Topics Computer Science
Topics Computer Science COSC 6397
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This 31 page Class Notes was uploaded by Lowell Harris on Saturday September 19, 2015. The Class Notes belongs to COSC 6397 at University of Houston taught by Staff in Fall. Since its upload, it has received 5 views. For similar materials see /class/208177/cosc-6397-university-of-houston in Chemistry at University of Houston.
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Date Created: 09/19/15
Medium Access Control Logic Collision detected mmmm ham1r Ink mu um min max 39 I L39 Interframe Space IFS Immediate access when mediun is free gt DIFS DIFS G D PlFS DIFS SlFS 1 A r Busy Medium 1 BackqffWlndow Next Frame I Slottime Defer Access Select Slot and Decrement Backoff as long as medium is idle Contention Window Short IFS SIFS 1 Shortest IFS used for ACK CTS poll response Ci Used for immediate response actions I Point coordination function IFS PIFS all Midlength IFS 23 Used by centralized controller in PCF scheme when using polls I Distributed coordination function IFS DIFS Ci Longest IFS data RTS Ci Used as minimum delay of asynchronous frames contending for access 73 Hidden Terminal Problem 74 I L39 Virtual Carrier Sensing I Network allocation vector NAV maintains a prediction of future tra c on the medium based on duration information that is announced in RT SC T S frames prior to the actual exchange of data DIFS Source Destination RTS SIFS CTS Data SIFS SIFS ACK Other NAV RTS l DIFS Pontentiun Window r 1 NAV CTS Defer Access Eackoff After Defer 75 LiExposed Terminal 5robem 76 39I I L39aFactors that affect the throughput I Header overhead Iii eg MAC header 09 I I ACK oat I Backoff mechanisms 1 RTSCTS 0 Works best for large packet size I Number of nodes Channel Utilizatvon OF Ihe Standard 802 H DCF I I I I I Average Pay oad 2 S SI Dttimes Average Payoad 50 stonian Average Payload 100 Slouimes rrrr Channel Utilization D I I I I I I I 30 TDD 124 140 169 180 200 Number UV actvve stations 77 Authentication I Open authentication null authentication 1 Request for authentication 2 authentication I Shared key authentication 1 Step 1 shared key is delivered over a secured channel l Step 2 the requester sends an request for authentication frame l Step 3 the responder sends a challenge text generated by WEP l Step 4 the requester copy the challenge text and transmit the new frame after encryption by WEP using share secret key L l Step 5 the responder decrypts the frame using the shared secret key 78 39 I L39 Wired Equivalent Privacy WEP I Goal to prevent eavesdropping from unintended stations ii Encryption EkP C ii Decryption DkC P Key Management Service Key Original Plaintext i Ciphertext Plaintext gtE E avesdropper 79 I WEP Encipherment Initialization Vector IV Secret Key Plaintext Integrity Algorithm Integrity Check Value ICV Message lIII El Concatenation Integrity check value ICV to prevent authorized data modification Initial vector IV can be changed periodically sent in cleartext After encipherment IV frame body ICV I WEP PRNG uses RC4 that takes a finite input and output variable length streams 80 I WEP Decipherment SecretKey I Wt Key Sequence quotquot S d PRNG L Clphnrln r F39Iarntext Irma rit N orithm ICV ICV Message I Secret key distributed earlier through certain key management schemes I IV extracted from packets 81 Problems with WEP I Flipping a bit i CRC is a linear algo l Flipping a bit results changes to a determinstic set of bits flipped in CRC checksum I XOR and short IV field in clear text make possible stochastic attacks ti Ex considertwo streams using the same key amp IV eld by XOR them gt one can get the XOR ofthe text strings I IV is 24 bits a busy access point sending messages at 11Mbps will exhaust the space in 150081110quot62quot24 18000s 5hr I WEP key is shared by all users of the group 82 39 I 39 Physical Media in Original IEEE 80211 I Directsequence spread spectrum 1 Operating in 24 GHz ISM band il Data rates of l and 2 Mbps I Frequencyhopping spread spectrum 1 Operating in 24 GHz ISM band il Data rates of l and 2 Mbps I Infrared U 1 and 2 Mbps 1 Wavelength between 850 and 950 nm 83 39 I 39 IEEE 8021 la and IEEE 8021 lb I IEEE 80211a 1 Makes use of 5GHz band iil Provides rates of 6 9 12 1824 36 48 54 Mbps 1 Uses orthogonal frequency division multiplexing OFDM 31 Subcarrier modulated using BPSK QPSK l6QAM or 64QAM I IEEE 80211b quotA Provides data rates of 55 and 11 Mbps 1 Complementary code keying CCK modulation scheme 84 39 L39 Requirements for WLAN vs IEEE 80211 I Throughput I Number of nodes Channei Ullltzatznn of he Standald 50211 DCF I I I I I Average Paonad 5 S otlimes Average Pay oa lt0 smmmes Average Payload 100 Siotlimes Cha nnel UtIl Izatro n a I I I I I I I I I 20 40 SD 40 160 1GB 200 SD I00 120 Number of acme slanens 85 Requirements for WLAN vs IEEE 80211 Connection to backbone LAN Portal