CMPS 447 Computer Networks: The Physical Layer - Spring 2024/2025 PDF

Computer Networks Spring 2024/2025 CMPS 447 Dr. May Itani The Physical Layer  Objectives Theoretical Basis for Data Communication Guided Transmission Wireless Transmission (Un-Guided) Modulation...

Computer Networks Spring 2024/2025 CMPS 447 Dr. May Itani The Physical Layer  Objectives Theoretical Basis for Data Communication Guided Transmission Wireless Transmission (Un-Guided) Modulation: Change of the signal from Analog to Digital and vice versa Digital Modulation and Multiplexing Multiplexing: Share the same link (path) to send multiple signals 2  The Physical Layer The physical layer or layer 1 is the first and lowest layer. Application It is closely associated with the physical connection between devices. Transport Network The physical layer provides an electrical, mechanical, and procedural Link interface to the transmission medium. Physical Foundation on which other layers Properties of wires, fiber, wireless build Key problem is to send (digital) bits This is called modulation using only (analog) signals 3  Physical Layer Sender Receiver The Physical Layer takes The signal goes to the receiver the packet or frame from the which has a physical layer view Data Link Layer Removes the header Put its header (control information) The signal passes through a channel on it which has to do with Sends it to the next layer modulation and multiplexing. 4  Analog and Digital Data can be analog or digital ❑ Analog data refers to information that has continuous values ❑ Analog signals can have an infinite number of values in a range ❑Digital data refers to information that has discrete states ❑ Digital data take on discrete values (finite or distinct) ❑ Digital signals can have only a limited number of values In data communications, we commonly use periodic analog signals and nonperiodic digital signals. ❑ Periodic: How many times the signal is being repeated. 5 Comparison of analog and digital signals Nonperiodic digital signal 6  PERIODIC ANALOG SIGNALS Periodic analog signals can be classified as simple or composite. ➡ A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. ➡ A composite periodic analog signal is composed of multiple sine waves. ➡ It can be decomposed using Fourier Series to multiple sign waves composite periodic analog signal simple periodic analog signal 7 PERIODIC ANALOG SIGNALS A sine wave can be represented by three parameters: Peak Amplitude Frequency Phase  Signal Amplitude The peak amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. Amplitude (volt): Related to voltage or power 9  Frequency Frequency is the rate of change with respect to time. It is measured in HZ Time: Related to Frequency or Bandwidth Change in a short Change over a long If a signal does not If a signal changes span of time means span of time means change at all, its immediately, its high frequency. low frequency. frequency is zero frequency is infinite. As Frequency increases, the amount of information carried in this signal increases too. 10  Frequency and Period Frequency and period are the inverse of each other. Units of period and frequency 11 Two signals with the same amplitude, but different frequencies 12  Examples The power we use at home has a frequency of 60 Hz. What is the period of this sine wave ? The period of a signal is 100 ms. What is its frequency in kilohertz? 13  Phase Phase describes the position of the waveform relative to time 0 If we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift. It indicates the status of the first cycle. Phase is measured in degrees or radians Three sine waves with the same amplitude and frequency, but different phases 14  Wavelength and period Wavelength is another characteristic of a signal traveling through a transmission medium. Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of the medium Wavelength = Propagation speed (the speed of light) x Period = Propagation speed / Frequency The speed of light: 3 x 108 m/s 15 Time-domain and frequency-domain plots of a sine wave A complete sine wave in the time domain can be represented by one single spike in the frequency domain. 16  Frequency-domain The frequency domain is more compact and useful when we are dealing with more than one sine wave. A single-frequency sine wave is not useful in data communication o We need to send a composite signal, a signal made of many simple sine waves. 17  Fourier analysis Information can be transmitted on wires by varying some physical property such as voltage or current. Frequency Periodic Component Function Fourier Analysis, where f = 1/T is the fundamental frequency, an and bn are the sine and cosine amplitudes of the nth harmonics (terms), and c is a constant. A time-varying signal can be equivalently represented as a series of frequency components (harmonics): = 18 A composite periodic signal Decomposition of the composite periodic signal in the time and frequency domains Frequency 19  Bandwidth Definition The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is 5000 - 1000, or 4000. The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium. The bandwidth is the capacity of the channel 20  Bandwidth Example If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V. Solution Let fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then: The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz:  Bandwidth example Having less bandwidth (harmonics) degrades the signal An example: the transmission of the ASCII character “b” encoded in an 8-bit byte. The bit pattern that is to be transmitted is 01100010. 