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Indoor Wireless LAN (Technology)

Indoor Wireless LAN (Technology). Contents. IEEE 802.11 PHY Spread Spectrum Modulation CCK OFDM IEEE 802.11 MAC Functions IEEE 802.11 MAC Architecture Terminology Frames Operational processes. OSI Reference Model: Phy. Network Oper. System Network Layer Guarantees delivery data

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Indoor Wireless LAN (Technology)

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  1. Indoor Wireless LAN (Technology)

  2. Contents • IEEE 802.11 PHY • Spread Spectrum • Modulation • CCK • OFDM • IEEE 802.11 MAC • Functions • IEEE 802.11 MAC Architecture • Terminology • Frames • Operational processes

  3. OSI Reference Model: Phy • Network Oper. System • Network Layer • Guarantees delivery data • Drivers • LLC Layer • send/receive data • LAN Controller • MAC Layer • data into/out frame • MODEM • Physical Layer • frame into/out phy frame Network Layer IEEE: LLC Layer IEEE: MAC Layer Physical Layer

  4. Wireless LAN technologies (overview) Wireless LAN Technologies Infrared Narrow Band Spread Spectrum Direct Sequence Frequency Hopping

  5. Frequency Spectrum (MHz) 88 2400 103 FM Band Power Power 2400 2500 Frequency Frequency Spread Spectrum Transmission Standard Radio Transmission Wireless LAN technologies (Spread Spectrum) • Unlicensed usage (2.4GHz and 915 MHz ISM band) • No line of sight requirement (indoor) • High link reliability • Built-in transmission security • Two techniques used: • Direct Sequence • Frequency Hopping

  6. COMPLETE WAVEBAND ALLOCATED Time Time Spread Spectrum Technologies (DS vs. FH) • Direct Sequence • Each symbol is transmitted over multiple frequencies at the same time • Very efficient (no overhead) • Higher speed than FH at comparable distances • System capacity (multiple channels) higher than FH • Frequency Hopping • Sequential use of multiple frequencies • Hop sequence and rate will vary • “End hop waste time”

  7. Digital Signal (Bits) Multiplier Source andChannelCoding RFModulator X FrequencySpectrum f Code Bits (Chips) f “Spread” FrequencySpectrum CodeGenerator Spread Spectrum Technologies (Direct Sequence transmitter) • Spreading: Information signal (I.e. a “symbol”) is multiplied by a unique, high rate digital code which stretches (spreads) its bandwidth before transmission. • Code bits are called “Chips”. • Sequence is called “Barker Code”

  8. Symbol time ts “1” “0” “symbol” X = “Barker” sequence Result of multiplication Chip time tc 22 Mhz 2 Mhz Spread Spectrum Technologies (“spreading”) • Due to the multiplication of a symbol with Barker code, the “rate-of-change” increases with a factor 11 • This means that cycle rate increases from 1 MHz to 11 MHz • In terms of spectrum this means that after RF modulation the signal is spread from 2 MHz bandwidth to 22 MHz bandwidth

  9. Digital Signal (Bits) Multiplied RFDemodulator ChannelandSourceDecoding X De-SpreadSignal f “Spread” FrequencySpectrum f Code Bits (Chips) CodeGenerator Spread Spectrum Technologies (Direct Sequence receiver) • At the receiver, the spread signal is multiplied again by a synchronized replica of the same code, and is “de-spread” and recovered • The outcome of the process is the original “symbol”

  10. Direct Sequence Spread Spectrum Signal : 11 chip code Data Symbol time Spread Spectrum Technologies (De-spreading) • When the incoming signal is de-spread, it results in either a positive (+) or a negative (-) “spike” • These “spikes” arrive at intervals equal to the symbol time • A positive spike represents a “1” symbol, a negative spike represents a “0” symbol

  11. Symbol time Spread Spectrum Technologies (effect of echoes) • Echoes may arrive at the receiver, fluctuations can be noticed at positions other than at the symbol time boundaries • These fluctuations are ignored as the receiver will only interpret the spike at the synchronization points (separated from each other by the symbol time)

  12. Modulation - DBPSK (Differential Binary Phase Shift Keying)

  13. Modulation - DQPSK (Differential Quadrature Phase Shift Keying)

  14. CCK CCK = Complementary Code Keying • IEEE 802.11 standard for high speed • 11 and 5.5 Mbps data rates • Outstanding high multi-path performance • Outstanding low-SNR performance • Seamless interoperability with existing DS • Maintains QPSK chips at 11 MHz chip rate • Maintains 3 frequency channels • FCC and MKK regulations satisfied