Service area lt 250m Battery power consumption power save mode Transmission robustness amp security WEP lC Colocated network operation 3 channels License free operation yes 39 industrial scienti c and medical ISM band 900MHz 24GHz 58GHz I Roaminghandoff association Dynamic configuration beacon association services 86 Lecture 2 Wireless Channel Propagation amp Modulation Techniques 13 I 1 L Police Radar L udernand39iarg Pnh39cw mm 5 RADAR at mm 39 Rammed Wave 5 RADAR Trammar I 39 I39m ir39r39 x 1 39 I L39 Basics m I Random variable X ll If a probability distribution has density fx then intuitively the infinitesimal interval X X dx has probability fx dx Ll Cumulative distribution function Fm J fxdx foo W Mean EGO jix xwx iii Variance w o2 l xEx2fxdx W Ex Gaussian distribution a E a a a n o Probability Density Function 1 x 2 Zexp 5 gt O39 m fx 72390quot 391 111 L39 Basics Fs j fxeXp i27rxsdx Fourier w Transformation f x T F s expi27z xsds I Time domain I Frequencydomain i m 4quot i i i V i i I Sme39Wave Signal Stuugh T H 11 n I I l I 439 II t x I I II I II I 439 I I 1 J 1 r I 39 I lamp II u pm n n I I k I h I I I v I 39 I I x I I l I II I I quot39 l quot I 1 1 I I t n 39I I l I 2 I l I I I II I 39a l Iquot I I t I I DI I II I I I k I I t 39l I I I II I I I I I 1 l I It I I I t I If R reflection Ex 3e824e9 1250m Di diffraction S Scattering 39 Propagation Model I 30 35 A 8 4o 5 E 45 a 3950 E 55 M 60 65 14 15 16 17 18 19 20 21 22 23 24 25 2539 27 28 T R Separation motor I Largescale propagation model the average received signal strength at a given distance from the transmitter t Useful for estimating the radio coverage area I Smallscale propagation model the variability of the signal strength in close spatial proximity to a particular location or short time durations 18 39 I l V Freespace Model I Friis free space equation i GHGV are the antenna gains at the transmitter and receiver 3 k is the wavelength PGG 12 7 1361 rr r oi d is the distance r 4 2d2L Cl L is a loss factor not related to propagation Free Space Model 7 Path loss PLdB 1010g 1010g 676W 47x2d2 I Only valid beyond farfield distance 212 f 2 df gtgt Ddf gtgt L d Bltdgt11ltddltjgtid2dozdf d D is the transmitter antenna aperture 20 I L Ground Reflection TwoRay Model 39I39tlrulmniuer Elm 105 44 R receiver Figure 47 Tworay ground reflection model Ad d39hhr2d2 h hr2d2 zLZZhr when d is large compared to h h 2 2 h h 1 pr PthGr ta 3 fordgt 207th 31 21 I L Lognormal Shadowing PLddB Em X Ego 10n10gdi X0 X0 is a zeromean Gaussian distributed random variable in dB with standard deviation 6 also in dB Table 42 Path Loss Exponents for Different Environments Environment Path Loss Exponent n Free gt7LtCC 2 Urban urcu cellular rudiu 27 to a i Shudmxcd urbttn ccllulttr rttdio 3 to 5 In building lineaol39wight 111 to 18 Obstructed in building 4 to 6 Obstructed in factories 2 to 3 22 Smallscale Fading I Factors that contribute to smallscale fading II Multipath propagation El Speed of the mobile I Speed of surrounding objects El The transmission I1f 1113 TU Figure 54 An example of the time varying discrete time impulse response model for a multipath radio channel Discrete models are useful in simulation where modulation data must be convolved with the channel impulse response Tra02 23 39 Parameters of Mobile Multipath Channels I Time Dispersion relative to direct lineof sight 1 1 Mean excess delay RMS delay spread Excess delay spread X dB Coherence bandwidth Measures the range of frequencies where the channel can be considered flat 0C 1RMS delay spread l Frequency dispersion l TD 0C fm O 4 l RMS Delay Spread 4640 ns 2 o o in in 3 0 39 L a g A u g 10 M A Maximum ExcessDelaylt1odBB4 ns 2 1 J I E Threshold Laval 20 dB 8 20 A MA A A E W Wquot quot DIV 39W WW 0 E Marin Excess Delay 4505 ns 0 Z 30 50 0 50 100 150 200 250 300 350 400 450 Excess Delay n5 Figure 510 Example of an indoor power delay profile rrns delay spread mean excess delay maximum excess delay 10 dB and threshold level are shown 1 Doppler Shift Geometry X w Figure 51 Illustration of Doppler effect 1 A v cos fd 27r At xi 25 lrl EHxPolice Radar fr 39 5quot Yr Rammed v39 I a Wave r39 f quot RADAR v39 e Tramma39 39 39 quot r I W s uquot g 139 r39 v 39 39rJJr39 P b xq r u 3 39 j I 2v fre eeted 39 flransm illed f f 26 LiTwo independent fading issues SmallScale Fading Based on multipath time delay spread i Flat Fading Frequency Selective Fading 1 BW of signal lt BW of channel 1 BW of signal gt BW of channel 2 Delay spread lt Symbol period 2 Delay spread gt Symbol period SmallScale Fading Based on Doppler spread Fast Fading Slow Fading 1 High Doppler spread 1 Low Doppler spread 2 Coherence time lt Symbol period 2 Coherence time gt Symbol period 3 Channel variations faster than base 3 Channel variations slower than band signal variations baseband signal variations Figure 511 Types of smallscale fading I
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