MOB 4 wireless LAN 2010 lập trình mobi

18 Introduction IEEE 802.11  Frequencies are chosen in the 2,4 GHz Band as for Bluetooth technology – No need for licensing – Band in not completely free in many countries  Commu

Internet & Mobile Communications

2

Need for standardisation

  Mobility  frequency regulation between

countries is necessary : common frequency band

  Limitation of battery usage

  Limitation of interferences among different

equipments (antennas can help)

  configuration as seamless as possible

  Compatibillity with existing LAN technologies

  Seamless for users and applications (location aware applications…)

3

Characteristics of wireless LANs

  Advantages

–  very flexible within the reception area

–  Ad-hoc networks without previous planning possible

–  (almost) no wiring difficulties (e.g historic buildings, firewalls)

–  more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug

4

Design goals for wireless LANs

–  global, seamless operation

–  low power for battery use

–  no special permissions or licenses needed to use the LAN

–  robust transmission technology

–  simplified spontaneous cooperation at meetings

–  easy to use for everyone, simple management

–  protection of investment in wired networks

–  security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low

radiation)

–  transparency concerning applications and higher layer protocols, but also location awareness if necessary

7.2.1

–  coverage of larger areas possible (radio can

penetrate walls, furniture etc.)

–  WaveLAN, HIPERLAN, Bluetooth

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7

Wifi Research

  Research shows 802.11 (aka Wi-Fi)

is becoming an essential part of our everyday lives

–  Research in the US reveals some factoids about the impact of Wi-Fi …

  80% say Wi-Fi is more essential than their iPod

  81% would rather see their favourite team lose than give up Wi-Fi for a week

  90% would rather do without their daily Starbucks than their Wi-Fi

–  … the bottom line is that Wi-Fi is affecting real people in their everyday lives

Source: WFA/Kelton Research, July & October 2006

Andrew Miles - CISCO Andrew Miles - CISCO

8

Wifi Research

  Wi-Fi has become popular based on products that are not optimised for wireless use …

–  with apologies to all those

Andrew Miles - CISCO

9

Wifi Research

  … and yet the Wi-Fi market reached over 200 millions chipsets per year

in 2006 –  In 1997, the first IEEE 802.11 standard was ratified

–  In 2005, over 150 million Wi-Fi devices were sold

–  In 2006, over 200 million Wi-Fi devices were sold (33% growth rate)

Enterprise APsHome/SOHOCE

  Wi-Fi is now being implemented in a wide

variety of more interesting devices …

Andrew Miles - CISCO

12

Wifi Research

  … with some of the new devices actually used for voice (and not just data) …

–  Numerous Wi-Fi carrier voice and data deployments are underway, and others expected during 2007 and

2008

–  Wi-Fi with UMA is the predominant voice approach today, although SIP solutions also exist

–  Examples of voice deployments include:

  BT Fusion:voice & data in the home/office network at more than 2,000 Openzone hotspots

  Orange Unik for Professionals: provides Wi-Fi to GSM handoff, and includes unlimited use for Wi-Fi calls

  NTT DoCoMo: SIP-based voice for large enterprise customers

Andrew Miles - CISCO

–  143k+ hot spots in 132 countries

  Source: JiWire (12 March 2007)

  Other sources indicate 200k+ hot spots

–  500+ muni deployments in 29 countries

  Source: Tropos & WFA

–  82% of US hotels offer Wi-Fi

  Source: American Hotel & Lodging Assn

Melbourne

Andrew Miles - CISCO

14

Wi-Fi Research

0 100

Phones PCs

  … which means the promise of one billion chipsets sold in a year might not be far off!

–  Both CE and Voice are forecast to make a big impact by 2010

–  They will enable even more use of Wi-Fi both in all market segments

–  One billion chipsets is forecast by

2012

CE Voice Not far from 1B!

Andrew Miles - CISCO

Internet & Mobile Communications

–  Token Bus (IEEE 802.4)

–  Token Ring (IEEE 802.5)

17

Introduction IEEE 802.11

  1990: launching of the project to create a

wireless LAN - WLAN (Wireless Local Area Network)

–  Goals:

  to offer a wireless connectivity to fixed workstations of mobile workstations

  to allow fast deployment inside a local area

  To permit the use of different frequency bands

–  2001: publication of the first International standard for wireless LAN developped by the IEEE

organisation

18

Introduction IEEE 802.11

  Frequencies are chosen in the 2,4 GHz Band (as for Bluetooth technology)

–  No need for licensing

–  Band in not completely free in many countries

  Communications

–  Can be direct from terminal to terminal

  It is then impossible to relay frames from one terminal to another

–  With an Access Point which relays all the traffic

  Transmission rates vary depending on coding technics which are used and of the bandwidth allocated

19

Introduction IEEE 802.11

  Access Method to the physical layer (MAC

protocol - Medium Access Control)

