Wireless data networks More on 802.11 Martin Heusse A closer look - - PowerPoint PPT Presentation

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Jan 13, 2023 258 likes •423 views

Wireless data networks More on 802.11 Martin Heusse A closer look at 802.11 1 Channels 802.11b 1 6 11 2412 MHz 2437 MHz 2462 MHz f 22MHz A closer look at 802.11 2 802.11a ) &#)*+, &#)*+, ! ! ! ! ! ! ! !

SLIDE 1

A closer look at 802.11 — 1

Wireless data networks More on 802.11

Martin Heusse

SLIDE 2

A closer look at 802.11 — 2

Channels 802.11b

22MHz 2412 MHz 2437 MHz f 2462 MHz 1 6 11

SLIDE 3

A closer look at 802.11 — 3

802.11a

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SLIDE 4

A closer look at 802.11 — 4

802.11a (cont.)

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SLIDE 5

A closer look at 802.11 — 5

Infrastructure network planning

If the access points do not hear each other, it does not mean

the cells are disjoint!

Paving with 3 channels:
Paving with 4 channels:

SLIDE 6

A closer look at 802.11 — 6

Roaming

Clients need to scan other channels (maybe helped by current

AP) ← they have to leave the channel for some time. (Can also leave for good and then scan)

✓ The client can declare that it’s going into power save mode…

Active (probe request) or passive scanning
Reassociation handshake (contains address of former AP)
Frame sent on the DS to update switch tables

SLIDE 7

A closer look at 802.11 — 7

Roaming (cont.)

Disassociation req. Disassociation resp. (optional) Probe Req/resp Reassociation req. Reassociation resp. Unicast frame, src = STA MAC addr.

SLIDE 8

A closer look at 802.11 — 8

Security

Historical: WEP: Wired Equivalent privacy (weak!)
WPA-EAP (Extensible Authentication Protocol)(uses 802.1x

— RADIUS server)

WPA-PSK (Pre-Shared Key)
WPA2 (RSNA, aka 802.11i)

SLIDE 9

A closer look at 802.11 — 9

WEP

Seed WEP Key Sequence PRNG Initialization Vector (IV) Secret Key Plaintext Integrity Check Value (ICV) Integrity Algo. XOR Ciphertext IV Message

SLIDE 10

A closer look at 802.11 — 10

TCP over WLAN

5 10 15 20 25 5 10 15 20 25 30 Data Transferred (MB) Time(s) Uploads Download

SLIDE 11

A closer look at 802.11 — 11

TCP over WLAN (cont.)

wired WLAN cell AP stations N=3 k=2

ACK segments DATA segments

SLIDE 12

A closer look at 802.11 — 12

Solutions?

PCF
802.11e
Another access method?
Uplink-aware queueing

SLIDE 13

A closer look at 802.11 — 13

802.11e — HCF Hybrid Coordination Function

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SLIDE 14

A closer look at 802.11 — 14

802.11e — HCF Hybrid Coordination Function (cont.)

EDCF replaces DCF

✓ Various AIFS (instead of DIFS) ✓ Various CWmin ✓ When a STA wins contention, it can transmit several frame up to TXOP

HCCA (HCF Controlled Channel Access)(enhances PCF)

✓ Polling (negotiation AP/STA) ✓ PCF:

◮ Contention free period (after the beacon)

✓ CAP: Controlled Access Phase, may be more frequent than beacons! ✓ TXOP are distributed by AP (CF-Poll frame), ✓ The coordinator (AP) accesses the channel after PIFS

SLIDE 15

A closer look at 802.11 — 15

Power save

DTIM: Delivery Traffic Indication Message

✓ DTIM is in the beacon ✓ DTIM period > beacon period

AP buffers broadcasts until next DTIM

Beacon-Interval DTIM Interval

TIM (in Beacon):

AP activity: Busy medium:

DTIM: Broadcast: