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

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 ) &#)*+, &#)*+, ! ! ! ! ! ! ! ! !"!# !"$# !%## !%%# !%(# !%'# !%$# !&## !&%# !&!# -./01);234)D4C0 7AA01);234)D4C0 %#)*+, %#)*+, ! ! ! ! !E%! !E(! !E'! !E$! !$#! !$%! -./01);234)D4C0 7AA01);234)D4C0 A closer look at 802.11 — 3

802.11a (cont.) 67")&'./#+89&//"%+ <9&//"%+8"/'")+ !"#$%&'()*+,(-&./ 0&/,+12345 /$-:"); =)">$"/8.";+1?345 H*%+)('X+0+)& HIJKK'8,:).'40*( M" ;ECL' ];BE;^;B!;_ YL ;!LL' YY ;!!L' YC ;!YL' H*%+)('X+0+)& HIJKK'3%((8)'40*( ;! ;!"L' ];B!;^;BM;_ ;" ;!CL' "L ;MLL' "Y ;M!L' H*%+)('X+0+)& HIJKK'1--).'40*( EYF ;PY;' ];BP!;^;BC!;_' E;M ;P";' E;P ;PC;' E"E ;CL;' A closer look at 802.11 — 4

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: A closer look at 802.11 — 5

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 A closer look at 802.11 — 6

Roaming (cont.) Unicast frame, src = STA MAC addr. Disassociation req. Reassociation req. Probe Req/resp Disassociation resp. (optional) Reassociation resp. A closer look at 802.11 — 7

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) A closer look at 802.11 — 8

WEP IV Initialization Key Vector (IV) WEP Seed Sequence Ciphertext PRNG Secret Key XOR Message Plaintext Integrity Algo. Integrity Check Value (ICV) A closer look at 802.11 — 9

TCP over WLAN 25 20 Data Transferred (MB) 15 Uploads 10 5 Download 0 0 5 10 15 20 25 30 Time(s) A closer look at 802.11 — 10

TCP over WLAN (cont.) DATA ACK segments k =2 segments AP N =3 wired WLAN cell stations A closer look at 802.11 — 11

Solutions? • PCF • 802.11e • Another access method? • Uplink-aware queueing A closer look at 802.11 — 12

802.11e — HCF Hybrid Coordination Function EFG'HI%H-J .I>>AJB'5=' $KKGFF'/I5GB=@? &@IJFLA5'MNGNGF' O=@'$/F #G@;MNGNG'"0/$'' ONJK5A=JF'PA5C' AJ5G@JIE'K=EEAFA=J' @GF=EN5A=J A closer look at 802.11 — 13

802.11e — HCF Hybrid Coordination Function (cont.) • EDCF replaces DCF ✓ Various AIFS (instead of DIFS) ✓ Various CW min ✓ 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 A closer look at 802.11 — 14

Power save • DTIM: Delivery Traffic Indication Message ✓ DTIM is in the beacon ✓ DTIM period > beacon period • AP buffers broadcasts until next DTIM DTIM Interval Beacon-Interval AP activity: TIM (in Beacon): Busy medium: DTIM: Broadcast: PS station Poll A closer look at 802.11 — 15