–  Quite complex

–  Many available options on the radio interface

–  Access Technics derived from CSMA/CD

  Carrier Sense Multiple Access/Collision Detection, used to define the access in wired networks

  Problem: In wireless networks it is impossible to detect collisions as for Ethernet LANs

–  Introduction of a new protocol : CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)

20

IEEE 802.11

  802.11 - Main Standard (june 1997)

–  Le groupe de travail concentre maintenant ses efforts pour produire des standards pour des WLAN à grande vitesse

  802.11x - Amendements

–  802.11b - speed up to 11 Mbits/s (ISM band)

–  802.11a - speed up to 54 Mbits/s (UN-II band)

–  802.11g - speed up to 54 Mbits/s (bande ISM)

–  802.11h – dynamic selection of frequencies and power control (UN-II band)

–  802.11e - Quality of service

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Comparison: infrastructure vs ad-hoc networks

infrastructure network

ad-hoc network

AP

AP

AP wired network

AP: Access Point

802.11 - Architecture of an infrastructure network

  Station (STA)

–  terminal with access mechanisms to the wireless medium and radio contact to the access point

  Basic Service Set (BSS)

–  group of stations using the same radio frequency

  Access Point

–  station integrated into the wireless LAN and the distribution system

on several BSS

STA1

ESS

24

802.11 - Architecture of an ad-hoc network

  Direct communication within a limited range –  Station (STA):

terminal with access mechanisms to the wireless medium

–  Independent Basic Service Set (IBSS):

group of stations using the same radio frequency

802.11 LAN

IBSS2

802.11 LAN

IBSS1STA1

STA4

STA5STA2

STA3

26

Equipments: 802.11 cards

27

Equipments: Access points

28

Equipments: Antennas

802.11 PHY 802.11 MAC

IP

802.3 MAC 802.3 PHY

application TCP

802.3 PHY 802.3 MAC

IP

802.11 MAC 802.11 PHY

LLC

infrastructure network

30

802.11 - Layers and functions

  PLCP Physical Layer Convergence Protocol

–  clear channel assessment signal (carrier sense)

  PMD Physical Medium Dependent

PMD PLCP MAC

LLC

MAC Management PHY Management

  MAC

–  access mechanisms, fragmentation, encryption

  MAC Management

–  synchronization, roaming, MIB, power management

7.8.1

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32

Frequency bands for 802.11x

  For 802.11, Wi-Fi (IEEE 802.11b) and 802.11g

–  ISM Band (Instrumentation, Scientific, Medical) - No licence required - 2,4 GHz

–  Bandwidth: 83 MHz

  For Wi-Fi5 (IEEE 802.11a)

–  UN-II Band - without Licence in the 5,2 GHz

–  Bandwidth: 300 MHz in the US/ France recently

33

Reglementation of the ISM band

2,400 – 2,4835 GHz 2,471 – 2,497 GHz 2,400 – 2,4835 GHz 2,400 – 2,485 GHz

Frequencies Band Country

5) 2432

6) 2437 7) 2442

8) 2447 9) 2452

10) 2457

11) 2462 12) 2467

35

Reglementation in the ISM band

  ISM Band

–  Band divided in 14 channels of 20 MHz each

–  Transmission (between sender and receiver) operated only on 1 channel

–  Co-localisation of 3 networks in a same space

13

83 MHz

37

802.11 - Physical layer

  3 versions: 2 radio (typ 2.4 GHz), 1 IR

–  data rates 1 or 2 Mbit/s

  FHSS (Frequency Hopping Spread Spectrum)

–  spreading, despreading, signal strength, typ 1 Mbit/s

–  min 2.5 frequency hops/s (USA), two-level GFSK modulation

  DSSS (Direct Sequence Spread Spectrum)

–  DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK)

–  preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s

–  chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)

  Infrared

–  850-950 nm, diffuse light, typ 10 m range

–  carrier detection, energy detection, synchonization

–  synch with 010101 pattern

  SFD (Start Frame Delimiter)

–  0000110010111101 start pattern

  PLW (PLCP_PDU Length Word)

–  length of payload incl 32 bit CRC of payload, PLW < 4096

  PSF (PLCP Signaling Field)

–  data rate of payload (1 or 2 Mbit/s)

  HEC (Header Error Check)

–  synch., gain setting, energy detection, frequency offset compensation

  SFD (Start Frame Delimiter)

–  1111001110100000

  Signal

–  data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)

  Service Length

–  future use, 00: 802.11 compliant length of the payload

–  protection of signal, service and length, x16+x12+x5+1

41

802.11b –physical layer

  ISM Band

  Based on DSSS

spread spectrum with direct sequence

  Throughput between 1 and 11 Mbits/s

  Mechanism of throughput variation depending

on the quality of the radio environment

43

SYNC 56-bits

SDF 16-bits

802.11b – Physical Layer

SYNC 128-bits

SDF 16-bits

signal 8-bits

Service 8-bits

longueur 16-bits

CRC

Long Preamble

(Scrambled 1s)

