IEEE 802.11 Basic Connectivity Manuel Ricardo Faculdade de - PowerPoint PPT Presentation

WLAN 1 IEEE 802.11 Basic Connectivity Manuel Ricardo Faculdade de Engenharia da Universidade do Porto

WLAN 2 Acknowledgements ♦ Based on Jochen Schiller slides ♦ Supporting text » Jochen Schiller, “Mobile Comunications”, Addison-Wesley » Section 7.3 – Wireless LAN

WLAN 3 Characteristics of Wireless LAN ♦ Advantages over wired LANs » Terminal is free to move » Network uses less cabling » Possibility of forming unplanned, ad-hoc, networks ♦ Disadvantage » Smaller and variable bitrates

WLAN 4 Transmission - Radio and Infrared ♦ Radio ♦ Infrared » Band ISM, 2.4 GHz and 5 GHz » Diods, multiple reflection ♦ Advantages ♦ Advantages » Planning similar to cellular » Simple networks » Large coverage ♦ Disadvantages » Interferences ♦ Disadvantages – Solar light, heat sources » Limited resources » Smaller bitrates » ISM, noisy channels

WLAN 5 Infrastructure Networks vs Ad-Hoc Networks Infrastructure AP: Access Point AP AP wired network AP Ad-hoc

WLAN 6 IEEE 802.11 – Infrastructure Network ♦ Station 802.11 LAN 802.x LAN » Terminal with radio access ♦ Basic Service Set (BSS) STA 1 » Set of stations in the same band BSS 1 Portal ♦ Access Point (AP) Access Point » Interconnects LAN to wired network Distribution System » Provides access to stations ♦ Stations communicate with AP Access ESS Point ♦ Portal  bridge to other networks ♦ Distribution System BSS 2 » Interconnection network » Logical network – EES, Extended Service Set STA 2 STA 3 802.11 LAN – Based on BSSs

WLAN 7 IEEE 802.11 –Ad-Hoc Network ♦ Direct communication between 802.11 LAN stations STA 1 ♦ Independent Basic Service Set, IBSS STA 3 IBSS 1 » Set of stations working the the same carrier (radio channel) STA 2 IBSS 2 STA 5 STA 4 802.11 LAN

WLAN 8 IEEE 802.11 – Protocol Stack fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

WLAN 9 802.11 – Protocol Stack

WLAN 10 802.11 – Layers and Functionalities ♦ Data plane » MAC  medium access, fragmentation, encryption » PLCP - Physical Layer Convergence Protocol  carrier detection » PMD - Physical Medium Dependent  modulation, codification ♦ Management plane » PHY Management  channel selection, MIB » MAC Management  synchronisation, mobility, power, MIB » Station Management  coordenation management functions Station Management LLC DLC MAC MAC Management PLCP PHY PHY Management PMD

WLAN 11 To Think About? ♦ How to minimize collision in a wireless, shared, medium? Access 802.11 LAN Point BSS 2 STA 1 STA 3 IBSS 1 STA 2 STA 3 802.11 LAN STA 2

WLAN 12 MAC Layer – Access Methods DCF – Distributed Coordination Function PCF - Point Coordination Function ♦ MAC-DCF CSMA/CA – Carrier sense, collision avoidance using back-off mechanism – ACK packet required for confirmations (except for broadcast packets) – mandadory ♦ MAC-DCF with RTS+CTS – Used to avoid hidden terminal problem – Optional ♦ MAC- PCF – Access Point asks stations to transmit – Optional

WLAN 13 MAC-DCF CSMA/CA ♦ Station having a packet to transmit senses the medium ♦ If the medium is free during one Inter-Frame Space (IFS) » Station starts sending the frame contention window (randomized back-off DIFS DIFS mechanism) medium busy next frame t direct access if medium is free ≥ DIFS slot time ♦ If medium is busy » Station waits for the medium to become free + one IFS + random contention period (collision avoidance, múltiplo de slot  n* 20 us) ♦ If other station accesses to the medium during the contention time » Waiting timer is suspended

WLAN 14 MAC-DCF CSMA/CA – Concurrent Stations DIFS DIFS DIFS DIFS bo e bo r bo e bo r bo e busy station 1 bo e busy station 2 busy station 3 bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t medium not idle (frame, ack etc.) elapsed backoff time busy bo e packet arrival at MAC bo r residual backoff time

