Data Transmission: Analog vs. Digital, Types and Transmission Methods - Prof. Joseph Bulli, Exams of Data Communication Systems and Computer Networks

Download Data Transmission: Analog vs. Digital, Types and Transmission Methods - Prof. Joseph Bulli and more Exams Data Communication Systems and Computer Networks in PDF only on Docsity! Chapter 3 Outline 3.1 - Introduction 3.2 - Circuits Configuration, Data Flow, Multiplexing (FDM, TDM, STDM, Inverse Mux, WDM), DSL 3.3 - Communication Media Guided and wireless media, media selection 3.4 - Digital Transmission of Digital Data Coding, Transmission Modes, Ethernet 3.5 - Analog Transmission of Digital Data (D to A) Modulation, Circuit Capacity, Modems 3.6 - Digital Transmission of Analog Data (A to D) Translating, Voice Data Transmission, Instant Messenger Transmitting Voice Data, VOIP 3.7 – Implications for Management 3.1 Introduction Includes network hardware and circuits Network circuits: physical media (e.g., cables) and special purposes devices (e.g., routers and hubs). Types of Circuits Physical circuits connect devices & include actual wires such as twisted pair wires Logical circuits refer to the transmission characteristics of the circuit, such as a T-1 connection refers to 1.544 Mbps Physical and logical circuits may be the same or different. For example, in multiplexing, one physical wire may carry several logical circuits. Physical Layer Network Layer Data Link Layer Types of Data Transmitted Analog data Produced by telephones Sound waves, which vary continuously over time, analogous to one’s voice Can take on any value in a wide range of possibilities Digital data Produced by computers, in binary form Information is represented as code in a series of ones and zeros All digital data is either on or off, 0 or 1 3 - * Types of Transmission Analog transmissions Analog data transmitted in analog form Examples of analog data being sent using analog transmissions are broadcast TV and radio Digital transmissions Made of discrete square waves with a clear beginning and ending Computer networks send digital data using digital transmissions Data converted between analog and digital formats Modem (modulator/demodulator): used when digital data is sent as an analog transmission Codec (coder/decoder): used when analog data is sent via digital transmission Data Type vs. Transmission Type Digital Transmission: Advantages Produces fewer errors Easier to detect and correct errors, since transmitted data is binary (1s and 0s, only two distinct values) A weak square wave can easily be propagated again in perfect form, allowing more crisp transmission than analog Permits higher maximum transmission rates e.g., Optical fiber designed for digital transmission More efficient Possible to send more digital data through a given circuit, circuit can be “packed” More secure Easier to encrypt digital bit stream Simpler to integrate voice, video and data Easier mix and match V, V, D on the same circuit, since all signals made up of 0’s and 1’s 3.2 Circuits Basic physical layout of the circuit Configuration types: Point-to-Point Configuration Goes from one point to another Sometimes called “dedicated circuits” Multipoint Configuration Many computer connected on the same circuit Sometimes called “shared circuit” Copyright 2011 John Wiley & Sons, Inc Point-to-Point Configuration Used when computers generate enough data to fill the capacity of the circuit Each computer has its own circuit to reach the other computer in the network (expensive) Multipoint Configuration + Cheaper (not as many wires) and simpler to wire - Only one computer can use the circuit at a time Used when each computer does not need to continuously use the entire capacity of the circuit Data Flow (Transmission) data flows in one direction only, (radio or cable television broadcasts) data flows both ways, but only one direction at a time (e.g., CB radio, it requires control info) data flows in both directions at the same time Selection of Data Flow Method Main factor: Application If data required to flow in one direction only Simplex Method e.g., From a remote sensor to a host computer If data required to flow in both directions Terminal-to-host communication (send and wait type communications) Half-Duplex Method Client-server; host-to-host communication (peer-to-peer communications) Full Duplex Method Half-duplex or Full Duplex Capacity may be a factor too Full-duplex uses half of the capacity for each direction Shielded twisted pair also exists, but is more expensive Coaxial Cable Less prone to interference than TP due to shielding More expensive than TP, thus quickly disappearing Used mostly for cable TV Source: Tony Freeman/ PhotoEdit Fiber Optic Cable Light created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber Has extremely high capacity, ideal for broadband Works well under harsh environments Not fragile, nor brittle; Not heavy nor bulky More resistant to corrosion, fire, water Highly secure, know when is tapped Fiber optic cable structure (from center): Core (v. small, 5-50 microns, ~ the size of a single hair) Cladding, which reflects the signal Protective outer jacket Types of Optical Fiber Multimode (about 50 micron core) Earliest fiber-optic systems Signal spreads out over short distances (up to ~500m) Inexpensive Graded index multimode Reduces the spreading problem by changing the refractive properties of the fiber to refocus the signal Can be used