Wireless Ethernet
802.11 Presentation 2




                        1
802.11 Physical Layer
   Frequency Allocation 802.11
   Direct-Sequence (DS) spread-spectrum radio
   Frequency-hopping...
Frequency-Hopping
 79 channels from 2.402 to 2.480 GHz with 1
  MHz channel space
 78 hopping sequences with minimum 6 M...
Direct Sequence Spread Spectrum
 Spreads signal by expanding the signal over a broad portion of the radio band.
 DBPSK D...
Orthogonal Frequency Division
Multiplexing
 OFDM – Uses all 48 channels.
 Each Parallel bit stream is modulated differen...
MAC Access Modes
 DCF – Distributed coordination function.
 PCF – Point coordination function.




   Image acquired fr...
Data Link Layer 802.11
   Two sub layers: Logical Link Control (LLC), Media Access Control (MAC)
   Same 802.2 LLC and 4...
802.11 MAC Frame
    NOTE: This frame structure is common for all data sent by a 802.11 station

      control info (WEP,...
Wired Equivalent Privacy
   Implemented at the MAC layer.
   RC4 Stream Cipher
   Provides encryption to data frames on...
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802 11 2

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Transcript of "802 11 2"

  1. 1. Wireless Ethernet 802.11 Presentation 2 1
  2. 2. 802.11 Physical Layer  Frequency Allocation 802.11  Direct-Sequence (DS) spread-spectrum radio  Frequency-hopping(FH) spread-spectrum radio  Infrared Light (IR) Image acquired from http://www.pulsewan.com/data101/802_11_b_basics.htm 2
  3. 3. Frequency-Hopping  79 channels from 2.402 to 2.480 GHz with 1 MHz channel space  78 hopping sequences with minimum 6 MHz hopping space, each sequence uses every 79 frequency elements once 3
  4. 4. Direct Sequence Spread Spectrum  Spreads signal by expanding the signal over a broad portion of the radio band.  DBPSK Differential Binary Phase Shift Keying  Ideal for high bandwidth apps.  Uses higher power to transmit and more expensive to implement compared to FHSS. Image acquired from http://www.wireless-telecom.com/images/dsss_pic.gif  4
  5. 5. Orthogonal Frequency Division Multiplexing  OFDM – Uses all 48 channels.  Each Parallel bit stream is modulated differently; BPSK QPSK or QAM.  Multipath propagation environment. mage acquired from http://iaf-bs.de/img/con/projects/3glte-mimo-ofdm-testbed/3GLTE_Testbed_fading_adapt.jpg 5
  6. 6. MAC Access Modes  DCF – Distributed coordination function.  PCF – Point coordination function.  Image acquired from www.cisco.com/.../ 119001-120000/119161.jpg  www.cisco.com/.../ 119001-120000/119161.jpg rich phelps 802.11 framing 6
  7. 7. Data Link Layer 802.11  Two sub layers: Logical Link Control (LLC), Media Access Control (MAC)  Same 802.2 LLC and 48-bit MAC addressing as for all other 802 LANs  MAC layer similar but different to Ethernet 802.3  Similarities: Contention-based ◦ Medium is free for all ◦ A node senses the free medium and occupies it as long as data packet requires it  Differences: CSMA/CD vs CSMA/CA, CRC, packet fragmentation  Ethernet 802.3 - CSMA/CD (Carrier Sense Multiple Access / Collision Detect) ◦ Collision detection possible because can send/receive simultaneously  Ethernet 802.11 - CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) ◦ Collision detection not possible because of near/far problem for radios ◦ Steps: sense the air, wait, send, wait for ACK, retransmit if needed ◦ Optional RTS/CTS feature to solve “hidden node” problem  “Hidden node”: where two stations on either side of an AP can’t hear each other broadcast.  CRC – error checking at layer 2 (Ethernet assumes this is layer 3 task)  Packet fragmentation: breaking large packets into smaller ones at MAC layer  Overhead of MAC layer reduces throughput by 40-50% 7
  8. 8. 802.11 MAC Frame  NOTE: This frame structure is common for all data sent by a 802.11 station control info (WEP, data type as management, control, data ...) frame ordering next frame duration info for RX -Basic service identification frame specific, -source/destination address variable length -transmitting station -receiving station frame check sequence (CRC) 8
  9. 9. Wired Equivalent Privacy  Implemented at the MAC layer.  RC4 Stream Cipher  Provides encryption to data frames only http://i.msdn.microsoft.com/Aa503279.Native_802_11_wep(en-us,MSDN.10).gif http://www.cs.wustl.edu/~jain/cse574-06/ftp/wireless_security/fig14.gif 9

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