Computer Networks Lecture 1 PDF

Systems and Architecture (COMP 1030) Computer Networks Lecture 1 (B) Lecturer : KR. SELVARAJ Computer Communications and Networks  An interconnection of autonomous computers.  An interconnected group of independent computer systems that communicate wit...

Systems and Architecture (COMP 1030) Computer Networks Lecture 1 (B) Lecturer : KR. SELVARAJ Computer Communications and Networks  An interconnection of autonomous computers.  An interconnected group of independent computer systems that communicate with one another for the purpose of sharing hardware and software resources. CCN.2 Computer Networks  Data communications: - deals with the transmission of signals in a reliable and efficient manner.  Networking - deals with the technology and architecture of the communications networks used to interconnect communicating devices. CCN.3 A Communications Model The fundamental purpose of a communications system is the exchange of data between two parties. CCN.4 Data Communications Model CCN.5  Networks  Local Area Network (LAN) - that spans a single building or campus  Metropolitan Area Network (MAN) - that spans a single city  Wide Area Network (WAN) - that spans sites in multiple cities, countries, or continents  Internet CCN.6 Local Area Networks Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet. CCN.7 Metropolitan Area Networks A metropolitan area network based on cable TV. CCN.8 Wide Area Networks WAN that connects three branch offices in Australia CCN.9 Internet Elements - Evolved from ARPANET first operational packet network applied to tactical radio & satellite nets also had a need for interoperability led to standardized TCP/IP protocols CCN.10 Differences between Local and Wide Area Networks  Speed - bit rate for digital & baud rate for analogue  Management  Security  Reliability  Billing  Heterogeneity (and standards) CCN.11 Transmission Media (Communication Channels)  Transmission media may be classified as guided or unguided.  In both cases, communication is in the form of electromagnetic waves.  With guided media, the waves are guided along a physical path. eg. twisted pair, coaxial cable, optical fiber  Unguided media, also called wireless, provide a means for transmitting electromagnetic waves but do not guide them.  Examples are propagation through air, vacuum, and seawater. CCN.12 Guided and unguided media  guided - wire / optical fibre  unguided – wireless  characteristics and quality determined by medium and signal in unguided media - bandwidth produced by the antenna is more important in guided media - medium is more important  key concerns are data rate and distance  the greater the data rate and distance the better. CCN.13 Transmission Terminology  Simplex signals are transmitted in only one direction; one station is transmitter and the other is receiver  eg. Television  Half duplex both stations may transmit, but only one at a time  eg. police radio  Full duplex both stations may transmit simultaneously, and the medium is carrying signals in both directions at the same time.  eg. telephone CCN.14 Signals  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 CCN.15 Analogue & Digital Signals CCN.16 Concepts Related to Channel Capacity  Data rate - rate at which data can be communicated (bps)  Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz)  Noise - average level of noise over the communications path  Error rate - rate at which errors occur Error = transmit 1 and receive 0; transmit 0 and receive 1  Frequency (f) Rate, in cycles per second, or Hertz (Hz) at which the signal repeats  Wavelength (λ) - distance occupied by a single cycle of the signal Or, the distance between two points of corresponding phase of two consecutive cycles CCN.17 Time-Domain Concepts  Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts  Frequency (f) Rate, in cycles per second, or Hertz (Hz) at which the signal repeats  Period (T) - amount of time it takes for one repetition of the signal T = 1/f  Phase (ϕ) - measure of the relative position in time within a single period of a signal  Wavelength (λ) - distance occupied by a single cycle of the signal Or, the distance between two points of corresponding phase of two consecutive cycles CCN.18 Analog and Digital Data Transmission  Wave CCN.19 Relationship between Data Rate and Bandwidth  The greater the bandwidth, the higher the information-carrying capacity  Conclusions Any digital waveform will have infinite bandwidth BUT the transmission system will limit the bandwidth that can be transmitted AND, for any given medium, the greater the bandwidth transmitted, the greater the cost HOWEVER, limiting the bandwidth creates distortions CCN.20 Attenuation of Digital Signals CCN.21 Examples of Analog and Digital Data  Analog Video Audio  Digital Text Integers CCN.22 Analog Signals  A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency  Examples of media: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation  Analog signals can propagate analog and digital data CCN.23 Digital Signals  A sequence of voltage pulses that may be transmitted over a copper wire medium  Generally cheaper than analog signaling  Less susceptible to noise interference  Suffer more from attenuation  Digital signals can propagate analog and digital data CCN.24 Reasons for Choosing Data and Signal Combinations  Digital data, digital signal Equipment for encoding is less expensive than digital- to-analog equipment  Analog data, digital signal Conversion permits use of modern digital transmission and switching equipment  Digital data, analog signal Some transmission media will only propagate analog signals Examples include optical fiber and satellite  Analog data, analog signal Analog data easily converted to analog signal CCN.25 Analog and Digital Signaling of Analog and Digital Data CCN.26 Analog Transmission  Transmit analog signals without regard to content  Attenuation limits length of transmission link  Cascaded amplifiers boost signal’s energy for longer distances but cause distortion Analog data can tolerate distortion Introduces errors in digital data CCN.27 Digital Transmission  Concerned with the content of the signal  Attenuation endangers integrity of data  Digital Signal Repeaters achieve greater distance Repeaters recover the signal and retransmit  Analog signal carrying digital data Retransmission device recovers the digital data from analog signal Generates new, clean analog signal CCN.28 Effect of Noise on Digital Signal CCN.29 Nyquist Bandwidth  For binary signals (two voltage levels) C = 2B  With multilevel signaling C = 2B log M 2 M = number of discrete signal or voltage levels CCN.30 Signal-to-Noise Ratio  Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission  Typically measured at a receiver  Signal-to-noise ratio (SNR, or S/N) signal power ( SNR) dB 10 log10 noise power  A high SNR means a high-quality signal, low number of required intermediate repeaters  SNR sets upper bound on achievable data rate CCN.31 Shannon Capacity Formula  Equation: C B log 2 1  SNR   Represents theoretical maximum that can be achieved  In practice, only much lower rates achieved Formula assumes white noise (thermal noise) Impulse noise is not accounted for Attenuation distortion or delay distortion not accounted for CCN.32 Reference  Chapter – 4 Computer Networks and Internets, Douglas E. Comer, Prentice Hall. 3rd edition. CCN.33