WLAN
802.11b 802.11a
Johan Montelius
johan@jmt2.com
IEEE 802
• The IEEE link layer standards:
– 802.3 Ethernet
– 802.5 Token Ring
– 802.11 Wireless LAN
– 802.15 Bluetooth
802.11
• 802.11
– 1 and 2 Mbps on the 2.4 GHz band
– FHSS, DSSS, IrDA
• 802.11b
– DSSS, upgrade to 5.5 and 11 Mbps
• 802.11a
– the 5 GHz band
– up to 54 Mbps
– OFDM, up to 64-QAM
• 802.11h
– 11a with power control, frequency selection
WLAN vs LAN
- what’s the difference
Ethernet hub/switch
WLAN vs LAN
- what’s the difference
WLAN AP
ESS
BSS
WLAN AP
BSS
WLAN vs LAN
- what’s the difference
• Identification
– The mobile station must be able to identify the
right AP-
• Registration
– The AP must know which mobile stations it should
serve.
• Collision Avoidance
– it’s hard to do collision detection
• Security
– How do we protect our network?
802.11b
•
•
•
•
Physical layer
MAC layer
Power saving
Encryption
802.11b Spectrum
• ISM band
– 2.4 GHz
– total 83 MHz
• Direct sequence spread spectrum
• Carriers
– 14 carriers (11 US, 13 Europe)
– bandwidth per carrier 22 MHz
802.11b Modulation
•
1 Mbps
–
–
•
2 Mbps
–
–
•
BPSK 11 Mcps
Barker spreading sequence 11 chips wide
QPSK 22 Mcps
Barker spreading sequence 11 chips wide
5,5 and 11 Mbps
–
–
QPSK 11 Mcps
spreading code 8 chips wide, encode 4 or 8 bits
CSMA/CA
• Carrier Sense
– listen to the media before you send
• Multiple Access
– you’re not alone
• Collision Avoidance
– the line should be idle for some time
– send a short request, wait for clearance
Why not Collision Detection?
AP
Collision Avoidance
Physical Carrier Sense
DIFS
Data
ACK
DIFS
Backoff
Data
SIFS
Collision Avoidance
Virtual Carrier Sense
DIFS
RTS
SIFS
Reserved
CTS
Data
ACK
SIFS
Go to sleep if you can
• All mobile stations are synchronized by
the AP.
• Mobile stations can enter doze mode.
• The AP will buffer incoming data for
stations in doze mode.
• The AP periodically send information
about which stations have data waiting.
• Mobile stations will poll the AP for data.
It’s not secure!
• WEP (wired equivalent privacy):
encryption at link level.
• Shared key in a BSS
• Not secure!
• AP should be located in a DMZ.
802.11a
• Same MAC layer as 802.11
• Physical layer
– 5 GHz band
– Orthogonal Frequency Division Multiplexing
– Each carrier 20 MHz wide
– up to 54 Mbps raw data rate
• Forward Error Correction
– increase throughput for low signal levels
5 GHz in the US
• 5150 – 5250 MHz
– < 40 mW
• 5250 – 5350 MHz
– < 200 mW
• 5725 – 5825 MHz
– < 800 mW
5 GHz In Europe
• Hiperlan 1
– 5150 – 5250 MHz
– < 200 mW
– Indoor only
• Hiperlan 2
–
–
–
–
5150 – 5350 and 5470 –5727 MHz
< 1W
Transmit power Control
Dynamic Frequency Selection
802.11a in Europe
•
•
•
•
5150 – 5250 (5150-5250/5125-5135 MHz
< 200 mW (50/250 mW in the US)
indoor usage
seller should
in the US)
– if possible limit the device
– inform the user about limitations
• … in reality
– same stuff, software controlled, it’s up to you