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Technologies radio Wifi

L'essentiel à connaître sur les fréquences, modulations WIFI et les normes 802.11a/b/g tient en quelques lignes. On trouvera ici un récupatilatif et on se reférera à d'autres documents en ligne pour l'intelligence de la matière.

Détails théoriques


Pour les détails théoriques on lira les documents suivants :


802.11b does not use FHSS (Frequency Hopping Spread Spectrum) !

Multipath Distortion
- definition : High signal Strength and low quality
- DSSS is more sensible than OFDM

802.11b/g -2.4GHz


Maximum Power level 802.11b
- in Europe : 20 dBm (100 mW)
- in USA : 24 dBm (250 mW)

Maximum 3 non-overlapping channels (33Mbps in 802.11b and 162Mbps in 802.11g)

Bandwidth of each channels
- 802.11g : 22MHz

Total bandwidth of 2.4Ghz = 83MHz (2.400 to 2.483)

Data Rate (Mbps) Transmission Type Modulation Scheme

54

OFDM

64 QAM

48

OFDM

64 QAM

36

OFDM

16 QAM

24

OFDM

16 QAM

18

OFDM

QPSK1

12

OFDM

QPSK

11

DSSS

CCK2

9

OFDM

BPSK3

6

OFDM

BPSK

5.5

DSSS

CCK

2

DSSS

QPSK

1

DSSS

BPSK

QPSK Quadrature Phase Shift Keying

CCK Complementary Code Keying

BPSK Biphase Shift Keying

DSSS Direct Sequence Spread Spectrum

QAM Quadrature Amplitude modulation


802.11a - 5GHz

Aironet client maximum outpout : 20mW (UNII-1 and UNII-2)

UNII bands
1. UNII-1 5.150GHz to 5.250GHz

  • indoor only
  • 40mW max transmit power
  • 4 non-overlapping channels
  • integrated antenna 6dBi in Cisco AP

2. UNII-2 5.25G0Hz to 5.350GHz

  • indoor/outdoor
  • 200mW max transmit power
  • 4 non-overlapping channels

3. UNII-3 5.725GHz to 5.825GHz

  • outdoor only
  • 1W with 6dBi antenna (multipoint)
  • 1W with 23dBi antenna (point-to-point)
  • 4 non-overlapping channels

Each channel of 20MHz is divided in 52 subchannels (each 300KHz of bandwidth). 48 of 52 are used for data transmission

8 non-overlapping channels (4 UNII-1 + 4 UNII-2)

802.11a Orthogonal Frequency Division Multiplexing (OFDM)

Modulation Scheme Data Rate (Mbps)

64QAM

48/54 Mbps

16QAM

24/36 Mbps

QBPSK

12/18 Mbps

BPSK

6/9 Mbps



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