DQPSK : 2 Mbit/s 5,5 à 11 Mbit/s

signal 8-bits

Service 8-bits

llengthr 16-bits

CRC

Short Preamble

(Scrambled 0s)

5,5 à 11 Mbit/s DQPSK à 2 Mbit/s

PPDU PPDU PLCB Preamble PLCB header

Fréquencies

IEEE 802.11a – Physical Layer

–  In the US: Frequency of 5 GHz in the UNII Band

  Unlicensed National Information Infrastructure : no need for a license

  8 channels in the low frequencies - 4 for the higher ones

–  In Europ:

  Band of 5,15 to 5,35 GHz: 8 possible channels

  Band of 5,47 to 5,735 GHz : 11 possible channels

  Not authorised outside in France without a resquest to the French ART regulation department:

–  This frequency band is used for the army, meteorological aironautic radars

–  It uses a dynamic method of frequencies selection which is not authorised in France/Europe

One OFDM channel

47

OFDM

–  > give a unique high speed channel

  No overlapping of disjoint channels

>  8 IEEE 802.11a networks (instead of 3 networks 802.11b)

disadvantages

  OFDM requires more power than spread spectrum technics

  At higher speed the loss probability increases

49

802.11g

  Common points and differences between b and g

–  Common points

  Signal takes about 20MHz

–  Differences :

  OFDM Modulation added to the basic 802.11b modulations

(versus 5MHz for 802.11b) to operate without too many interferences

50

802.11g

  Throughputs and modulations

51

IEEE 802.11e

  Improvement of the quality by introducing

–  Some quality of service

–  Security and authentication functionnalities

  Aim: to have VoIP and multimedia data sent

over the shared network

–  Definition of classes of service

–  Terminals choose the right priority depending on the type of application to transmit

52

802.11a vs 802.11b

  ISM Band becomes more and more saturated (802.11b, 802.11g, Bluetooth, etc.)

  More possible Co-location in 802.11a

  Better throughputs for 802.11a but smaller

coverage area

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802.11 - MAC layer I - DFWMAC

  Traffic services

–  Asynchronous Data Service (mandatory)

  exchange of data packets based on “best-effort”

  support of broadcast and multicast

–  Time-Bounded Service (optional)

  implemented using PCF (Point Coordination Function)

  Access methods

  collision avoidance via randomized „back-off“ mechanism

  ACK packet for acknowledgements (not for broadcasts)

–  DFWMAC-DCF w/ RTS/CTS (optional)

  Distributed Foundation Wireless MAC

  avoids hidden terminal problem

–  DFWMAC- PCF (optional)

  access point polls terminals according to a list 7.12.1

55

Mac sub layer of IEEE 802.11

  2 modes:

–  only ad-hoc Mode: DCF

–  Infrastructure mode both with DCF and PCF

  Distributed Coordination Function (DCF)

–  Contention-based access method

  Point Coordination Function (PCF)

–  Contentionless access method

inter-–  Time Intervals between the transmission of 2 frames

–  IFS intervals = periods of idleness the transmission support

–  There is different types of IFS

  EIFS –  DCF lowest priority

Backoff ACK

59

DFWMAC DCF

  Listening to the medium

–  Terminals from a same BSS can listen to detect if other stations in the same BSS are busy or not

–  To limit collisions risks when a station sends a frame

  Other stations update a timer called NAV (Network Allocation Vector)

  The NAV allows to delay all the waiting transmissions

  NAV is calculated based on the information located in the time to live field of the sent frames

60

DFWMAC DCF

  SENDER :

–  The station that wants to transmit listen on the medium

  If the medium is free during a DIFS interval, immediat transmission of the data frame

  If the medium is busy :

–  The station listen until the medium gets free –  The station delays its transmission using a backoff algorithm before it transmits

  RECEIVER:

–  If the data have been received properly (checks the frame’s CRC)

  the receiver waits until SIFS then transmits an ACK

62

DFWMAC DCF

Backoff Algorithm to reduce contention risks

Principle:

  When a station listens to the medium before

transmitting and hears the medium is busy:

1.  It waits until the medium gets free

2.  It calculates a random timer, unless if it already got

one

3.  It decreases its timer by 1 until :

-  Its timer reaches 0, then it transmits its frame

-  The medium becomes busy before its timer expires

-  then it stores the timer value and goes back to 1

63

DFWMAC DCF

  Timer calculation

–  Initially, a station chooses a random value between 0 and 7 timeslots

–  When the medium is free, stations decrease their timer until the medium is busy or until it reaches 0

–  If 2 or more stations reach the 0 at the same time, a collision occurs and each station must regenerate a new timer :random value

transmission reuse the same algorithm

–  drawback: no guarantee of minimum delay

 Harder to handle synchronous media such as voice or video

65

802.11 - competing stations - simple version

t busy

elapsed backoff time

bor residual backoff time busy medium not idle (frame, ack etc.)

busy busy