Network and communications fundamentals

Transcription

1 Network and communications fundamentals أساسيات اتصاالت وشبكات (Net222) 1/11/1439 AH Lecture 1: Ch1 -Classifications of Signals- What does a Signal mean? a function representing a physical quantity or variable, typically contains information about the behavior or nature of the phenomenon. What does a System mean? viewed as transformation (mapping) of x into y, to process input signals to produce output signals. Types of Signals: - Electrical. e.g: voltage & current. - Electromagnetic. e.g: radio & light. - Sound. e.g: humans sounds & music. Data: entities that convey info Signals: electric or electromagnetic representation of data Signaling: physically propagates along a medium Transmission: communication of data by propagation and processing of signals Types of Signals: - Continuous-Time: Signal x(t) is continuoustime signal if the independent variable t is continuous. it can be defined at every instant of time. it contains values for all real numbers along the X-axis. for e.g: Analog Signals. (intervals : ) data: Audio and video. - Discrete-Time: A signal x(t) is defined at discrete times, it s often identified as a sequence of numbers, denoted by x[n]. for e.g: Digital Singles. (only finite number of values) data: text, numbers and IRA(ASCII). Advantages: cheaper and less susceptible to noise interference. Disadvantages: suffer more from attenuation. - Even Signals: any signal x such that x(t) = x(-t), symmetric around y axis. - Odd signals: any signal x such that x(-t) = - x(t), symmetric around origin. - Periodic Signals: pattern repeated over time. - Aperiodic signals: no pattern repeated. Lecture 2: Ch1 -Classifications of Signals cont.- - Sinusoidal Signals: Sine wave is the fundamental periodic signal. 1

2 + it s the simplest periodic signal.represented by: 1. Peak Amplitude (A) < Highest value in y axis. measured in: volts. 2. Frequency (f) < # of cycle/s. measured in: Hz or cycle/s. period (T)= 1/f. 3. Phase (φ) < relative position in time within a single period of signal. s(t) = A sin(2π f t +Φ) > see e.g in slide 19. Time domain: shows changes in signal amplitude with respect to time. - wave length: distance occupied by a single cycle. - light speed in free space = 3*10^8 m/s 1/11/1439 AH Frequency domain: show the relationship between amplitude and frequency. -more useful- - measured in Hz (cycle/s) - Spectrum: Range of frequencies that a signal spans form minimum to maximum. - Bandwidth: Absolute value of the difference between the lowest and highest frequencies of a signal. > check e.g slide 26. Lecture 2: Ch1 -Transformations of the independent variable (time)- Time Reversal Time Shifting Time Scaling Lecture 2: Ch1 -The Unit impulse and the Unit steps function- Unit step function Unit impulse function - Used in signal processing to represent a signal that switches on at a specified time and stays switched on indefinitely. - It called also Heaviside function. - Used in signal processing as an infinitesimally narrow pulse of unit area centers around 0. - It called also Dirac delta function. 2

3 Lecture 3: Ch1 -Continuous-Time and Discrete-Time Systems- Cont.-time and Dust.-time Systems Cascade(series) Interconnection Parallel Interconnection Series-parallel Interconnetion Feedback Interconnection for more clear pictures of systems > check slide 3-5. System with memory Without memory (memory-less) n y [ n] = x[ k] k = Invertible System A system S is invertible if the input signal can always be uniquely recovered from the output signal. Y(t) = 2x(t) The Inverse System foramally written as, S^-1, such that the cascade interconnection in the figure below is equivalent to the identity system, which leaves the input unchanged. y(t) = 1/2 x(t) Linear System (L): when an input to given system is scaled by a value, the output of the system is scaled by the if the output at anytime depends on only the input at that same time. Otherwise, the system is said to have memory. same amount. - it obeys the principle of superposition. (that means if two inputs are added together and passed through a linear system, the output will be the sum of the individual inputs outputs. 3

