PHYSICAL LAYER

PHYSICAL LAYER. COMPUTER NETWORKS.

Chapter 2: roadmap. 2.1. Introduction Packet encapsulation Basic for data communication 2.2. Functionality 2.3. Physical layer communication media 2.3.1. Wired 2.3.2. Wireless 2.4. Line coding 2.5 Multiplexing/De-multiplexing.

2.1 Introduction. Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical 10010111001011010010110101011110101 segments packets frames Data Data Data Data.

[image] Application Presentation Session Transport Network Data Link Physical Source Application Presentation Session Transport Network Data Link Physical.

2.1.2. Basic for data communication. Relevant matters How to convert information into digital data. Types of transmission channels. How to connect communication devices. How to transmit a bit from the sending device to the receiving device..

Analog / Digital Analog signal - signal intensity varies in a smooth fashion over time No breaks or discontinuities in the signal Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level.

Periodic / Aperiodic Periodic signal - analog or digital signal pattern that repeats over time s(t +T ) = s(t ) -∞< t < +∞ where T is the period of the signal (the smallest value that satisfies the equation ) Aperiodic signal - analog or digital signal pattern that doesn't repeat over time.

Wireless and Mobile Networks. 15. Time Domain Concepts.

Wavelength () - distance occupied by a single cycle of the signal Electromagnetic waves travel at the speed of light (c) c= f c: speed of light in free space = 3 x 108 m/s f: signal frequency (Hz) : wave length (m).

In practice, an electromagnetic signal is made up of many frequencies. Fourier Analysis o Any signal is made up (được tạo nên từ) of components at various frequencies, in which each component is a sinusoid (sóng hình sin). Fundamental frequency - when all frequency components of a signal are integer multiples of one (the lowest) frequency, it’s referred to as the fundamental frequency The period of the total signal is equal to the period of the fundamental frequency.

Frequency Domain Concepts. Wireless and Mobile Networks.

Spectrum (phổ) - range of frequencies that a signal contains Absolute Bandwidth - width of the spectrum of a signal = fMAX - fMin Many Signals have an infinite bandwidth but most of the energy is contained in a relatively narrow band of frequencies Effective Bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in. e.g. Voice Signal (20 Hz to 20 kHz) we use only (300Hz to 3.4 KHz) Example Fundamental Freq = Max_Freq = BW =.

Data Rate - rate at which data can be communicated (bps = bits per second) Consider the square wave shown in the figure Suppose that the positive pulse represent binary 0, and the negative pulse represent binary 1. The data rate = 2 x f bits per second (bps) This waveform consists of infinite number of frequency components and hence an infinite bandwidth..

o What is the Data Rate? Data Rate = 4 Mbps Case-III o Assume a signal has the following components: f, 3f; f= 2 MHz.

Data - entities that convey meaning, or information ( người ngồi ) Signals - electric or electromagnetic representations of data ( xe ô tô ) Transmission - communication of data by the propagation and processing of signals Examples of Analog and Digital Data Analog Video Audio Digital Text Integers Both analog and digital data can be represented, and hence propagated, by either analog or digital signals.

Analog and Digital Data Transmission. Wireless and Mobile Networks.

Wireless and Mobile Networks. 24. Decibel Notation.

For power, dBm is used to denote a power level with respect to 1mW as the reference power level. Let’s say that a transmit power of a system is 100W. Question: What is the transmit power in unit of dBm? Answer: transmit power(dBm) = 10log(100W/1mW) = 10log(100,000mW/1mW) = 50dBm ( Khuếch đại bao nhiu lần ) For power, dBW is used to denote a power level with respect to 1W as the reference power level. Let say transmit power of a system is 100W. Question: What is the transmit power in unit of dBW? Answer: transmit _power(dBW) = 10log(100W/1W) = 10log(100) = 20dBW..

e.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second What if the number of signal levels is more than 2 (M levels): C = 2B log2(M) bps e.g. a 3000 Hz channel, with 8 discrete signal elements, can transmit data at a rate of at most 18000 bits/second For a given bandwidth, the capacity can be increased by increasing the number of different signal elements (M) Increasing M, increases receiver sensitivity to noise and other channel impairments..

The presence of noise can corrupt one or more bits. If the data rate is increased, then the bits become "shorter" in time, so that more bits are affected by a given pattern of noise. Thus, at a given noise level, the higher the data rate, the higher the error rate. For a given level of noise, we would expect that a greater signal strength would improve the ability to receive data correctly in the presence of noise.

Represents theoretical maximum channel capacity that can be achieved In practice, only much lower rates can be achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or delay distortion not accounted for We can also use Shannon’s theorem to calculate the noise that can be tolerated to achieve a certain rate through a channel..

Channel Capacity. Wireless and Mobile Networks. 29.

2.2. Functionality of Physical layer. The physical layer involves the transmission of raw data (dữ liệu gốc) bits on the transmission channel. Amplitude, Frequency, Phase, Bandwidth, Spectrum Data rate, throughput, delay (latency) It is required is a transmission mechanism, interface, voltage level, that are compatible with the transmission media below. Wired Wireless.

