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MAC layer

MAC layer. Taekyoung Kwon. Media access in wireless - start with IEEE 802.11. In wired link, C arrier S ense M ultiple A ccess with C ollision D etection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) In wireless

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MAC layer

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  1. MAC layer Taekyoung Kwon

  2. Media access in wireless- start with IEEE 802.11 • In wired link, • Carrier Sense Multiple Access with Collision Detection • send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) • In wireless • Signal strength decreases in proportional to at least square of the distance • Collision detection only at receiver • Half-duplex mode • Furthermore, CS is not possible after propagation range

  3. Hidden terminal problem • 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 A B C

  4. Exposed terminal problem • Exposed terminals • B sends to A, C wants to send to D • C has to wait, CS signals a medium in use • but A is outside the radio range of C, thus waiting is not necessary • C is “exposed” to B A B C D

  5. Multiple access methods • FDMA • TDMA • CDMA • SDMA

  6. ALOHA, Slotted-ALOHA • Mechanism • random, distributed (no central arbiter), time-multiplex • Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries • Aloha • Slotted Aloha collision sender A sender B sender C t collision sender A sender B sender C t

  7. MACA (multiple access 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 • aka, virtual carrier sense

  8. A A C C MACA operation • 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 RTS CTS CTS B RTS RTS CTS CTS B

  9. PRMA • Implicit reservation (PRMA - Packet Reservation MA) • a certain number of slots form a frame, frames are repeated • stations compete for empty slots according to the slotted aloha principle • once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send • competition for this slots starts again as soon as the slot was empty in the last frame reservation 1 2 3 4 5 6 7 8 time-slot ACDABA-F frame1 A C D A B A F ACDABA-F frame2 A C A B A AC-ABAF- collision at reservation attempts frame3 A B A F A---BAFD frame4 A B A F D ACEEBAFD frame5 A C E E B A F D t

  10. IEEE 802.11 wireless LAN infrastructure network AP: Access Point AP AP wired network ad-hoc network

  11. Portal Distribution System 802.11 infrastructure mode • Station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • Basic Service Set (BSS) • group of stations using the same radio frequency • Access Point • station integrated into the wireless LAN and the distribution system • Portal • bridge to other (wired) networks • Distribution System • interconnection network to form one logical network (ESS: 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

  12. 802.11 MAC requirements

  13. 802.11 MAC • Traffic services • Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” • support of broadcast and multicast • Time-Bounded Service (optional) • implemented using PCF (Point Coordination Function) • Access methods • DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized back-off mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) • DFWMAC-DCF w/ RTS/CTS (optional) • Distributed Foundation Wireless MAC • avoids hidden terminal problem • DFWMAC- PCF (optional) • access point polls terminals according to a list

  14. MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management 802.11 layers • PLCP Physical Layer Convergence Protocol • clear channel assessment signal (carrier sense) • PMD Physical Medium Dependent • modulation, coding • PHY Management • channel selection, MIB • Station Management • coordination of all management functions

  15. Infrastructure mode 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

  16. 802.11 MAC • Priorities • defined through different inter frame spaces • no guaranteed, hard priorities • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS

  17. 802.11 CSMA/CA contention window (randomized back-offmechanism) DIFS DIFS • station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) • if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) • if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) • if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) medium busy next frame t direct access if medium is free  DIFS slot time

  18. 802.11 CSMA/CA: contention resolution DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 boe busy boe bor station4 boe bor boe busy boe bor station5 t medium not idle (frame, ack etc.) busy boe elapsed backoff time packet arrival at MAC bor residual backoff time

  19. 802.11 CSMA/CA: detailed • Sending unicast packets • station has to wait for DIFS before sending data • receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) • automatic retransmission of data packets in case of transmission errors DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention

  20. 802.11: RTS & CTS • Sending unicast packets • station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) • acknowledgement via CTS after SIFS by receiver (if ready to receive) • sender can now send data at once, acknowledgement via ACK • other stations store medium reservations 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 contention

  21. 802.11 beaconing (infrastructure mode) beacon interval B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame

  22. 802.11 beaconing (ad hoc mode) beacon interval B1 B1 station1 B2 B2 station2 busy busy busy busy medium t B value of the timestamp beacon frame random delay

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