4 Time-Invariant System (TI): has the property that a certain input will always give the same output, without regard to when the input was applied to the system. - Because the system is (TI), the input x(t) and x(t-to) produce the same output. The only difference is that the output due to x(t-to) is shifted by a time to. LTI Systems in Series if two or more LTI systems are in series with each other, their order can be interchanged without affecting the overall output of the system. also called cascaded systems. LTI Systems in Parallel if two or more LTI systems are in parallel with one another, an equivalent system is one that is defined as the sum of these individual systems. Linear Time-Invariant System (LTI): It obeys the principle of superposition (if two inputs are added together and passed through a linear system, the output will be the sum of the individual inputs outputs). Lecture 4: Ch3 -Data Transmission- Physical Layer: Foundation on which other layers build. Properties of wires, fiber, wireless limit what the network can do. Modulation: Key problem is to send digital bits using only analog signals. Transmission medium and physical layer: Transmission media: anything that carry info between a source to a destination. Located below the sender physical layer and is connected to the other receiver physical layer. The successful transmission of data depends on: 4

5 1. Quality of the signal being transmitted. 2. Characteristics of the transmission medium. 1/11/1439 AH Types of connections: 1. Point-to-point (direct link between 2 devices) 2. Multipoint (more then 2 devices share the same medium) Direction of data flow: - Simplex: signals transmitted in 1 direction < eg: television - Half duplex: both stations transmit, but only 1 at a time. < e.g.: police radio - Full duplex: simultaneous transmissions. < eg: telephone Time domain and frequency domain: - How to find frequency? Number of cycles per sec Data rate: amount of data that is moved from one place to another in a given time. Bandwidth: -see lecture 2- There is a direct relationship between data rate & bandwidth. Lecture 5: Ch3 -Data Transmission- Transmission Impairments: - Signal received may differ from signal transmitted causing: 1. Analog - degradation of signal quality. 2. Digital - bit errors - Most significant impairments are: 1. Attenuation 2. Delay distortion 3. Noise Attenuation: - Signal strength falls off with distance over transmission medium. - we can use amplifiers/repeaters to increase strength. - Signal strength varies with frequency(attenuation is greater at higher frequencies and this causes distortion. - we can equalize attenuation by using loading coils/amplifiers. Received signals must be: - Strong enough to be detected - Higher than noise to be received without error. - Attenuation: Db = 10 log10 (Pd / Ps) answer = - > loss of data, answer = + > gain of data Thermal noise Intermodulation noise Crosstalk Impulse noise - agitation of electrons - referred to as white noise - can t be eliminated - significant for satellite 5 communication. - produced by nonlinearities in the transmitter and receiver. - signal from one line picked up by another. - caused by external electromagnetic interference (lighting, faults, flaws). - consisting of irregular pulses or spikes of short duration and high amplitude.

6 Delay Distortion occurs because propagation velocity of a signal through a guided medium varies with frequency. Noise: unwanted signals inserted between transmitter and receiver. < major limiting factor. Noise categories: - SNR(Signal-to-Noise-Ratio): SNR = Signal / Noise - SNR(db) = 10 log10 (SNR) Channel Capacity: maximum (largest) rate at which data can be transmitted over a given communications channel under given conditions. Nyquist Bandwidth: C = 2B log2 M - where M = number of discrete signal/voltage levels. Capacity: C = B log2 (1+SNR) Lecture 6.1: Ch3 -Transmission Media- guided media - wire: electromagnetic waves are guided along a solid medium. unguided media - wireless: transmission occurs through the atmosphere. Transmission characteristics of guided media: - just to compare between them numbers are not important> Twisted pair: - decrease cross talk - cheaper - easier to work with unshielded (UTP) - telephone wire - cheapest - easier to install - suffers from external EM interference shielded (STP) - metal braid or seating that reduces interference - more expensive - harder to handle because it s thick and heavy. - Application: - Telephone network - between house and local exchange - within building - local area network (LAN) 6