  15. 5.5 MBps CCK 2 bits encoded to 4 complex code words; 2-QPSK 11 MBps CCK 8 chips 6 bits encoded to 64 complex code words; 2-QPSK 8 chips CCK - How it works • Data bits are encoded to a symbol which is transmitted in the form of 8 chips • For Data-Rate = Medium Encoding means: • mapping 2 data bits to I or Q channel (in-Phase, Quaternary Phase) • mapping 2 data bits to one of 4 Complex Codewords • For Data-Rate = High Encoding means: • mapping 2 data bits to I or Q channel (in-Phase, Quaternary Phase) • mapping 6 data bits to one of 64 Complex Codewords • Codewords are complex complementary codes selected from a code set

  16. CCK - Operating at medium speed I OUT Pick One of 4 Complex Codes * 2 1 Q OUT 1 DATA IN 1 MUX Scrambler 1:8 *= Code Set: 747B 47B7 8B7B B8B7 see next slide 1 1.375 MHz 8 chips clocked with 11 MHz 11 MHz Data Rate = 4 bits/symbol * 1.375 MSps = 5.5 MBps

  17. CCK - Operating at high speed I OUT Pick One of 64 Complex Codes 6 1 Q OUT 1 DATA IN 1 MUX Scrambler 1:8 1 Code Set is defined by formula: 1.375 MHz 11 MHz Data Rate = 8 bits/symbol * 1.375 MSps = 11 MBps

  18. CCK - Data rates and symbol rates Bit-rates: • The 11 chips Barker sequence in Standard DSSS carries one symbol clocked at 1MHz, which results in a symbol rate of 1Msymbol/sec. • The 8 chips sequence in CCK clocked at 1 MHz, results in a symbol rate of 1.375 Msymbol/sec (I.e. 11/8) • At date rate = medium, 4 data bits are mapped on one symbol, which results in 5.5 Mbps (I.e. 1.375 * 4) • At date rate = high, 8 data bits are mapped on one symbol, which results in 11 Mbps (I.e. 1.375 * 8)

  19. 11 MBps CCK 802.11 DSSS BPSK 802.11 DSSS QPSK 5.5 MBps CCK 1 MBpsBarker BPSK 2MBps Barker QPSK 6 bits encoded to 64 complex code words; 2-QPSK 2 bits encoded to 4 complex code words; 2-QPSK 1 bit used to BPSK code word 2 bits used to QPSK code word I, Q I, Q I, Q I, Q 11 chips 11 chips 8 chips 8 chips 1 MSps 1 MSps 1.375 MSps 1.375 MSps CCK - From DSSS BPSK to 11 Mbps CCK

  20. 5180 5220 5260 5300 5350 MHz 5150 5200 5240 5280 5320 OFDM • Orthogonal Frequency Division Multiplexing (OFDM) modulation • Each “channel” composed out of 52 sub-carriers of which 48 can carry information • Each sub-carries is in fact a narrow band transmission • Data rates • 54, 48, 36, 24, 18, 12, 6 Mbit/s; auto fall back Eight channels in lower 5 GHz band 52 Carriers total ... 20 MHz One Channel (detail) Each carrier is ~300kHz wide

  21. 90 0 Vector: 90 90 Angle = Phase Length = Amplitude 11 01 Decision Decision 180 Decision Decision 0 180 0 BPSK QPSK Thresholds Thresholds Thresholds Thresholds 0 180 180 0 00 10 1 Example 270 16 QAM 64 QAM Example 270 270 OFDM - modulation techniques • Different modulation techniques used to map bits to symbols • QAM = Quadrature Amplitude Modulation • QPSK = Quadrature Phase Shift Keying • BPSK = Binary Phase Shift keying • Data capacity per modulated sub-carrier: • BPSK - 1 bit (2 decision points) • QPSK - 2 bits (4 decision points) • 16 QAM - 4 bits (16 decision points) • 64 QAM - 6 bits (64 decision points • Selected modulation technique depends on distance between stations and environmental conditions

  22. Data_rate = Modulation * Coding_rate * Symbol_rate * #-of_sub-carriers Data_rate Modulation Coding_rate Measured throughput 54 64QAM 3/4 18 48 64QAM 2/3 18 36 16QAM 3/4 16 24 16QAM 1/2 13.6 18 QPSK 3/4 Data not available 12 QPSK 1/2 8.4 9 BPSK 3/4 Data not available 6 BPSK 1/2 4.6 OFDM - coding rates (FEC) • Certain modulated patterns are avoided to allow Forward Error Correction (FEC) under varying conditions, by applying “Convolutional coding with Viterbi decoding” • Coding Rates (R) • 1/2 for every 2 bits, 1 bit is data, 1 bit is FEC • 2/3 for every 3 bits, 2 bits are data, 1 bit is FEC • 3/4 for every 4 bits, 3 bits are data, 1 bit is FEC • Trade-off between data rate and robustness • 1/2 coding provides more redundancy but slower data rates than 3/4 coding • Symbol rate is 0.25 Msymbols/second • Data rates • 54, 48, 36, 24, 18, 12, 6 Mbit/s; auto fall back

