special course in computer science - Åbo akademi | startsida · · 2012-03-16special course in...
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Lecture 2 16.3.2012
Special Course in Computer Science: Local Networks
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Roadmap of the course Last time LAN and networking introduction Models for data communication Data transmission issues
Today Transmission media Error detection methods
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Transmission media
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Transmission Media For communication, data is represented with signals Signals are transmitted as electromagnetic energy Electromagnetic energy can travel through vacuum,
air, or other transmission media Electromagnetic spectrum:
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Classes of transmission media
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Reality Check: Storage media
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
• Send data on tape / disk / DVD for a high bandwidth link Mail one box with 1000 800GB tapes (6400 Tbit) Takes one day to send (86,400 secs) Data rate is 70 Gbps.
• Data rate is faster than long-distance networks! • But, the message delay is very poor.
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Guided media Provide a conduit between devices A signal traveling through such media is directed
and contained by the physical limits of the medium
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Signals through guided media Twisted pair and coaxial cable Metalic (copper) conductors Signals as electrical current
Optical fiber Glass or plastic cable Signals as light
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Unshielded Twisted Pair (UTP) cable Made of 2 wires (copper), each separately insulated The wires are twisted around each other Between 2-12 twists per foot
Cheap and easy to use
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UTP grades Electronics Industries Association (EIA) has standards
to grade UTP cables 5 categories are used and categories 6 and 7 are
coming Category 1: basic, previously used in telephone systems –
fine for voice Category 2: voice and data transmission, up to 4 Mbps Category 3: voice and data transmission, up to 10 Mbps Min 3 twists/foot Standard cable for most telephone systems
Category 4: voice and data transmission, up to 16 Mbps Min 3 twists/foot + other properties
Category 5: data transmission up to 100 Mbps Categories 6 and 7: data transmission (250 and 600 Mbps,
respectively) Cat 6: most installed cable in Finland’s LANs (2002)
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Category 5 UTP cable with four twisted pairs
Cat 5: usually 4 UTP grouped together in a plastic sheath 100 Mbps Ethernet: uses just two out of the four pairs 1 Gbps Ethernet uses all four pairs in both directions
simultaneously
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Shielded Twisted Pair (STP) cable Has metalic foil that encases the insulated conductors This prevents electromagnetic noise Also prevents crosstalk Introduced by IBM in the 1980s UTP Cat 7 is shielded!
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Twisted pair usage Telephone systems Networking Temporary network connections (TP very flexible) Short and medium length connections (UTP) Video applications (security cameras) Bandwidth of UTP improved to match the baseband of
television signals FDDI networks, token rings (STP) Ethernet (10G) (STP)
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Coaxial cable (coax) Carries higher-frequency signals than TPs Better shielding and more bandwidth for longer
distances and higher rates than twisted pair.
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Coax standards Categorized by RG ratings (radio governments) Each RG number denotes a unique set of physical
specifications Each cable defined by RG ratings is adapted for a
specialized function: RG-8,RG-9, RG-11: Thick Ethernet RG-58: Thin Ethernet RG-59: TV
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More on coax Two kinds 50-ohm cable used for digital transmission 75-ohm cable used analog transmission and cable TV
Better shielding than TP => it can span longer distances at higher speeds
Construction => high bandwidth, excellent noise immunity Bandwidths of up to a few GHz are common
Used widely for long-distance telephone systems in the past (now fiber on long-haul routes)
Still widely used in cable TV and other MANs
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Power-line networking Use power lines for data communication Not new, see X10 for instance Now focus on high-rate communication Inside the home as a LAN Outside the home as broadband Internet access
Difficulties Household electrical wire designed to distribute (low-frequency)
power signals 50-60 Hz, wiring attenuates the much higher frequency (Mhz)
signals needed for high-rate data communication Practical to send 100 Mbps With communication schemes that resist impaired frequencies
and bursts of errors May products use proprietary standards International standards actively under development
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Household electrical wiring
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Optical fiber Made of glass/plastic and transmits signals in the
form of light Light is a form of electromagnetic energy Max speed in vacuum: 300000 km/h Travels in a straight line in a single uniform
substance Refraction: change of direction at border between
substances Speed also changes
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Refraction and reflection
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Using reflection Optical fiber uses reflection for guiding light
through a channel A glass/plastic core is surrounded by a cladding
of less dense material Difference in density so chosen that reflection
occurs instead of refraction How is information encoded into a beam of light On-off flashes represent 1-0 bits
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Fiber – how it works
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Common for high rates and long distances Long distance ISP links, Fiber-to-the-Home Light carried in very long, thin strand of glass
Light source (LED, laser) Photodetector Light trapped by
total internal reflection