over distances of up to about 1000 meters Single mode (about 5 micron core) Transmits a single direct beam through the cable Signal can be sent over many miles without spreading Expensive (requires lasers; difficult to manufacture) Optical Fiber Different parts of signal arrive at different times, signal dispersion Center light likely to arrive at the same time as the other parts Source: Hugh Threlfall/ Alamy Wireless transmission of electrical waves through air Each device has a radio transceiver with a specific frequency Low power transmitters (few miles range) Often attached to portables (Laptops, PDAs, cell phones) Includes AM and FM radios, Cellular phones Wireless LANs (IEEE 802.11) and Bluetooth Microwaves and Satellites, Low Earth Orbiting Satellites Radio Waves Microwave Radio High frequency form of radio communications Extremely short (micro) wavelength (1 cm to 1 m) Requires line-of-sight Performs same functions as cables Often used for long distance, terrestrial transmissions (over 50 miles without repeaters) No wiring and digging required Requires large antennas (about 10 ft) and high towers Possesses similar properties as light Reflection, refraction, and focusing Can be focused into narrow powerful beams for long distance Some effect from water, rain and snow Source: Matej, Pribelsky listock photo 03Satellite Communications Special form of microwave communications Signals travel at speed of light, yet long propagation delay due to great distance between ground station and satellite Factors Used in Media Selection Type of network LAN, WAN, or Backbone Cost Always changing; depends on the distance Transmission distance Short: up to 300 m; medium: up to 500 m Security Wireless media is less secure Error rates Wireless media has the highest error rate (interference) Transmission speeds Constantly improving; Fiber has the highest Media Summary 3.4 Digital Transmission of Digital Data Computers produce binary data Standards needed to ensure both sender and receiver understands this data Codes: digital combinations of bits making up languages that computers use to represent letters, numbers, and symbols in a message Signals: electrical or optical patterns that computers use to represent the coded bits (0 or 1) during transmission across media Coding Coding is the representation of a set of characters by a string of bits Letters (A, B, ..), numbers (1, 2,..), special symbols (#, $, ..) ASCII: American Standard Code for Information Interchange Originally used a 7-bit code (128 combinations), but an 8-bit version (256 combinations) is now in use Found on PC computers EBCDIC: Extended Binary Coded Decimal Interchange Code An 8-bit code developed by IBM Used mostly in mainframe computer environment ASCII Chart Transmission Modes Bits in a message can be sent on: a single wire one after another (Serial transmission) multiple wires simultaneously (Parallel transmission) Serial Mode Sends bit by bit over a single wire Serial mode is slower than parallel mode Parallel mode Uses several wires, each wire sending one bit at the same time as the others A parallel printer cable sends 8 bits together Computer’s processor and motherboard also use parallel busses (8 bits, 16 bits, 32 bits) to move data around Parallel Transmission Example Used for short distances (up to 6 meters) since bits sent in parallel mode tend to spread out over long distances Serial Transmission Example Can be used over longer distances since bits stay in the order they were sent Signaling of Bits Digital Transmission Signals sent as a series of “square waves” of either positive or negative voltage Voltages vary between +3/-3 and +24/-24 depending on the circuit Signaling (encoding) Defines how the voltage levels will correspond to the bit values of 0 or 1 Examples: Unipolar, Bipolar RTZ, NRZ, Manchester Data rate: describes how often the sender can transmit data 64 Kbps  once every 1/64000 of a second Signaling (Encoding) Techniques Unipolar signaling Use voltages either vary between 0 and a positive value or between 0 and some negative value Bipolar signaling Use both positive and negative voltages Experiences fewer errors than unipolar signaling Signals are more distinct (more difficult for interference to change polarity of a current) Return to zero (RZ) Signal returns to 0 voltage level after sending a bit Non return to zero (NRZ) Signals maintains its voltage at the end of a bit Manchester encoding (used by Ethernet) Manchester Encoding Used by Ethernet, most popular LAN technology Defines a bit value by a mid-bit transition A high to low voltage transition is a 0 and a low to high mid-bit transition defines a 1 Data rates: 10 Mb/s, 100 Mb/s, 1 Gb/s 10- Mb/s  one signal for every 1/10,000,000 of a second (10 million signals or bits every second) Less susceptible to having errors go undetected If there is no mid-bit voltage transition, then an error took place Digital Transmission Types number of symbols transmitted per second General formula: b = s x n where b = Data Rate (bits/second) s = Symbol Rate (symbols/sec.) n = Number of bits per symbol Example: AM n = 1  b = s Example: 16-QAM n = 4  b = 4 x s Bandwidth of a Voice Circuit Difference between the highest and lowest frequencies in a band or set if frequencies Human hearing frequency range: 20 Hz to 14 kHz Bandwidth = 14,000 – 20 = 13,800 Hz Voice circuit frequency range: 0 Hz to 4 kHz Designed for most commonly used range of human voice