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Lecture 4: IEEE 802.11

Lecture 4: IEEE 802.11 . ECE 591. IEEE 802.11 Requirements. Wi-Fi often used by the public as a synonym for IEEE 802.11-wireless LAN (WLAN). Design for small coverage (e.g. office, home) Low/no mobility High data-rate applications

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Lecture 4: IEEE 802.11

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  1. Lecture 4: IEEE 802.11 ECE 591

  2. IEEE 802.11 Requirements • Wi-Fi often used by the public as a synonym for IEEE 802.11-wireless LAN (WLAN). • Design for small coverage (e.g. office, home) • Low/no mobility • High data-rate applications • Ability to integrate real time applications and non-real-time applications • Use un-licensed spectrum

  3. Portal Distribution System 802.11: Infrastructure Mode • Architecture similar to cellular • networks station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • access point (AP) • station integrated into the wireless LAN and the distribution system • basic service set (BSS) • group of stations using the same AP • portal • bridge to other (wired) networks • distribution system • interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  4. 802.11 - Architecture of an ad-hoc network • Direct communication within a limited range • Station (STA):terminal with access mechanisms to the wireless medium • Independent Basic Service Set (IBSS):group of stations using the same radio frequency 802.11 LAN STA1 STA3 IBSS1 STA2 IBSS2 STA5 STA4 802.11 LAN

  5. IEEE standard 802.11 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

  6. IEEE 802.11 Physical Layer • Family of IEEE 802.11 standards: • unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 300 MHz 5.15-5.35 GHz 5.725-5.825 GHz and 802.11b/g 802.11a

  7. The IEEE 802.11 Family

  8. 802.11a Modulation • Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (312.5 KHz each) • 48 data, 4 pilot • Adaptive modulation • BPSK: 6, 9 Mbps • QPSK: 12, 18 Mbps • 16-QAM: 24, 36 Mbps • 64-QAM: 48, 54 Mbps

  9. 802.11 MAC Layer: Access Methods • DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized “back-off“ • ACK packet for acknowledgements • DFWMAC-DCF w/ RTS/CTS (optional) • additional virtual “carrier sensing: to avoid hidden terminal problem • DFWMAC- PCF (optional) • access point polls terminals according to a list

  10. Hidden and Exposed Terminals • Hidden terminals • A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails) • collision at B, A cannot receive the collision (CD fails) • A is “hidden” for C • Exposed terminals • B sends to A, C wants to send to another terminal (not A or B) • C has to wait, CS signals a medium in use • but A is outside the radio range of C, therefore waiting is not necessary • C is “exposed” to B A B C

  11. Near and Far Terminals • Terminals A and B send, C receives • signal strength decreases proportional to the square of the distance • the signal of terminal B therefore drowns out A’s signal • C cannot receive A • If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer • Also severe problem for CDMA-networks - precise power control needed! A B C

  12. MACA - collision avoidance • MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance • RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet • CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive • Signaling packets contain • sender address • receiver address • packet size • Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

  13. A A C C MACA examples • MACA avoids the problem of hidden terminals • A and C want to send to B • A sends RTS first • C waits after receiving CTS from B • MACA avoids the problem of exposed terminals • B wants to send to A, C to another terminal • now C does not have to wait for it cannot receive CTS from A RTS CTS CTS B RTS RTS CTS B

  14. MACA variant: DFWMAC in IEEE802.11 sender receiver idle idle packet ready to send; RTS data; ACK RxBusy time-out; RTS wait for the right to send RTS; CTS time-out  data; NAK ACK time-out  NAK; RTS CTS; data wait for data wait for ACK RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy

  15. 802.11 CSMA/CA • CSMA: Listen before transmit • Collision avoidance • when transmitting a packet, choose a backoff interval in the range [0, CW] • CW is contention window • Count down the backoff interval when medium is idle • count-down is suspended if medium becomes busy • Transmit when backoff interval reaches 0

  16. B1 = 25 wait data data wait B2 = 10 B2 = 20 Congestion Avoidance: Example busy B1 = 5 B2 = 15 busy B1 and B2 are backoff intervals at nodes 1 and 2

  17. 802.11 – RTS/CTS + ACK • Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) • Receiver acknowledges via CTS (if ready to receive) • CTS reserves channel for sender, notifying possibly hidden stations • Sender can now send data at once, acknowledgement via ACK • Other stations store NAV distributed via RTS and CTS DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

  18. Not END

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