  23. Band Operating Channel Numbers Channel Center Frequencies U-NII lower band 5.15-5.25 GHz 36 40 44 48 5,180 MHz 5,200 MHz 5,220 MHz 5,240 MHz U-NII middle band 5.25-5.35 GHz 52 56 60 64 5,260 MHz 5,280 MHz 5,300 MHz 5,320 MHz OFDM - channels • 8 channels in lower and middle U-NII band • IEEE 802.11a standard also defines upper U-NII band (5.725-5.825 GHz), which is not yet supported by this kit • 20 MHz spacing between center frequencies of adjacent channels • Channel width: • 22 MHz @ -20dBr (first sidelobes overlap) • 40 MHz @ -28dBr (second sidelobes overlap) • 60 MHz @ -40dBr (full separation) • First and last carriers 30 MHz from band edges to get enough out-of-band attenuation (-57 dB FCC requirement)

  24. OFDM - 2XTM Mode • Doubles the data rate of 802.11 standard • Up to 108 Mbps • 3 Channels available in low-mid U-NII band • Channel 42 (5.21 GHz), 50 (5.25 GHz), 58 (5.29 GHz) • Each Channel is 40 MHz • Twice the size of a standard 802.11a channel • Not compliant with 802.11a standard • User-selectable • All devices must be same mode (802.11a or 2X) to communicate

  25. 6-54 Mbps 1-11 Mbps OFDM - distances compared to 2.4 GHz

  26. Contents • IEEE 802.11 PHY • Spread Spectrum • Modulation • CCK • OFDM • IEEE 802.11 MAC • Functions • IEEE 802.11 MAC Architecture • Terminology • Frames • Operational processes

  27. 4-22 MHz HERMES Chip RADIO MODEM MDI P H O S T M A C C B u f f e r & THESEUS MMI M I / F C o n t r o l F r a g m e n t D I S C C F u n c t i o n M a n a g e m e n t RADIO I H W GPSIO A F l a s h E P R O M S e r i a l E E P r o m S R A M BootFlash 128K*8 m i n 3 2 K B , m a x 2 M B Implementation of IEEE 802.11 • Digital Signal Processor (Theseus) • IEEE 802.11 MAC chip (Hermes)

  28. 4-22 MHz HERMES Chip RADIO MODEM MDI P H O S T M A C C B u f f e r & THESEUS MMI M I / F C o n t r o l F r a g m e n t D I S C C F u n c t i o n M a n a g e m e n t RADIO I H W GPSIO A F l a s h E P R O M S e r i a l E E P r o m S R A M BootFlash 128K*8 m i n 3 2 K B , m a x 2 M B Implementation of IEEE 802.11 • Digital Signal Processor (Theseus) • IEEE 802.11 MAC chip (Hermes)

  29. 4-22 MHz HERMES Chip RADIO MODEM MDI P H O S T M A C C B u f f e r & THESEUS MMI M I / F C o n t r o l F r a g m e n t D I S C C F u n c t i o n M a n a g e m e n t RADIO I H W GPSIO A F l a s h E P R O M S e r i a l E E P r o m S R A M BootFlash 128K*8 m i n 3 2 K B , m a x 2 M B Implementation of IEEE 802.11 • Protocol functions programmed in FW, so flexible. • For use in station and access points (additional FW loaded when operating as access point) • Functions can be added over time, via upgrade utilities

  30. IEEE 802.11 features • Sharing Medium • ACK protocol • Medium reservation (RTS/CTS) • Fragmentation • Multi-channel roaming • Automatic data-rate fall-back • Cell size / Multi-rate applications • In-cell relay • Power Management • Wired Equivalent Privacy (WEP) • Wireless Distribution System (WDS)

  31. CRS defer CRS defer CRS CRS collision Sharing the medium - The way Ethernet works (CSMA/CD) • Adapters that can detect collisions (e.g. Ethernet adapters) • Carrier Sensing: listen to the media to determine if it is free • Initiate transmission as soon as carrier drops • When collision is detected station defers • When defer timer expires: repeat carrier sensing and start transmission station A station B station C