7 - Types: Near end Crosstalk: occurs when transmit signal entering the link couples back to receiving pair. Optical Fiber: uses reflection to guide light through a channel. a glass or plastic core is surrounded by a cladding of less dense glass or plastic. - used in: 1. Cable TV network 2. Local area networks 3. Backbone networks. Propagation modes using fiber optics: 1. Multimode Fiber: any light ray incident on thou boundary above the critical angle will be reflected internally, many 2. Single-mode Fiber: light can propagate only in a straight line, without bouncing. Benefits: - greater capacity - smaller size & weight - lower attenuation - electromagnetic isolation - greater repeater spacing. - - just to compare between them numbers are not important > Radio wave Microwave Infrared wave - omnidirectional - point-to-point communication - for satellite communication - local point-topoint and multipoint application Lecture 6.2: Ch3 -Transmission Media- Electromagnetic waves are formed when an electric field (shown in blue) couples with a magnetic field (red arrows). - travels at light speed - don t require a medium. Antenna: electrical conductor used to radiate or collect electromagnetic energy. Types: 1. Horn antenna 2. Dish antenna(parabolic reflective antenna) 7

8 Isotropic antenna Omnidirectional Directional antenna - a point in space that radiate power in all directions equally. - doughnut shape - Broadside 1/11/1439 AH - Endfire Types of radiation patterns: Broadcast Radio Microwave Infrared - omnidirectional - Communication need not line of sight - can penetrate walls - less sensitive to rainfall - suffers form multipath interference. - Terrestrial Microwave - Satellite Microwave - Satellite communication configuration: - Point-to-point link - Broadcast link - Infrared communications is achieved using transmitters/receivers that modulate non-coherent infrared light. - Transceivers must be within the line of sight - blocked by walls - no licenses required - typical uses: TV remote control, IRD port. Antenna Gain: measure of directionality of an antenna. G = 4(Pi) Um / PT Um : maximum radiation intensity. PT : total input power isotropically. Radiation Efficiency: ratio of total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter. er = Pr/Pin Pin : portion of input power. Antenna Effective Aperture: Pt = P Ae Impairments specific to wireless line-of-sight transmission: - Free space loss - Atmospheric absorption - Multipath - Refraction. 8

9 Lecture 7.1: Ch5 -Signal Encoding Techniques- - for digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). - the basis for analog signaling is a continuous constant-frequency signal AKA the carrier signal. - Digital data, digital signals: simplest form of digital encoding of digital data. < equipment less complex and less expensive than digital-to-analong modulation equipment. Digital signal is a sequence of discrete voltage pulses. Line coding schemes: - unipolar > 1 voltage level - polar > 2 voltage levels (+,-): NRZ-I,NRZ-L, Manchester, Differential Manchester - bipolar > 3 voltage levels (+,0,-) Encoding scheme: mapping from data bits to signal elements. include: NRZ-L NRZ-I Bipolar AMI Pseudoternary Manchester Manchester Differential 1/11/1439 AH 0 : + 1 : - 0 : no transition 1 : high to low/low to high 0 : no line signal 1 : -/+ 0 : -/+ 1 : absence of line signal 0 : high to low 1 : low to high 0 : transition at start 1 : no transition at start lack of synchronizaition when data contain long streams of 0/1 loss of synchronizaition when data contain long streams of 0 no loss of sync if long string of 1s Pros + easy to engineer + make good use of bandwidth Cons - dc component - lack of synchronization capability < same - long runs of zeroes still a problem - no net dc component - lower bandwidth - easy error detection Multilevel Binary issues: - synchronization with long runs of 0s or 1s - scramble data - not as efficient as NRZ usage of scrambling: to replace sequences that would produce constant voltage. Biphase: Pros + synchronization on mid bit transition (self clocking) + has no dc component + has error detection Cons - at least one transition per bit time and possibly two - maximum modulation rate is twice NRZ - requires more bandwidth Modulation rate: rate at which signal elements are generated. 9