WLAN 15 MAC Layer – Guard Time Intervals DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free ≥ DIFS » DIFS (DCF IFS) – Lowest priority, used for asynchronous data » PIFS (PCF IFS) – Medium priority, used for real time traffic /QoS » SIFS (Short Inter Frame Spacing) – Maximum priority  used for signalling: ACK, CTS, answers to polling

WLAN 16 MAC-DCF CSMA/CA ♦ Sending a frame in unicast » Station waits DIFS before sending the packet » If packet is correctly received (no errors in CRC)  Receiver confirms reception immediatly, using ACK, after waiting SIFS » In case of errors, frame is re-transmitted » In case of retransmission  Maximum value for the contention window duplicates  Contetion window has minimum and maximum values (eg.: 7 and 255) DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention

WLAN 17 Virtual Carrier Sensing – Network Allocation Vector ♦ How does a station know if the medium is free? » Usually, by listening the carrier ♦ IEEE 802.11 also uses Network Allocation Vector (NAV) » 802.11 frames contain a duration field; used to reserve the medium » Stations have a timer NAV – Updated with the values seen in the frames – Decremented in real-time – If != zero  medium not free

WLAN 18 To Think About ♦ How to enable hidden terminals to sense the carrier? D A B C Hidden node: C is hidden to A

WLAN 19 MAC DCF with RTS+CTS ♦ Sending a frame in unicast » Station sends RTS with a reserve parameter, after waiting DIFS – Reserve time includes RTS+SIFS+CTS+SIFS+DATA+SIFS+ACK » Receiver confirms with CTS, after waiting SIFS » Transmitter sends frame, after waiting SIFS. Confirmation with ACK » Other stations become aware of reserved time by listening RTS and CTS DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other NAV (CTS) stations t defer access contention

WLAN 20 MAC- PCF t 0 t 1 SuperFrame medium busy PIFS SIFS SIFS D 1 D 2 point SIFS SIFS coordinator U 1 U 2 wireless stations NAV stations‘ NAV

WLAN 21 MAC-PCF II t 2 t 3 t 4 PIFS SIFS D 3 D 4 CF end point SIFS coordinator U 4 wireless stations NAV stations‘ contention free period t NAV contention period

WLAN 22 MAC – Frame Format ♦ Frame types » Data, control, management ♦ Sequence number ♦ Addresses » destination, source, BSS identifier, ... ♦ Others » Error control, frame control, data bytes 2 2 6 6 6 2 6 0-2312 4 Frame Duration/ Address Address Address Sequence Address Data CRC Control ID 1 2 3 Control 4 bits 1 1 1 1 1 1 2 2 4 1 1 Protocol To From More Retry Power More Type Subtype Data WEP Order version DS DS Frag Mgmt

WLAN 23 To Think About ♦ STA 1 needs to send a frame to STA 2 . In the Infrastructure mode, the frame is sent via the AP. What MAC addresses are required in the frame sent by STA 1 to the AP? Access Point BSS 2 STA 1 STA 2 802.11 LAN

WLAN 24 Addresses in MAC scenario to DS from address 1 address 2 address 3 address 4 DS ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure 1 1 RA TA DA SA network, within DS DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

WLAN 25 Special Frames- ACK, RTS, CTS 2 2 6 4 bytes Frame Duration Receiver ♦ Acknowledgement ACK CRC Control Address bytes 2 2 6 6 4 Frame Duration Receiver Transmitter ♦ Request To Send CRC RTS Control Address Address bytes 2 2 6 4 Frame Duration Receiver CTS CRC ♦ Clear To Send Control Address (Fig. 7.17 do livro está errada)

WLAN 26 Station Management LLC DLC MAC MAC Management MAC Management PLCP PHY PHY Management PMD ♦ Synchronization – Station discovers a LAN; station associates to an AP – stations synchronize clocks; Beacon is generated by AP ♦ Power management – Save terminal’s power  terminal enters sleep mode  Periodically  No frame loss; frames are stored ♦ Roaming – Station looks for new access points – Station decides about best access point – Station (re-)associates to new AP ♦ MIB - Management Information Base

WLAN 27 Synchronization by Beacon – Infrastructure Network ♦ Stations must be synchronised. E.g. – To preview PCF cycles – To change state: sleep  wake ♦ Infrastructure networks – Access Point sends (almost) periodically a Beacon with timestamp e BSSid sometimes medium is busy – Timestamp sent is the correct – Other stations adjust their clocks beacon interval B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame