Local Networks. Lecture 2 23-Mar-2016

Transcription

1 Local Networks Lecture 2 23-Mar-2016

2 Roadmap of the course Last time LAN and networking introduction Models for data communication Data transmission issues Today Transmission media Error detection methods

3 Transmission media

4 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:

5 Classes of transmission media

6 Reality Check: Storage media Send data on tape / disk / DVD for a high bandwidth link Mail one box with GB 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. CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

7 Guided media Provide a conduit between devices A signal traveling through such media is directed and contained by the physical limits of the medium

8 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

9 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

10 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)

11 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

12 Shielded Twisted Pair (STP) cable Has metallic foil that encases the insulated conductors This prevents electromagnetic noise Also prevents crosstalk Introduced by IBM in the 1980s UTP Cat 7 is shielded!

13 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)

14 Coaxial cable (coax) Carries higher-frequency signals than TPs Better shielding and more bandwidth for longer distances and higher rates than twisted pair.

15 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

16 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

17 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 Data signal superimposed on the low-frequency power signal Difficulties Household electrical wire designed to distribute (low-frequency) power signals 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 Many products use proprietary standards International standards actively under development

18 Household electrical wiring

19 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: km/s = 3*10 8 m/s Travels in a straight line in a single uniform substance Refraction: change of direction at border between substances Speed also changes

20 Refraction and reflection

21 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

22 Fiber how it works 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) Light trapped by total internal reflection Photodetector CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

23 Propagation modes Multimode, step-index Multimode, graded-index Single mode

24 Cable composition

25 Fiber Cables Single-mode Core so narrow (10μm) 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 (50μm core) Used with LEDs for cheaper, shorter distance links CN5E by Tanenbaum & Wetherall, Pearson Education- Prentice Hall and D. Wetherall, 2011 Fibers in a cable

26 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 => lower installation costs 1000 twisted pairs, 1 km long weigh 8000 kg 2 fibers have more capacity and weigh 100 kg Fiber does not leak light; hard to tap 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

27 Comparison of the properties of wires and fiber 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 50 Tbps bandwidth achievable with fiber technology; we at about 100 Gbps! (due to interfaces processing) CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

28 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

29 Radio frequency allocation

30 Radio communication Section of the electromagnetic spectrum used for various wireless transmissions Radio waves: are easy to generate can travel long distances can penetrate buildings easily => widely used for communication, indoors and outdoors are omnidirectional (receiver and transmitter do not need to be aligned physically) their properties depend on their frequencies low: pass through obstacles well, power falls off sharply with distance from source high: travel in straight lines and bounce off obstacles; absorbed by rain are subject to interferences (eg, from electrical equipment)

31 Electromagnetic Spectrum (1) Different bands have different uses: Radio: wide-area broadcast; Infrared/Light: line-of-sight Microwave: LANs and 3G/4G; Networking focus Microwave CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

32 Electromagnetic Spectrum (2) 300 MHz To manage interference, spectrum is carefully divided, and its use regulated and licensed, e.g., sold at auction. 3 GHz WiFi (ISM bands) 3 GHz Source: NTIA Office of Spectrum Management, GHz Part of the US frequency allocations CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

33 Electromagnetic Spectrum (3) Fortunately, there are also unlicensed ( ISM ) bands: Free for use at low power; devices manage interference Widely used for networking; WiFi, Bluetooth, Zigbee, etc b/g/n a/g/n CN5E by Tanenbaum & Wetherall, Pearson Education-Prentice Hall and D. Wetherall, 2011

34 Unlicenced bands Roaring success over past decades FCC allows unlicenced use of white spaces around 700 MHz (2009) White space Frequency bands that are allocated but not used locally Analog to all-digital TV in US in 2010 freed up white spaces around 700 MHz Difficulty Unlicenced devices have to be able to detect licenced devices that have first rights to use that band FCC opens GHz band in 2001 for unlicenced operation More bandwidth than all ISM combined Can support high-speed networks for high-def TV At 60 GHz, signals do not propagate far => suitable for short-range networks Millimeter band Products appear

35 Types of propagation

36 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

37 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

38 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

39 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

40 Space propagation Satellite relays take the place of atmospheric refraction Basically line-of-sight with an intermediary (the satellite) Great distances are thus covered

41 Propagation of VLF waves Surface waves, through air or seawater Used in long-range radio navigation and submarine communication Susceptible to atmospheric noise Heat, electricity

42 Propagation of LF waves Surface waves Attenuation greater during daytime Used in long-range radio navigation, radio beacons, navigational locators

43 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

44 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

45 Propagation of VHF waves Is mostly line-of-sight Used for VHF TV, FM radio, aircraft AM radio, aircraft navigational aid

46 Propagation of UHF waves Is line-of-sight Used for UHF TV, mobile telephony, cellular telephony, paging, microwave links

47 Propagation of SHF waves Is mostly line-of-sight and sometimes in space Uses: terrestrial and satellite microwave and radar communication

48 Propagation of EHF waves Uses the space Uses: predominantly scientific: radar, satellite, and experimental communication

49 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

50 Illustration of repeaters

51 Satellite communication

52 Satellites in geosynchronous orbit

53 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!

54 Transmission impairment Transmission media are not perfect => impairments in the signal sent through the medium

55 Attenuation Means loss of energy => amplifiers needed Decibel: shows if a signal has lost/gained strength (negative/positive): db=10log 10 (P 2 /P 1 )

56 Adding decibels

57 Distortion Means the signal changes its form Occurs in composite signals Each signal has its own propagation speed through the medium => its own delay

58 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

59 Illustration of noise

60 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

61 Illustration of throughput

62 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

63 Error Detection and correction Error codes add structured redundancy to data so errors can be either detected, or corrected

64 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 Interference can change shape/timing of signal Reliable systems must have mechanisms for detecting and correcting errors

65 Error types

66 Single-bit errors Only 1 bit of a data unit is changed Least likely to appear in serial transmission Can happen in parallel transmission

67 Burst errors 2 or more bits in the data unit are changed Length of burst: from 1 st to last corrupted bit; in between uncorrupted bits are possible Likely in serial transmissions

68 How to detect errors?

69 Types of redundancy in LANs

70 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

71 Illustration of VRC

72 Performance of VRC Detects single-bit errors It can also detect burst errors if total number of bits changed is odd Exp: 1 error, ; detected, sum is wrong Exp: 3 errors, ; detected, sum is wrong Exp: 2 errors, ; not detected, sum is right! Error can also be in the parity bit itself Random errors are detected with probability ½

73 Longitudinal Redundancy Check

74 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

75 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

76 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

77 Deriving the CRC

78 CRC generator Uses modulo-2 division

79 CRC checker Uses modulo-2 division in the same way

80 Polynomials CRC generator typically represented as an algebraic polynomial This is useful Short Proves the concept mathematically

81 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

82 Standard polynomials 12,16, 32 size of CRC remainders CRC divisor s size is hence 13, 17, 33

83 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