Tài liệu Lecture-06-intro 802.11

Introduction to 802.11 Easy as a, b and g Dr. Nguyen Tuan Nam [email protected] 802.11 Modes •  Infrastructure (Access Point) mode: –  Communicate between wireless node –  Communicate with wired network –  All through Access Point –  Ex: Coffee shop, corporate WLAN •  Ad hoc: –  Wireless node communicate directly –  No need access point Nguyen Tuan Nam/WNC 2010 2 Infrastructure Mode Dynamic IP Node A Static IP Communication Range? INTERNET Node B Access Point_ Provide connection to wired network _ Relay function _ Not mobile Can node A talk directly to node B without going through Access Point? Nguyen Tuan Nam/WNC 2010 3 When Is Such Infrastructure Not Available? •  Single point of failure •  No single trusted AP •  Special area (jungle) •  Quick deployment (emergency, fire) Nguyen Tuan Nam/WNC 2010 4 Ad hoc Mode Communication Range? Node A Routing? Node B Node C Nguyen Tuan Nam/WNC 2010 5 802.11 (Infrastructure Mode) •  802.11 (1997): –  1 to 2 Mbps –  2.4 GHz –  20m •  802.11a (1999): –  54 Mbps –  5GHz –  15m -> 30m •  802.11b (1999): –  –  –  –  11 Mbps 2.4 GHz 45m à 90m More interference •  802.11g (2003): –  54 Mbps –  2.4 GHz –  45m à 90m •  802.11n (2009) –  600 Mbps –  2.4 GHz or 5GHz –  91m à 182m •  802.11y Nguyen Tuan Nam/WNC 2010 6 Radio Transmission •  FHSS (Frequency Hopping Spread Spectrum): –  Hop from frequency to frequency –  Wide band of frequencies –  FCC: 75+ frequencies, max 400ms/hop •  DSSS (Direct Sequence Spread Spectrum): –  Divides up the spectrum into a number of channels (802.11b) –  A data signal combined with chipping code –  Error can be recovered due to the redundancy of the transmission •  OFDM (Orthogonal Frequency Division Multiplexing): –  Split signal into smaller sub-signals –  Transmitted simultaneously at diff frequencies •  IR (infrared) Differences between FHSS and DSSS? Nguyen Tuan Nam/WNC 2010 7 802.11 Frequency Hopping Spread Spectrum (FHSS) PHY 1, 2 Mbps Direct Sequence Spread Spectrum (DSSS) PHY 1,2 Mbps Higher rate (DSSS) PHY 54 Mbps High rate (DSSS) PHY 11, 5.5 Mbps 802.11g 802.11b Infrared (IR) PHY 1,2 Mbps Orthogonal Frequency Division Multiplexing (OFDM) PHY 6,9,12,18,24,36,48,54 Mbps 802.11a 5.7 GHz 2.4 GHz Nguyen Tuan Nam/WNC 2010 8 802.11b Nguyen Tuan Nam/WNC 2010 9 802.11g •  Backward compatible with 802.11b –  Same channel assignments –  OFDM Nguyen Tuan Nam/WNC 2010 10 802.11a •  5 GHz range –  Less crowded –  Bigger chunk of frequencies –  More room for data on each channel –  More channels –  More non-overlapping channels (8 vs. 3) –  Not as popular as 802.11g. Why? Nguyen Tuan Nam/WNC 2010 11 802.11 MAC •  CSMA/CA •  Interference •  Hidden nodes •  Power Saving •  Privacy and Access Methods Nguyen Tuan Nam/WNC 2010 12 CSMA/CD •  Carrier Sensing Media Access –  Listen for the carrier –  Carrier à Busy, not OK –  No carrier à Available, OK to transmit Nguyen Tuan Nam/WNC 2010 13 Collision Detection •  Detect 2 devices attempt to use data channel simultaneously •  32 bit CRC at end of transmission •  Monitoring its own transmission –  If current above what generated à collision –  Send 32 bit jam sequence instead of CRC Nguyen Tuan Nam/WNC 2010 14 Random Exponential Backoff •  Collision, wait •  Wait how long? –  R x slot_time –  Slot_time = constant –  R = random number between 1 and 2N –  N = min (10, trial attempt) •  Maximum trial = 16 •  Why random? •  Why exponential? •  Why 10? Nguyen Tuan Nam/WNC 2010 15 Ethernet Capture •  If one NIC sends excessive number of frames, it may dominate the LAN •  Some nodes may be locked out •  Why? Nguyen Tuan Nam/WNC 2010 16 CSMA/CD •  No longer use in the 10 Gigabit Ethernet specification Nguyen Tuan Nam/WNC 2010 17 CSMA/CA •  Wireless: –  Avoid but not detect –  Why? –  DCF Busy Medium DIFS R slots Next Frame DIFS = DCF Interframe Space Nguyen Tuan Nam/WNC 2010 18 CSMA/CA with ACK Source needs to send DATA to Dest ACK return immediately DIFS Source No ACK: retransmit after backoff Data SIFS ACK Dest DIFS R slots Others Next Frame SIFS = Short Interframe Space Nguyen Tuan Nam/WNC 2010 19 Problem with CSMA •  Exposed terminal problem •  Hidden terminal problem Nguyen Tuan Nam/WNC 2010 20