  32. backoff backoff CRS CRS backoff (rest) defer CRS CRS Sharing the medium - Coordinating access using CSMA/CA • Wireless LAN adapters cannot detect collisions, so different coordination schemes have to be devised • DCF (Distributed Coordination Function) • Implemented as CSMA/CA (Carrier Sensing Multiple Access with Collision Avoidance) • Contention based (using “random” back-off timers to resolve contention) • ORiNOCO systems implement DCF station A station B station C

  33. Sharing the medium - Coordinating access using PCF Contention Free Repetition Interval Contention Free Period • PCF (Point Coordination Function) • Optional additional medium access control method • Contention free operation with single Point Coordinator in a cell (typically residing the AP) • Point Coordinator controls the medium by polling stations in the BSS • ORiNOCO systems do not implement PCF but are sensitive for PCF presence SIFS SIFS SIFS PIFS SIFS Contention Period PC beacon D1+poll D2+Ack+ Poll D3+Ack +Poll D4 +poll STA U1 + Ack U2+ Ack U4+ Ack No response to CD-Poll CF End PIFS SIFS SIFS SIFS Reset NAV NAV CF_Max_Duration

  34. Free access when medium DIFS is free longer than DIFS Contention Window PIFS DIFS SIFS Backoff-Window Next Frame Busy Medium Slot time Select Slot and Decrement Backoff as long as medium is idle. Defer Access Sharing the medium - Inter-Frame Spacing • Inter frame spacing required for MAC protocol traffic • SIFS = Short interframe space • PIFS = PCF interframe space • DIFS = DCF interframe space • Back-off timer operates in the contention window • Back-off time is expressed in terms of number of time slots

  35. DIFS Src Data SIFS Dest Ack DIFS Contention Window Other Next MPDU Backoff after Defer Defer Access Sharing the medium - Sharing the medium with low level ACKs • Collisions still can occur (interference; incapability of sensing other’s carrier) • IEEE 802.11 defines “low-level” ACK protocol • Provides faster error recovery • Makes presence of high level error recovery less critical • Acknowledgment are to arrive at within the SIFS • The DCF interframe space is observed before medium is considered free for use

  36. A B C A sends to B C doesn’t detect that, so C might also start sending to B Collision of messages at B: both messages lost “Hidden stations” - the problem • Situation that occurs in larger cells (typical outdoor) • Loss of performance • Error recovery required

  37. “Hidden stations” - the solution A B C • IEEE 802.11 defines: • MAC level RTS/CTS protocol (Request to Send / Clear to Send) • Can be switched off to reduce overhead (when no hidden nodes exist) • More robustness, and increased reliability • No interruptions when large files are transmitted RTS: I want to send to B 500 bytes CTS: OK A, go ahead, so everybody quiet Data: the 500 bytes of data from A to B ACK: B received the data OK, so an ACK

  38. Hit A hit in a large frame requires re-transmission of a large frame Fragmenting reduces the frame size and the required time to re-transmit Fragmentation • IEEE 802.11 defines: • MAC level function to transmit large messages as smaller frames (user definable) • Improves performance in RF polluted environments • Can be switched off to avoid the overhead in RF clean environments

  39. Channel 11 Channel 1 Channel 6 Channel 1 Multi-channel roaming • ORiNOCO IEEE 802.11 systems, support multi-channel roaming • Access points are set to a fixed frequency • Stations do not need to be configured for a fixed frequency • Stations switch frequency when roaming between access points • Stations “associate” dynamically to the access point with best signal, on power on

  40. Automatic rate select • ORiNOCO PC Card, dynamically switches data-rate • Fall back to lower data-rate when communications quality decreases • out of range situations • Interference • Fall-back scheme: • 11 Mbps, 5.5 Mbps, 2 Mbps, 1 Mbps (802.11b devices) • 54, 48, 36, 24, 18, 12, 6 Mbit/s; auto fall back (802.11a devices) • ORiNOCO PC Card in APs is capable of supporting different data-rates “simultaneously”: • e.g. operates at “High” speed in communication to nearby station and at “Low” speed to station that is further away. • Data rate capability is maintained in “station association table” • Speed of IEEE Management - and Control frames use fixed speed determined as “IEEE Basic Rates”, and controlled by “Multi-cast Rate parameter”.

