Sine Waves

• CMPS 447 explores Computer Networks.
• Dr. May Itani presents the topic of The Physical Layer for Spring 2024/2025.

Objectives

• Understand the theoretical basis for data communication.
• Study guided and wireless transmission methods.
• Learn about digital modulation and multiplexing, where modulation changes the signal from analog to digital, and multiplexing shares a link to send multiple signals.

The Physical Layer

• It is the first and lowest layer (layer 1).
• It is closely tied to the physical connection between devices.
• This layer provides the electrical, mechanical, and procedural interface to the transmission medium.
• Serves as the foundation on which other layers are built.
• Considers properties of wires, fiber, and wireless connections.
• A key problem is sending digital bits using analog signals, a process called modulation.
• The physical layer takes packets/frames from the Data Link Layer.
• Puts header (control information) related to modulation and multiplexing.
• Transmits the signal through a channel.
• At the receiver end, extracts the signal.
• Provides a physical layer view.
• Removes the header.
• Passes data to the next layer.

Analog and Digital Data

• Data can be analog (continuous values) or digital (discrete states).
• Analog signals can have an infinite number of values within a range.
• Digital data takes on discrete values (either finite or distinct).
• Digital signals have a limited number of values.
• Data communications utilizes periodic analog signals and nonperiodic digital signals.
• Periodicity refers to how many times a signal repeats.

Periodic Analog Signals

• Classified as simple (sine wave) or composite.
• A simple periodic analog signal (sine wave) cannot be broken down into simpler signals.
• A composite signal consists of multiple sine waves.
• Composite signals can be decomposed using Fourier Series.
• Sine waves are defined by peak amplitude, frequency, and phase.

Signal Amplitude

• Peak amplitude is the absolute value of a signal's highest intensity and relates to the energy it carries.
• Amplitude, measured in volts, relates to voltage or power.

Frequency

• Frequency is the rate of change with respect to time.
• Measured in Hertz (Hz).
• Frequency relates to time or bandwidth.
• High frequency means change in a short span of time.
• Low frequency means change over a long span of time.
• Zero frequency means no change at all.
• Infinite frequency means immediate change.
• As frequency increases, information increases too.
• Frequency and period are inverse to each other.

Frequency and Period Units

• Seconds (s) = 1 s
• Milliseconds (ms) = 10^-3 s
• Microseconds (µs) = 10^-6 s
• Nanoseconds (ns) = 10^-9 s
• Picoseconds (ps) = 10^-12 s
• Hertz (Hz) = 1 Hz
• Kilohertz (kHz) = 10^3 Hz
• Megahertz (MHz) = 10^6 Hz
• Gigahertz (GHz) = 10^9 Hz
• Terahertz (THz) = 10^12 Hz

Phase

• Phase describes the position of a waveform relative to time zero-point.
• Indicates the status of the first cycle.
• Measured in degrees or radians.

Wavelength

• Wavelength relates the period or frequency of a sine wave to the propagation speed in a medium.
• Wavelength = Propagation speed (speed of light) x Period.
• Wavelength = Propagation speed / Frequency.
• The speed of light is approximately 3 x 10^8 m/s.

Time-Domain and Frequency-Domain

• A complete sine wave can be represented by one single spike in the frequency domain.
• The frequency domain is compact and useful when dealing with more than one sine wave.
• A single-frequency sine wave is not useful in data communication.
• Communications requires composite signal is created from many simple sine waves.

Fourier Analysis

• Information transmits on wires occurs by varying a physical property like voltage or current.
• Fourier Analysis uses the fundamental frequency, and sine and cosine amplitudes of nth harmonics to represent signals; a constant (c).
• A time-varying signal is represented as frequency component series (harmonics).

Bandwidth Definitions

• The range of frequencies in a composite signal is its bandwidth.
• Bandwidth is a difference between two numbers; i.e., the range 1000 - 5000 frequency has a bandwidth of 4000.
• Bandwidth relates to a transmission medium's physical property such as construction, thickness, length.
• Bandwidth is the capacity of the channel.

Bandwidth Example

• A periodic signal decomposed into five sine waves (100, 300, 500, 700, 900 Hz) has a bandwidth calculated by:
• Bandwidth = highest frequency - lowest frequency = 900 Hz – 100 Hz = 800 Hz.
• Less bandwidth has more signal degradation.

Maximum Data Rate

• Data Rate is related to a digital signal. Uses bits per second.
• Nyquist's theorem relates data rate to bandwidth (B) and signal discrete Level (V).
• Max. data rate = 2B log2V bits/sec.
• For two level digital signal (0 and 1), V = 2.
• Max. data rate = 2B bits/sec.

Guided Transmission

• This involves data transfer over physical media (wires and fiber).
• Guided transmission media (wired media) are physical channels connect devices in a network.
• Types include twisted pair cable, coaxial cable, and fiber optic cables.

Wires Using Twisted Pair Cable

• Insulated copper wires twisted together in pairs reduces electromagnetic interference.
• It is physical media made of cable pairs twisted with each other.
• Twisted pair cable is inexpensive.
• Installation is easy, and cable is lightweight.
• Frequency ranges from 0 - 3.5KHz.
• increased twisting decreases noise
• More twisted pairs enable greater bandwidth.
• Connects via RJ45 connector to a LAN.

