

Wired Communication

• Twisted Pair Wires:

  • Common in Ethernet and telephone lines, widely used for short to medium distance data and voice communication.

  • Consist of twisted pairs of wires to reduce crosstalk, which is interference between the signals on adjacent wires.

  • Twisting cancels out electromagnetic fields generated by the current flow, minimizing interference and improving signal quality.

  • Full-duplex links allow simultaneous transmission in both directions, using separate twisted pairs for each direction to send and receive data concurrently.

  • Half-duplex links allow communication in only one direction at a time, sharing the same wire for transmission and receiving, requiring coordination to avoid collisions.

  • Simplex links provide fixed communication in one direction only, such as unidirectional sensors or broadcast systems.

• Coaxial Cable:

  • Used for cable Internet and cable TV, designed to transmit high-frequency signals with minimal loss.

  • Features a copper core, insulation, and a braided conductor, providing better shielding and higher bandwidth compared to twisted pair wires.

  • The braided conductor is grounded, creating a Faraday cage that blocks electromagnetic noise from the outside, reducing interference and signal degradation.

  • Provides higher bandwidth due to the shielding, allowing for faster data transmission rates.

• Power Lines:

  • It is possible to send data signals along power lines, however, there is a lot of noise on these systems, which results in low data rates and unreliable communication.

Fiber Optics

• Uses light signals (photons) to transmit data, providing high bandwidth and low signal attenuation.

• A laser or LED source sends light pulses through a thin glass fiber to a photoreceptor on the other end, converting the light back into electrical signals.

• Less signal degradation over long distances compared to wired approaches, making it suitable for long-haul communication.

• Typically used by ISPs and increasingly for home internet connections, offering faster and more reliable internet access.

• Certain wavelengths (colors) of light are used, depending on the attenuation factors of the glass, optimizing transmission efficiency.

• The limiting factor is often the electrical signals on either end of the fiber optic link, which can introduce bottlenecks.

  • Single Mode Fiber:

    • Narrow core (approximately 10 micrometers), allowing only one mode of light to propagate, reducing modal dispersion.

    • Uses lasers for coherent light transmission, ensuring high precision and minimal signal distortion.

    • Enables long-distance communication (e.g., 100 kilometers without a repeater), ideal for long-haul networks.

    • The structure resembles coaxial cable, providing strength and preventing outside interference, enhancing durability and signal integrity.

  • Multicore Fiber:

    • Cheaper than single mode fiber, making it more cost-effective for shorter distances.

    • Larger diameter (approximately 50 micrometers), allowing multiple modes of light to propagate, leading to modal dispersion and lower bandwidth.

    • Light bounces around more, resulting in slower transmission speeds, limiting its use in long-distance applications.

    • Can use LEDs, which are more affordable than lasers, reducing overall system costs.

• Comparison: Wires vs. Fiber Optics:

  • Distance: Wires are short (hundreds of meters to a kilometer without repeaters), while fiber optics can span tens to a hundred kilometers with repeaters, making fiber optics ideal for long-distance communication.

  • Bandwidth: Fiber optics offer much higher bandwidth, supporting faster data rates and more capacity than wires.

  • Cost: Wires are less expensive than fiber optics, making them suitable for short-distance, lower-bandwidth applications.

  • Convenience: Wires are more convenient and forgiving to set up and take down, while fiber optics require more careful handling and precise connections.

  • Security: Wires are easier to tap into, while fiber optics are harder to intercept without detection, providing better data security.

Wireless Communication

• Utilizes the electromagnetic spectrum for data transmission, enabling communication without physical cables.

• Electromagnetic Spectrum:

  • Various frequencies are used, including radio (AM, FM), LAN, 3G, 4G, and 5G networks, each with different characteristics and applications.

  • Frequencies are categorized as VHF (very high frequency), UHF (ultra-high frequency), and SHF (super-high frequency), each suitable for different communication needs.

  • Fiber optics operate in the UV range of light, offering extremely high bandwidth and low signal loss.

  • Traditional communications use microwave frequencies, which provide a good balance between bandwidth and range.

  • Maritime radio uses very low frequencies for better water penetration, allowing communication with submerged vessels.

• US Frequency Allocations:

  • Governed and maintained by the government to prevent interference and ensure efficient use of the spectrum.

  • ISM (Industrial, Scientific, and Medical) bands are free for public use, such as WiFi and Bluetooth, allowing for unlicensed wireless communication.

  • The FCC regulates power limits in ISM bands to prevent interference and maintain spectrum quality.

  • Companies buy or rent portions of the electromagnetic spectrum for specific uses (e.g., cell phone companies), providing exclusive access to certain frequencies.

  • Bandwidth is the most limiting and expensive factor in wireless communication, as it determines the amount of data that can be transmitted.

• WiFi:

  • Operates in ISM bands, providing wireless internet access in homes, offices, and public spaces.

  • Standards include 802.11b, g, n, ac, and ax, using different frequency ranges (2.4 GHz and 5 GHz), each offering different data rates and ranges.

• Radio Transmission:

  • Direct: Line-of-sight communication over short distances (used in cell phone towers, AM/FM radio), providing clear and reliable transmission.

  • Ionospheric Bounce:

    • High-frequency signals bounce off the ionosphere, enabling communication around the Earth, allowing for long-distance radio communication.

    • This method proves the Earth is not flat through long-distance radio communication, as signals can travel beyond the horizon.

• Microwave Transmission:

  • Used for Wi-Fi, 3G, and satellites, providing high bandwidth and long-distance communication.

• Multipath Model:

  • Wireless signals bounce off reflectors in the environment, leading to multiple paths and signal fading, which can affect signal quality and reliability.

  • This must be accounted for to ensure reliable communication, using techniques such as diversity and equalization.

• Light Transmission:

  • A laser and photo detector are aligned for direct line-of-sight communication without wires, providing high bandwidth and secure communication.

  • Can be affected by atmospheric conditions, like heated roofs causing refraction, which can distort the signal and reduce reliability.