  41. Cell size / Multi Rate applications • Cell-size can be influenced by “Distance between APs” parameter: • Large • Medium • Small • Mini • Micro • Only for 802.11b devices (not for 802.11a radios) • Cell-size influences capacity per station in the cell • small cell physically accommodates smaller number of stations than large cell • bandwidth per station in small cell greater than in large cell • Cell size influences data-rate • larger distance between station and access-point may lead to lower data-rate

  42. 11 Mbits/sec 1 Mbits/sec Cell size / Multi Rate applications • Mixture of cell-sizes accommodate mixed applications: • Office workers: • High physical station density • High bandwidth requirement • Small cell operating at high data rate • Distance between APs is small • Warehouse operations (such as forklift truck) • Low physical station density • Low bandwidth requirement (transaction processing) • Large cell operating at low data rate • Distance between APs is large

  43. Power Management • IEEE 802.11, supports power management: • nothing to send: station in sleep mode • out-bound traffic stored in Access Point (out-bound = from AP to STA) • station wake up only for Traffic Information Map (TIM) • if messages: stay awake to receive them • This implies: • Prolonged battery life • Increase usability in hand-held equipment • Works best in application that have limited bandwidth requirements (transaction processing)

  44. Wired Equivalent Privacy • Optional security functionality (factory “installed”) • Encryption based on RC4 (1988 RSA algorithm) • Stream cipher 64 or 128 bits key • User defined keys can be 40 or 104 bits long • 24 bits varying for each packet called the IV (Initialization vector) • Used for data encryption • Used for shared key station authentication • ORiNOCO’s FW inside the PC Card (implementing WEP) contains unique protection against so-called “Weak Key” attacks • Sniffing (key capturing) programs such as AirSnort will are ineffective in “stealing” encryption keys

  45. Channel 11 Channel 1 Channel 6 Wireless Distribution System • IEEE 802.11, WDS means • Multiple (7) wireless “ports” inside the access-point for wireless operations • 1 port can be assigned to connect Wireless Stations • Up to 6 ports can be used to connect wirelessly to other Access Points • All done by one Wireless PC Card in the Access Point • All wireless links operate on the same channel • WDS allows: • Extending the existing infrastructure with wireless backbone links • Totally wireless system without any wired backbones, needed in locations where large areas are to be covered and wiring is not possible • Only implemented in 802.11b radios (not in 802.11a)

  46. Contents • IEEE 802.11 PHY • Spread Spectrum • Modulation • CCK • OFDM • IEEE 802.11 MAC • Functions • IEEE 802.11 MAC Architecture • Terminology • Frames • Operational processes

  47. PC-Card Hardware Radio Hardware 802.11 frame format WMAC controller with Station Firmware (WNIC-STA) 802.3 frame format Platform Computer Driver Software (STADr) Ethernet V2.0 / 802.3 frame format Protocol Stack IEEE 802.11 Terminology - STA (Station) Station (STA) Architecture: • Device that contains IEEE 802.11 conformant MAC and PHY interface to the wireless medium, but does not provide access to a distribution system • Most often end-stations available in terminals (work-stations, laptops etc.) • Implemented in ORiNOCO IEEE 802.11 PC-Card • Ethernet-like driver interface • supports virtually all protocol stacks • Frame translation according to IEEE Std 802.1H • IEEE 802.3 frames: translated to 802.11 • Maximum Data limited to 1500 octets • Transparent bridging to Ethernet

  48. PC-Card Hardware Radio Hardware 802.11 frame format WMAC controller with Access Point Firmware (WNIC-AP) 802.3 frame format Bridge Software Driver Software (APDr) Ethernet V2.0 / 802.3 frame format Kernel Software (APK) Bridge Hardware Ethernet Interface IEEE 802.11 Terminology - AP (Access Point) Access-Point (AP) Architecture: • Device that contains IEEE 802.11 conformant MAC and PHY interface to the wireless medium, providing access to a distribution system for associated stations • Most often infra-structure products that connect to wired backbones • Implemented in ORiNOCO IEEE 802.11 PC-Card inserted in AP-500, AP-1000, AP-2000 • STAs select an AP and “associate with it • APs : • Support roaming • Provide time synchronization (beaconing) • Provide Power Management support

  49. BSS IEEE 802.11 Terminology - Basic Service Set (BSS) Basic Service Set (BSS): • A set of stations controlled by a single “Coordination Function” (=the logical function that determines when a station can transmit or receive) • Similar to a “cell” in Cellular network terminology • A BSS can have an Access-Point (both in standalone networks and in building-wide configurations), or can run without and Access-Point (in standalone networks only) • Station-to-Station traffic is relayed by the Access Point

  50. IBSS IEEE 802.11 Terminology - Independent Basic Service Set (IBSS) Independent Basic Service Set (IBSS): • A Basic Service Set (BSS) which forms a self-contained network in which no access to a Distribution System is available • A BSS without an Access-Point • Station-to-station traffic flows directly without any relay action • All stations in the cell will be able to receive frames transmitted by another station in the cell (filtering of traffic for subsequent processing is based on MAC address of the receiver)

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