22  Maximum Data Rate of a Channel Data Rate is related to digital signal and is represented by bits per second Nyquist’s theorem relates the data rate to the bandwidth (B) and number of signal discrete levels (V): Max. data rate = 2B log2V bits/sec In case of two digital signal with two level (0 and 1), V =2 Max. data rate = 2B log22 bits/sec = 2B bits/sec 23 Guided Transmission (Wires & Fiber) 24  Guided Transmission (Wires & Fiber) Communication is an essential component of computer networks It depends on a variety of transmission methods to enable data exchange. Guided transmission media (wired media) are the actual channels that direct signals between connected devices in a network. Types of Guided Transmission Media Wires: 1. Twisted Pair Cable 2. Coaxial Cable 3. Fiber cables  Wires – Twisted Pair Cable To reduce electromagnetic interference, insulated copper wires are twisted together in pairs to create twisted pair cables. Twisted pair is a physical media made up of a pair of cables twisted with each other. A twisted pair cable is cheap as compared to other transmission media. Installation of the twisted pair cable is easy, and it is a lightweight cable. The frequency range for twisted pair cable is from 0 to 3.5KHz. As the twisting between wires increases, the noise in the signal decreases. More twisted pairs mean more bandwidth Category 5 UTP (Unshielded RJ45: connects a Twisted Pair) cable with four twisted 26 pairs computer to a LAN  Wires – Twisted Pair Cable UTP cable is a type of copper cable STP (Shielded Twisted Pair) cables widely used for networking purposes. have an extra layer of shielding compared to UTP (Unshielded UTP cables consist of pairs of insulated Twisted Pair) cables. wires that are twisted together to reduce interference and crosstalk. This shielding protects against electromagnetic interference but They are commonly used in Ethernet makes STP cables more expensive, networks for transmitting data signals. heavier, and harder to install. 27  Wires – Twisted Pair Different LAN standards may use the twisted pairs differently. For example, 100-Mbps Ethernet uses two (out of the four) pairs, one pair for each direction. To reach higher speeds, 1-Gbps Ethernet uses all four pairs in both directions simultaneously. Twisted pairs can be used for transmitting either analog (voice) or digital information. The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases. Category 5: 100 Mbps, 1 Gpbs Ethernet Category 6: 10Gbps 28  Link Terminology Full-duplex link ๏ Used for transmission in both directions at once, like a two-lane road ๏ e.g., use different twisted pairs for each direction Half-duplex link ๏ Both directions, but not at the same time, like a single-track railroad line ๏ e.g., senders take turns on a wireless channel Simplex link ๏ Only one fixed direction at all times, like a one-way street; not common 29  Wires – Coaxial Cable (“Co-ax”) Also common. Better shielding and more bandwidth for longer distances and higher rates than twisted pair. Low susceptibility to interference coaxial since it contains two Extremely cost-effective conductors parallel to each other. Easy to install Fast and efficient data transmissions It has a higher frequency as compared Long operating life to Twisted pair cable. 30  Wires – Coaxial Cable (“Co-ax”) Advantages Of Coaxial Disadvantages Of Coaxial cable: cable: The data can be It is more expensive as transmitted at high speed compared to twisted pair cable. It has better shielding as compared to twisted pair If any fault occurs in the cable cable causes the failure in the entire network. It provides higher bandwidth 31  Fiber Cables (1) Common for high rates and long distances An optical transmission system has three key components: ๏ the light source ๏ the transmission medium ๏ the detector Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. Light source Light trapped by Photodetector (LED, laser) total internal reflection 32  Fiber Cables (2) Single-mode ๏ Core so narrow (10um) light can’t even bounce around ๏ Used with lasers for long distances, e.g., 100km Multi-mode ๏ Other main type of fiber ๏ Light can bounce (50um core) ๏ Used with LEDs for cheaper, shorter distance links Fibers in a cable 34 Differences Comparison of the properties of wires and fiber Property Wires Fiber Distance Short (100s of m) Long (tens of km) Bandwidth Moderate Very High Cost Inexpensive Less cheap Convenience Easy to use Less easy Security Easy to tap Hard to tap 36  Wireless Transmission ELECTROMAGNETIC RADIO MICROWAVE SPECTRUM TRANSMISSION TRANSMISSION LIGHT WIRELESS VS. TRANSMISSION WIRES/FIBER 37  Electromagnetic Spectrum (1) Electromagnetic radiation is classified by wavelength according to frequency into radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays. Microwave 38  Electromagnetic Spectrum (2) To manage interference, spectrum is carefully divided, and its use regulated and licensed, e.g., sold at auction. 300 MHz 3 GHz WiFi (ISM bands) 3 GHz Source: NTIA Office of Spectrum Management, 2003 30 GHz Part of the US frequency allocations 39  Electromagnetic Spectrum (3) Fortunately, there are also unlicensed (“ISM: industrial, scientific and medical”) bands: ๏ Free for use at low power; devices manage interference ๏ Widely used for networking; WiFi, Bluetooth, Zigbee, etc. 802.11 802.11a/g/n b/g/n 40  Radio Transmission Radio signals penetrate buildings well and propagate for long distances with path loss Layer of the earth's atmosphere which contains a high concentration of ions and free electrons and reflects radio waves. It extends from about 80 to 1,000 km above the earth's surface. In the VLF, LF, and MF bands, radio waves follow the curvature of the earth More Power = More Distance More Frequency = More Data In the HF band, radio waves bounce off the ionosphere. 41  Microwave Transmission Microwaves have much bandwidth Microwave is a form of and are widely used indoors (WiFi) electromagnetic radiation with and outdoors (3G, satellites) wavelengths shorter than ๏ Signal is attenuated/reflected by other radio waves but longer everyday objects than infrared waves. ๏ Strength varies with mobility due to Its wavelength ranges from multipath fading, etc. about one meter to one millimeter, corresponding to frequencies between 300 MHz ๏ It has high frequency and thus can send more information and 300 GHz 42 Bands Half duplex 43  Wireless vs. Wires/Fiber Wireless: + Easy and inexpensive to deploy + Naturally supports mobility + Naturally supports broadcast − Transmissions interfere and must be managed using guard band − Signal strengths hence data rates vary greatly Wires/Fiber: + Easy to engineer a fixed data rate over point-to-point links − Can be expensive to deploy, esp. over distances − Doesn’t readily support mobility or broadcast 44