10 Lecture 7.2: Ch5 -Signal Encoding Techniques- Encoding is the conversion of streams of bits into signal (digital/analog). Categories of encoding techniques: Digital Transmission: - Digital data > Digital signal - Analog data > Digital signal Analog Transmission: - Digital data > analog signal - Analog data > analog signal Analog data > analog signal Digital data > analog signal Analog data > digital signal Modulation: process of encoding source data onto a carrier signal with frequency. it involves: - Amplitude (AM) -simplest- - Frequency (FM) - Phase (PM) -FM and PM requires greater bandwidth- -Amplitude modulation requires 2B < twice bandwidth- Modulation involves: - Amplitude Shift Keying (ASK) - susceptible to sudden gain changes - inefficient - Frequency Shift Keying (FSK) Binary Frequency Shift Keying (BFSK) - most common - less susceptible than ASK Multiple FSK - more than 2 f used - more bandwidth efficient - more prone to error - Phase Shift Keying (PSK) 1. Binary phase shift keying (BPSK) 2. Differential phase shift keying (DPSK) 0 : same phase as previous 1 : opposite to preceding one Multiple PSK - more efficient use of bandwidth - each signal represent more than 1 bit Quadrature PSK (QPSK) Digitization: conversion of analog data into digital data. it s done using a codec(coder-decoder) Pulse code modulation: Sampling theorem: *rate higher than twice the highest signal frequency eg: 4000Hz > 8000 samples/second Non-Linear Coding: PCM scheme is refined using a technique AKA nonlinear encoding, it means that quantization levels are not equally spaced. - reduces overall distortion - can significantly improve the PCM SNR ratio. Companding: a process that compresses the intensity of a signal by setting more gain to weak signals than to strong signals on input. Delta Modulation: analog input is approximated by a staircase function. 10

11 Lecture 8: Ch6 -Digital communication technique- Half Duplex(HDX) - provides communication in both directions - but only one direction (not simultaneously) - once a party begins receiving a signal, it must wait for the transmitter to stop transmitting before replying. - examples: walkie talkies Full Duplex(FDX) - or it called double-duplex - allows communication in both directions - unlike half-duplex, allows this to happen simultaneously. - examples: Land-line telephone networks. Benefits to using full-duplex over half-duplex 1. time is not wasted, since no frames need to be retransmitted, as there are no collisions. 2. the full data capacity is available in both directions because the send and receive functions are separated. 3. stations(nodes) don t have to wait until others complete their transmission, since therein only one transmitter for each twisted pair. Transmission mode is the manner in which data is sent over the underlying medium. Transmission modes can be divided into two fundamental categories: - Serial -one bit is sent at a time- - Parallel -multiple bits are sent at the same time- Parallel transmission - allows transfers of multiple data bits at the same time over separate media. - it s used with a wired medium - the signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same time - n wires are used to send n bits at one time - advantages: speed - disadvantages: cost; limited to short distances. Serial transmission - it sends one bit at a time - most communication systems use serial mode, because: serial networks can be extended over long distances at less cost using only one physical wire means that there is never a timing problem caused by one wire being slightly longer than another - advantages: reduced cost - disadvantages: requires conversation devices - in serial mode, when sending bits, which bit should be sent across the medium first? - consider an integer: should a sender transmit the Most Significant Bit or Least Significant Bit? Either form can be used, but the sender and receiver must agree. - Serial transmission mechanisms can be divided into two board categories(depending on how transmission are spaced in time): - Asynchronous > transmission can occur at any time - Synchronous > transmission occurs continuously 11

12 Asynchronous Transmission: - Asynchronous transmission allows the physical medium to be idle for an arbitrary amount of time between two transmissions. - it s well-suited to applications that generate data at random time intervals. for example: - a user typing on a keyboard - a user that clicks on a hyperlink - transfer of data with start and stop bits and a variable time interval between data units. - timing is not important - Start bit > alerts receiver that new group of data is arriving. - Stop bit > alert receiver that byte is finished. - Synchronization achieved through start/stop bits with each byte received. - The beginning of a character is signaled by a start bit with a value of binary 0. - followed by the 5 to 8 bits that actually make up the character. - Then the data bits are usually followed by a parity bit, set by the transmitter, the receiver uses this bit for error detection. - The final element is a stop element, which is a binary 1. - Characteristics: - Cheap and effective - ideal for low-speed communication when gaps may occur during transmission (ex: keyboard) - Asynchronous disadvantages: - Asynchronous technologies usually require the sender to transmit a few extra bits before each data item: - to inform the receiver that a data transfer is starting - extra bits (preamble or start bits) allow the receiver to synchronize with the incoming signal - Slower Synchronous Transmission: - Requires constant timing relationship - Bit stream is combined into longer frames, possibly contain multiple bytes - any gaps between bursts are filled in with a special sequence of 0s and 1s indicating idle - advantages: - speed, no gaps or extra bits - more efficient than asynchronous - block of data transmitted, sent as frames in a steady stream without start and stop codes. - clocks must be synchronized - can use separate clock line, or embed clock signal in data - need to indicate start and end of block - use preamble and post-amble (flags) 12

