WLAN b a Johan Montelius

IEEE 802 The IEEE link layer standards: –802.3 Ethernet –802.5 Token Ring – Wireless LAN – Bluetooth

–1 and 2 Mbps on the 2.4 GHz band –FHSS, DSSS, IrDA b –DSSS, upgrade to 5.5 and 11 Mbps a –the 5 GHz band –up to 54 Mbps –OFDM, up to 64-QAM h –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 BSS ESS WLAN AP

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 –BPSK 11 Mcps –Barker spreading sequence 11 chips wide 2 Mbps –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 SIFS ACK Data DIFS Backoff Data

Collision Avoidance Virtual Carrier Sense DIFS SIFS CTS RTS Data SIFS ACK Reserved

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 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 ( / MHz in the US) < 200 mW (50/250 mW in the US) indoor usage seller should –if possible limit the device –inform the user about limitations … in reality –same stuff, software controlled, it’s up to you