Switching & Data Networks Lecture ID: ET-IDA-067 Lecture 6 Media and Data Transmission Chapter 3 and 4 from textbook Data and Computer Communications by W. Stallings 01.04.2014 , v03 Prof. W. Adi

Chapter 4- from Textbook Data and Computer Communications Transmission Media Chapter 4- from Textbook Data and Computer Communications by W. Stallings

Overview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance

Design Factors Bandwidth Transmission impairments Interference Higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Many signals at the same time Number of receivers In guided media More receivers (multi-point) introduce more attenuation and distortion

Electromagnetic Spectrum

Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

Twisted Pair Two categories: Unshielded Shielded

Twisted Pair - Applications Most common medium Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps

Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate 16 ... 300 MHz Short range Attenuation ! 2.6 dB/100m at 1MHz 21.4 dB/100m at 300 MHz

Twisted Pair - Transmission Characteristics Analog Amplifiers (repeaters) every 5km to 6km Digital Use either analog or digital signals repeater every 2km or 3km Limited distance Limited bandwidth Limited data rate Susceptible to interference and noise

Unshielded and Shielded TP Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Metal shield that reduces interference More expensive Harder to handle (thick, heavy)

UTP Categories Cat 3 Cat 4 Cat 5 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm

Near End Crosstalk Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to receiving pair i.e. near transmitted signal is picked up by near receiving pair

Coaxial Cable

Coaxial Cable Applications Most versatile medium Television distribution Arial to TV Cable TV Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Short distance computer systems links Local area networks

Coaxial Cable - Transmission Characteristics Analog Amplifiers every few km Closer if higher frequency is used Up to 500MHz Digital Repeater every 1km Closer for higher data rates

Optical Fiber

Optical Fiber - Benefits Greater capacity Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing 10s of km at least

Optical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs

Optical Fiber - Transmission Characteristics Act as wave guide for 1014 to 1015 Hz Portions of infrared and visible spectrum Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Division Multiplexing ‘WDM’ (1000Gbps=1 Terabit/sec Bell-labs 1997)

Optical Fiber Transmission Modes

Wireless Transmission Unguided media Transmission and reception via antenna Directional Focused beam Careful alignment required Omnidirectional Signal spreads in all directions Can be received by many antennae

Frequencies 2GHz to 40GHz 30MHz to 1GHz 3 x 1011 to 2 x 1014 Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omnidirectional Broadcast radio 3 x 1011 to 2 x 1014 Infrared Local

Terrestrial Microwave Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates propagation source

Satellite Microwave Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit to stay still relative to a location on the earth Height of 35,784km Television Long distance telephone Private business networks

Broadcast Radio Omnidirectional, sends to all directions FM radio UHF and VHF television Line of sight Suffers from multipath interference Reflections

Infrared Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port

Chapter 3- from Textbook Data and Computer Communications Data Transmission Chapter 3- from Textbook Data and Computer Communications by W. Stallings

Basic Components Transmitter Receiver Medium Guided medium, e.g. twisted pair, optical fiber Unguided medium, e.g. air, water, vacuum Transmitter Medium Receiver Z0 Z0 Z0 Example: typical twisted pair connection

Art of Transmissions Simplex Half duplex Full duplex One direction e.g. Television Half duplex Either direction, but only one way at a time e.g. police radio Full duplex Both directions at the same time e.g. telephone

Transmitted Signals: Continuous & Discrete Signals

Wavelength Distance occupied by one cycle Or distance between two points of corresponding phase in two consecutive cycles  : Wavelength Assuming signal velocity v  = v T  f = v c = 3*108 ms-1 (speed of light in free space)

Signal Spectrum & Bandwidth The range of frequencies contained in a signal Absolute bandwidth width of spectrum Effective bandwidth Often just bandwidth: Narrow band of frequencies containing most of the energy DC Component Component of zero frequency

Addition of Frequency Components Fundamental frequency 3rd Harmonic frequency Simplified square wave

Signal with DC Component

Analog and Digital Data Transmission Definitions Data Entities that convey meaning Signals Electric or electromagnetic representations of data Transmission Communication of data by propagation and processing of signals

Data Analog Digital Continuous values within some interval e.g. sound, video Digital Discrete values e.g. text, integers

Acoustic Spectrum (Analog) Voice: 300 – 3400 Hz

Data and Signals Representation Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data Example: Modem Can use digital signal to carry analog data Example: Compact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Transmission Impairments Signal received may differ from signal transmitted (distortion) Analog --> degradation of signal quality Digital --> bit errors Caused by Attenuation and attenuation distortion Delay distortion Noise

Attenuation Signal strength falls off with distance Depends on transmission medium Attenuation is an increasing function of frequency Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error

Delay Distortion Only in guided media Propagation velocity varies with frequency

Noise (1) Additional signals inserted between transmitter and receiver Thermal Due to thermal agitation of electrons Uniformly distributed White noise Thermal Noise power is : N = K T B K: Boltzmann‘s constant 1.3803 10-23 J/oK T: teprature, degrees kelvin (273 + oC) B: Bandwidth in Hz Intermodulation Signals that are the sum and difference of original frequencies sharing a medium (due to nonlinearity)

Noise (2) Crosstalk Impulse A signal from one line is picked up by another Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude

Channel Capacity Data rate Question: Given - Channel Bandwidth Measured in bits per second bps Rate at which data can be communicated Question: Given - Channel Bandwidth - Channel quality (noise) What is the channel capacity ?

Nyquist Bandwidth and Channel Capacity Nyquist Formula: Given a noisless channel with bandwidth B Hz, the highest bit rate C that can be carried is: C = 2B log2 M Where M is the number of discrete signalling levels Example: A Modem operating at a telephone channel with a bandwidth of 3100 Hz. The number of different signal levels is 8. The Nyquist capacity is: C = 2 x 3100 log2 8 = 18 600 bps

Shannon Capacity (Maximum error-free Channel Transmission Rate) Shannon Formula: Given a channel with a signal-to-noise ratio of SNR, the theoretical error-free transmission rate is: C = B log2 (1 + SNR) Where C is the channel capacity in bits per second pbs (still no techniques are known today to reach this limit) ! Example: A Modem operating at a telephone channel with a bandwidth of 3100 Hz. The channel signal-to- noise ratio is 7. The maximum theoretical error-free transmission rate is: C = 3100 log2 (1+7) = 9300 bps