13 Transmissions Types: In serial transmission a character is converted from parallel to serial form when transmitting and form serial to parallel form when receiving. Lecture 9: Ch8 -Multiplexing- - Multiplexing process allows several transmission sources to share a larger transmission capacity. - multiple links on 1 physical line - most common use of multiplexing is in long-haul communication using coaxial cable, microwave and optical fiber. - the multiplexer combines (multiplexes) data from the n input lines and transmits over a single data link(medium). - The demultiplexer separates (demultiplexes) the data according to channel, and delivers data to the appropriate output lines. - Multiplexing: the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. types: 1. Frequency-Division Multiplexing (FDM) 2. Wavelength-Division Multiplexing (WDM) 3. Time-Division Multiplexing (TDM) 4. Code-Division Multiplexing (CDM) FDM FDM system overview - can be used with analog signals - put different signals on different frequency bands using modulation - each signal is modulated onto a different carrier - all the modulated signals are combined to form a composite signal for transmission - signals are carried simultaneously on the same medium - to prevent interference, the channels are separated by guard bands which are unused portions of the spectrum. - television and radio uses FDM to broadcast many channels over the same media. 13

14 WDM - theoretically identical to Frequency Division Multiplexing - used in optical systems while FDM is used in electrical systems - Requires more spacing between channels - also have Dense Wavelength Division Multiplexing (DWDM) TDM - multiple transmission can occupy a single link by subdividing them and interleaving the portions - TDM can be implemented in two ways: 1. Synchronous TDM 2. Asynchronous TDM Synchronous Time Division Multiplexing - can be used with digital signals or analog signals carrying digital data. - in this form of multiplexing, data from various sources are carried in repetitive frames. - Each frame consists of a set of time slots, and each source is assigned one or more time slots per frame. - Synchronous TDM is called synchronous not because synchronous transmission is used, but because the time slots are preassigned to sources and fixed. - the multiplexer allocates exactly the same time slot to each device at all times, wether or not a device has anything to transmit. - a frame consists of one complete cycle of time slots - thus the number of slots in frame is equal to the number of inputs. - How Synchronous TDM Works? > Asynchronous TDM(Statistical Time Division Multiplexing) - each slot in a frame is not dedicated to the fix device - the number of slots in a frame is not necessary to be equal to the number of devices - more than one slots in a frame can be allocated for an input device - allows maximum utilization of the link - it allows a number of lower speed input lines to be multiplexed to a single higher speed line Statistical TDM - in synch TDM many slots are wasted - Statistical TDM allocates time slots dynamically based on demand - multiplexer scans input lines and collects data until frame full - may have problems during peak periods (must buffer inputs). - How Asynchronous TDM Works? > 14

15 Synchrnous vs. Statistical TDM > 1/11/1439 AH Multiplexing Real Examples Asymmetrical Digital Subscriber Line(ADSL) - Digital Subscriber Line is the link between subscriber and network. - it uses currently installed twisted pair cable - is Asymmetric -bigger downstream than up- - uses frequency division multiplexing - has a range up to 5.5km Code Division Multiplexing - sends many signals or chips per bit - each sender uses a unique pattern of chips - may use multiple frequencies for spread spectrum communication - common with wireless systems. الحمدهلل الذي بنعمته تتم الصالحات. 15