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Propagation modes Multimode, step-index
Multimode, graded-index
Single mode
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Cable composition
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Fiber Cables
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Single-mode Core so narrow (10um) light
can’t even bounce around Used with lasers for long
distances, e.g., 100km
Multi-mode Other main type of fiber Light can bounce (50um core) Used with LEDs for cheaper,
shorter distance links
Fibers in a cable
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Attenuation of light in infrared
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Fiber has enormous bandwidth (THz) and tiny signal loss – hence high rates over long distances
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Transmission of light through Fiber Glass used for optical fiber: exceptionally
transparent Attenuation of light through glass Ratio of input to output signal
Chromatic dispersion Light pulses spread out in length as they propagate Depends on wavelength Solitons research Special pulses
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Optical fibers vs. Copper wires Advantages
Much higher bandwidths than copper Repeaters needed only every 50 km, compared to 5 km Not affected by power surges, electromagnetic interference, power
failures, or corrosive chemicals Thin and lightweight:
1000 twisted pairs, 1 km long weigh 8000 kg 2 fibers have more capacity and weigh 100 kg
Lower installation costs
Disadvantages Unfamiliar technology to common engineers Optical transmission is unidirectional => two fibers or two frequency bands
needed for two way communication Fiber interfaces cost more than electrical interfaces
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Comparison of the properties of wires and fiber
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Property Wires Fiber Distance Short (100s of m) Long (tens of km) Bandwidth Moderate Very High Cost Inexpensive Less cheap Convenience Easy to use Less easy Security Easy to tap Hard to tap
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Unguided media Provide for wireless communication Transport electromagnetic waves without using a
physical conductor Signals are broadcast through the air => available
to anyone having proper devices to receive them
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Radio frequency allocation
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Types of propagation
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Surface propagation Radio waves travel through the lowest
atmosphere (hug the earth) At lowest frequencies, signals emanate in all
directions and follow the curvature of the planet Distance depends on the signal power Can also take place in seawater
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Tropospheric Propagation Can work in 2 ways Line-of-sight: signal is directed in straight line from
antenna to antenna Receiver and transmitter placed in line-of-sight
Broadcast at an angle in upper troposphere and reflected back Allows greater distances to be covered
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Ionospheric Propagation Higher-frequency radio waves radiate upward to
the ionosphere where they are reflected back Greater distances are covered with lower input Density difference between “spheres” makes the
waves to bend back to earth
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Line-of-sight propagation
Very high frequency signals are transmitted in straight lines from antenna to antenna
Antennas should be directional, facing each other and tall enough or close enough
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Space propagation Satellite relays take the place of atmospheric
refraction Basically line-of-sight with an intermediary (the
satellite) Great distances are thus covered
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Propagation of VLF waves Surface waves, through air or seawater Used in long-range radio navigation and
submarine communication Susceptible to atmospheric noise
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Propagation of LF waves Surface waves Attenuation greater during daytime Used in long-range radio navigation, radio
beacons, navigational locators
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Propagation of MF waves In the troposphere, absorbed by ionosphere Most transmissions rely on line-of-sight antennas Used for AM radio, maritime radio, radio direction
finding, emergency frequencies
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Propagation of HF waves Uses ionosphere Used for amateur radio, citizen’s band radio,
international broadcasting, military communication, long-distance aircraft and ship communication, telephone, telegraph, fax
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Propagation of VHF waves Is mostly line-of-sight Used for VHF TV, FM radio, aircraft AM radio,
aircraft navigational aid
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Propagation of UHF waves Is line-of-sight Used for UHF TV, mobile telephony, cellular
telephony, paging, microwave links
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Propagation of SHF waves Is mostly line-of-sight and sometimes in space Uses: terrestrial and satellite microwave and
radar communication
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Propagation of EHF waves Uses the space Uses: predominantly scientific: radar, satellite,
and experimental communication
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Terrestrial microwave Require line-of-sight transmission and reception
equipment Microwave signals propagate in one direction at a
time Hence 2 frequencies are required for 2-way
communication (e.g., telephone calls) transceiver (transmitter and receiver)
Repeaters Basis for many telephone systems
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Illustration of repeaters
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Satellite communication
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Satellites in geosynchronous orbit
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Satellites versus Fiber Satellites good for rapid deployment Crises, military, disasters
Broadcast is cheaper with satellites Communication in places with hostile terrain or
poorly developed infrastructure Economics!