Phone lines transmission capacity is much bigger 1 MHz for lines up to 2 miles from a telephone exchange 300 kHz for lines 2-3 miles away Data Capacity of a Voice Circuit Fastest rate at which you can send your data over the circuit (in bits per second) Calculated as the bit rate: b = s x n Depends on modulation (symbol rate) Max. Symbol rate = bandwidth (if no noise) Maximum voice circuit capacity: Using QAM with 4 bits per symbol (n = 4) Max. voice channel carrier wave frequency: 4000 Hz = max. symbol rate (under perfect conditions) Data rate = 4 * 4000  16,000 bps A circuit with a 10 MHz bandwidth using 64-QAM could provide up to 60 Mbps. Data Compression in Modems Used to increase the throughput rate of data by encoding redundant data strings Example: Lempel-Ziv encoding Used in V.44, the ISO standard for data compression Creates (while transmitting) a dictionary of two-, three-, and four-character combinations in a message Anytime one of these patterns is detected, its index in dictionary is sent (instead of actual data) Average reduction: 6:1 (depends on the text) Provides 6 times more data sent per second 3.6 Digital Transmission of Analog Data Analog voice data sent over digital network using digital transmission Requires a pair of special devices called Codec - Coder/decoder A device that converts an analog voice signal into digital form Converts it back to analog data at the receiving end Used by the phone system Modem is reverse device than Codec, and this word stands for Modulate/Demodulate. Modems are used for analog transmission of digital data. Analog to Digital Conversion Analog data must be translated into a series of bits before transmission onto a digital circuit Done by a technique called Pulse Amplitude Modulation (PAM) involving 4 steps: Take samples of the continuously varying analog signal across time Measure the amplitude of each signal sample Encode the amplitude measurement of the signal as binary data that is representative of the sample Send the discrete, digital data stream of 0’s and 1’s that approximates the original analog signal Creates a rough (digitized) approximation of original signal Quantizing error: difference between the original analog signal and the replicated but approximated, digital signal The more samples taken in time, the less quantizing error PAM – Measuring Signal Sample analog waveform across time and measure amplitude of signal In this example, quantize the samples using only 8 pulse amplitudes or levels for simplicity Our 8 levels or amplitudes can be depicted digitally by using 0’s and 1’s in a 3-bit code, yielding 23 possible amplitudes PAM – Encoding and Sampling 000 – PAM Level 1 001 – PAM Level 2 010 – PAM Level 3 011 – PAM Level 4 100 – PAM Level 5 101 – PAM Level 6 110 – PAM Level 7 111 – PAM Level 8 For digitizing a voice signal, it is typically 8,000 samples per second and 8 bits per sample 8,000 samples x 8 bits per sample  64,000 bps transmission rate needed 8,000 samples then transmitted as a serial stream of 0s and 1s Minimize Quantizing Errors Increase number of amplitude levels Difference between levels minimized  smoother signal Requires more bits to represent levels  more data to transmit Adequate human voice: 7 bits  128 levels Music: at least 16 bits  65,536 levels Sample more frequently Will reduce the length of each step  smoother signal Adequate Voice signal: twice the highest possible frequency (4Khz x 2 = 8000 samples / second) RealNetworks: 48,000 samples / second PAM for Telephones Combined Modulation Techniques Combining AM, FM, and PM on the same circuit Examples QAM - Quadrature Amplitude Modulation A widely used family of encoding schemes Combine Amplitude and Phase Modulation A common form: 16-QAM Uses 8 different phase shifts and 2 different amplitude levels 16 possible symbols  4 bits/symbol TCM – Trellis-Coded Modulation An enhancement of QAM Can transmit different number of bits on each symbol (6,7,8 or 10 bits per symbol) PCM - Pulse Code Modulation local loop phone switch (DIGITAL) Central Office (Telco) Analog transmission To other switches trunk Digital transmission convert analog signals to digital data using PCM (similar to PAM) 8000 samples per second and 8 bits per sample (7 bits for sample + 1 bit for control)  64 Kb/s (DS-0 rate) DS-0 is the basic digital communications unit used by phone network DS-0 corresponds to 1 digital voice signal ADPCM Adaptive Differential Pulse Code Modulation Encodes the differences between samples The change between 8-bit value of the last time interval and the current one Requires only 4 bits since the change is small  Only 4 bits/sample (instead of 8 bits/sample), Requires 4 x 8000 = 32 Kbps (half of PCM) Makes it possible to for IM to send voice signals as digital signals using modems (which has <56 Kbps) Can also use lower sampling rates, at 8, 16 kbps Lower quality voice signals. 3.7 Implications for Management Digital is better Easier, more manageable, faster, less error prone, and less costly to integrate voice, data, and video Organizational impact Convergence of physical layer causing convergence of phone and data departments emerging new technologies such as VoIP accentuate these developments Impact on telecom industry Disappearance of the separation between manufacturers of telephone equipment and manufacturers of data equipment Continued financial turbulence among vendors requires care in technology selection by network managers