Transmission media. Wired transmission (Guided) Twisted pairs cáp xoắn đôi Coaxial cần trục Fibre cáp quan.

Twisted-pair cable. A twisted-pair cable consists of two insulated copper wires, typically about 1 mm thick, twisted to avoid crossover talk (interference). It is classified into 2 types: Unshield Twisted Pair (UTP) Shield Twisted Pair (STP).

Twisted-pair cable. Depending on how cables twisted (sparse or dense), the twisted-pair cable can be classified into Cat3, Cat4, Cat5, Cat5e, Cat6, Cat7….

Coaxial cable. A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh. The outer conductor is covered in a protective plastic sheath (Diameter of 1cm to 2.5cm) The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable quality and length. 50 ohm: used for digital signal 75 ohm: used for analog transmission and cable television.

Cable TV system.

Fiber Optics. At the center is the glass core through which the light propagates. In multimode fibers, the core is typically 50 microns in diameter, about the thickness of a human hair. In single-mode fibers, the core is 8 to 10 microns..

Fiber Optics. The core is surrounded by a glass cladding with a lower index of refraction than the core, to keep all the light in the core. Next comes a thin plastic jacket to protect the cladding. Fibers are typically grouped in bundles, protected by an outer sheath..

Light sources. Two kinds of light sources LEDs (Light Emitting Diodes) Semiconductor lasers..

Wireless transmission. When electrons move, they create electromagnetic waves that can propagate through space (predicted by James Clerk Maxwell 1865, first observed by Heinrich Hertz in 1887).

Different Types of Wireless Communication Technologies.

Wireless transmission. EMS.

Licensed Spectrum: Need to buy right to use spectrum allocation in a specific geographic location from the government. Prevents interference – licensee can control signal quality e.g.: GSM Frequency Spectrum. Unlicensed Spectrum Anyone can operate in the spectrum Can have interference problems e.g.: ISM-Band: Industrial, Scientific and Medical frequency band 2.4 GHz e.g. : Wi-Fi uses ISM band.

Wireless transmission. [image] Ground wave Earth's surface (a) Earth's sudace (b).

RF: Wi-Fi (IEEE 802.11). Radio Frequency (RF) 2.4GHz 5.0 GHz.

4G/5G. 1-38. [image] Existing Mobile Spectrum Coverage Capacity Now Mobile Spectrum Below 1 GHz Macro 1-6 GHz w,Fi Macro ard 30 GHz mm Wave band inks and cells too GHZ.

Description of communication satellite Microwave relay station Used to link two or more ground-based microwave transmitter/receivers Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) A single orbiting satellite will operate on a number of frequency bands, called transponder channels, or simply transponders. Applications Television distribution Long-distance telephone transmission Private business networks Optimum Freq. Range: 1 – 10 GHZ.

2.4. Line coding. Digitization data Sources: analog/digital -> digitization Encoding: convert into line codes for transmitting Unipolar, Polar, Bi-polar line codes.

2.4 Why line coding?. There should be self-synchronizing i.e., both receiver and sender clock should be synchronized. It requires clock recovery. – đồng bộ There should be no (low frequency) DC-component as long distance transfer is not feasible for DC-component signal. – không phù hợp khoảng cách lớn There should be bandwidth efficiency since the channel capacity is limited There should have some error-detecting capability. There should be immunity to noise and interference. There should be less complexity. There should be less base line wandering..

2.4. Line codes. Unipolar (line code) NRZ non return zero.

2.4. Line codes (2). Polar RZ NRZ. ‘1’: positive + zero ‘0’: negative + zero.

2.4. Line codes (3). Bipolar AMI. [image] Amplitude AMI Tme.

[image] 1 NRZ-L NRZI Bipolar-AMI (most recent preceding I bit 1 negative v oltage) Pseudoternary (most recent preceding O bit I negative voltage) I Manchester Differential Manchester.

2.5 Multiplexing/De-multiplexing. Why need multiplexing? The medium can only have one signal at a time. There are multiple signals to share one medium. There is a possibility of collision. Transmission services are very expensive => một tín hiệu, đắt The two basic types of multiplexing techniques: Time division multiplexing (TDM) => chia ra nhiều đường Frequency division multiplexing (FDM) In optical information, wavelength division multiplexing (WDM) is also the FDM..

2.5 Multiplexing/De-multiplexing. Multiplexing techniques are used only when the bandwidth of the transmission channel is higher than the bandwidth of data sources. E.g: the signals from three sources can be combined (multiplex) and sent over a single channel. At the receiving end, the combined signals are separated into 3 separated original signals..

Frequency Division Multiplexing. In the FDM, the signals are shifted into different frequency ranges and sent through the media ( mỗi tần số chiếm một khoảng khác nhau ) . Communication channels are divided into different bands, and each signal transmission band corresponds to one source..

Time Division Multiplexing. Trong TDM, các tín hiệu số hóa được kết hợp và gửi thông qua các kênh truyền thông. ..

Summary. Basic knowledge of data communication system Digitization forms of information Distinguish and calculate quantities related to the characteristics of a transmission channel such as bandwidth, frequency, data rate, noise, capacity and throughput of a transmission channel Describe and understand line codes Multiplexing/de-multiplexing techniques.