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Transmission impairment Transmission media are not perfect =>
impairments in the signal sent through the medium
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Attenuation Means loss of energy => amplifiers needed Decibel: shows if a signal has lost/gained
strength (negative/positive): dB=10log10(P2/P1)
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Adding decibels
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Distortion Means the signal changes its form Occurs in composite signals Each signal has its own propagation speed through the
medium => its own delay
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Noise Can corrupt the signal Thermal: random motion of electrons in a wire =>
extra signal created Induced: comes from sources such as motors,
appliances Crosstalk: effect of an wire over another Impulse: spike coming from power lines, lightning
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Illustration of noise
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Performance of a medium Measured by throughput, propagation speed,
propagation delay Throughput: how fast data can pass through a
point Propagation speed: the distance a signal or bit
can travel through the medium in one second Depends on medium and frequency of signal
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Illustration of throughput
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Propagation time Measures the time required for a signal/bit to
travel from one point of the media to another Propagation time = distance / propagation speed
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Error Detection and correction Error codes add structured redundancy to data so errors can be either detected, or corrected
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Error detection A system that cannot guarantee that the data
received is the data sent is useless Data can be corrupted Quite likely Heat, magnetism, other forms of electricity Noise
Reliable systems must have mechanisms for detecting and correcting errors
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Error types
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Single-bit errors Only 1 bit of a data unit is changed Least likely to appear in serial transmission Can happen in parallel transmission
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Burst errors 2 or more bits in the data unit are changed Length of burst: from 1st to last corrupted bit; in
between uncorrupted bits are possible Likely in serial transmissions
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How to detect errors?
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Types of redundancy in LANs
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Vertical Redundancy Check Called also parity check A redundant bit (the parity bit) is appended to
every data unit so that the total number of 1s in the unit (including the parity bit) is even
Most common and least expensive Odd number of 1s can also be used
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Illustration of VRC
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Performance of VRC Detects single-bit errors It can also detect burst errors if total number of
bits changed is odd Exp: 1 error, 11100101; detected, sum is wrong Exp: 3 errors, 11011001; detected sum is wrong Exp: 2 errors, 11101101; not detected, sum is right! Error can also be in the parity bit itself Random errors are detected with probability ½
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Longitudinal Redundancy Check
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Performance of LRC Better at detecting burst errors than VRC There is one pattern of errors that is still elusive If some bits in one data unit are damaged and the same
number of bits in the same position are damaged in another data unit, then LRC does not detect error
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Cyclic Redundancy Check Most powerful, based on binary reduction Predefined binary unit called the divisor The data unit (DU) is appended with a sequence of
redundant bits (CRC remainder) so that the resulted DU is exactly divisible by the divisor
At destination, the received DU is divided by the divisor If remainder is zero, ok
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More on CRC Required qualities of a CRC To have exactly one bit less than the divisor Appending it to the DU must make the resulting bit
sequence divisible by divisor Theory and application of CRC: straightforward The complication: deriving the CRC
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Deriving the CRC
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CRC generator Uses
modulo-2 division
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CRC checker Uses
modulo-2 division in the same way
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Polynomials
CRC generator typically represented as an algebraic polynomial
This is useful Short Proves the concept mathematically
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Polynomial properties Should not be divisible by x All burst errors of length equal to the polynomial’s
degree are detected Should be divisible by x+1 All burst errors affecting an odd number of bits are
detected
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Standard polynomials
12,16, 32 size of CRC remainders CRC divisor’s size is hence 13, 17, 33
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CRC performance If CRC respects the rules mentioned then: All burst errors of length equal to the polynomial’s
degree are detected All burst errors affecting an odd number of bits are
detected Burst errors of length greater than the degree of
polynomials are detected with high probability 32-bit CRC used in Ethernet, Token Ring
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Error Correction
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Error correction codes: Hamming codes Binary convolutional codes Reed-Solomon and Low-Density Parity Check
codes Mathematically complex, widely used in real systems
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Error Bounds – Hamming distance
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Code turns data of n bits into codewords of n+k bits
Hamming distance is the minimum bit flips to turn one valid codeword into any other valid one. Example with 4 codewords of 10 bits (n=2, k=8): 0000000000, 0000011111, 1111100000, and 1111111111 Hamming distance is 5
Bounds for a code with distance: 2d+1 – can correct d errors (e.g., 2 errors above) d+1 – can detect d errors (e.g., 4 errors above)