Wires Using Twisted Pair Varieties

• UTP (Unshielded Twisted Pair) is a copper cable used for networking.
• Wires consist of insulated pairs wires twisted to reduce interference and crosstalk.
• UTP commonly used in Ethernet networks for transmitting data.
• STP (Shielded Twisted Pair) cables have an extra shiedling to reduce electronic interference for higher cost; heavier and harder to install.

Twisted Pair Variations

• LAN standards use twisted pairs differently.
• 100-Mbps Ethernet uses two out of four.
• Higher speeds with 1-Gbps utilizes all four pairs in both directions.
• Twisted pairs transmit analog (voice) or digital information.
• Bandwidth relates to thickness of the wire and distance.
• Several megabits/sec achieved for a few kilometers.
• Category 5: 100 Mbps, 1 Gbps Ethernet.
• Category 6: 10 Gbps.

Link Terminology

• Full-duplex link: Transmission in both directions at once, like a two-lane road; i.e., use different pairs for each direction.
• Half-duplex link: Transmission in both directions, but not at the same time like a one-track railroad line; i.e., senders take turns on a wireless channel
• Simplex link: Transmission in one direction at all times like a one-way street; also uncommon.

Coaxial Cable "Co-ax"

• Better shielding and more bandwidth for longer distances and higher rates versus twisted pair.
• It is low susceptibility to interference
• Cost-effective.
• Easy to install and has a long operating life with fast, data transmission.
• Contains two conductors parallel to each other.
• Higher frequency frequency.

Coaxial Cable Advantages

• The data can be transmitted at high speed.
• Better shielding as compared to twisted pair cable.
• Delivers higher bandwidth.

Coaxial Cable Disadvantages

• More expensive than twisted pair cable.
• Faults result in the failure in the entire network.

Fiber Optic Cables (1)

• Uses high rates and covers long distances.
• An optical transmission system has three key components: a light source, transmission medium, and detector.
• Conventionally, a light pulse indicates a 1 bit.
• Absence of light indicates a 0 bit.
• Utilizes strands of pure glass/plastic.
• Fiber cables have special connectors.

Fiber Optic Cables (2)

• Single-mode cables have a core narrow enough (10μm) making light almost straight.
• These cables need lasers to transmit over long distances up to 100km.
• Multi-mode cables have a wider core (50um) in which light bounces.
• These need LEDs and covers shorter distances cheaper.

Fiber Optic Differences

• Single-mode is harder to terminate verses multimode due to smaller core size.
• Single-mode needs lower cable cost and higher electronic cost compared to multimode.
• Multimode uses short distance cable runs.
• Single-mode has is for long distancing with higher bandwidth support.
• Cables have special connectors.

Comparison of Wires vs Fiber

• Wires have a reach of short distances compared to long distnances with Fiber
• Wires has moderate bandwith while fiber very high.
• Wires are more expensive vs fiber.
• Wires ease of use is higher than fiber.
• Fiber security is that taping occurs less than with Wires.

Wireless Transmission

• Electromagentic and light spectrums are used.
• Also uses radio, and microwave transmission.
• All of that provides wireless transfer compared to a wired setup.

Electromagnetic Spectrum (1)

• Electromagnetic radiation types listed by wavelength: Radio, microwave, infrared, visible, ultraviolet, X-rays, and gamma rays.

Electromagnetic Spectrum (2)

• Spectrum is divided, regulated, licensed because it must regulate interface.
• Spectrum management is sold as auction
• WiFi (ISM) bands used, NTIA Office of Spectrum Management, 2003.
• Frequency allocations are a part of US law.

Electromagnetic Spectrum (3)

• Some bands are unlicensed, or "ISM" (industrial, scientific, medical).
• Free for use with low power devices.
• Used for networking: WiFi, Bluetooth, Zigbee.

Radio Transmission

• Radio signals penetrate buildings well and can travel long distances but encounter path loss.
• VLF, LF, and MF bands: Radio waves follow the Earth's curvature.
• HF Band: radio bounces off the ionosphere.

Microwave Transmission

• Microwaves have a lot of bandwidth
• Commonly used indoors with WiFi or outdoors with Satellites
• Signals can be reflected or attenuated by objects.
• High frequency means more information with mobility causes multipath fading.
• Frequencies reside at 300 MHz and 300 GHz
• Wavelength from about one millimeter to one meter.

Bands

• VLF 3-30 kHz uses ground propagation for long range.
• LF 30-300 kHz uses ground for radio beacons.
• MF 300kHz-3 MHz uses Sky for radio.
• HF 3-30 MHz uses Sky for communication.
• VHF 30-300 MHz uses both Sky and line-of-sght for TV.
• UHF 300 MHz-3 GHz uses Line-of-sight for TV and phones.
• SHF 3-30 GHz uses Line-of-sight for satellites.
• EHF 30-300 GHz uses Line-of-sight for radar.

Wireless vs. Wires/Fiber

• Wireless is Easy/inexpensive to deploy that it has a natural support as a broadcast.
• But transmissions intefere it is important it needs to be manage through guard band.
• Fiber is engineers for point-to-point links that has easy support but can be expensive that doesn’t readily maintain braodcast.