1. IEEE 802.11 MAC Ikjun Yeom

2. Computer Network Physical Layer: network card, wire/wireless Datalink Layer: data delivery in a link Network Layer: addressing, routing Transport Layer: TCP, UDP Application Layer: network applications

3. Data Link Layer

4. Multiple Access Protocols

5. Random Access  without carrier sense – ALOHA – Slotted ALOHA  with carrier sense – CSMA/CD – CSMA/CA

6. Pure ALOHA

8. Vulnerable Time for Pure ALOHA

9. Slotted ALOHA

10. Carrier Sensing

11. Persistence Methods

12. CSMA/CD

13. CSMA/CD in Wireless Network  hard to employ CSMA/CD directly since – cannot detect collision immediately – wastage due to collision is much more A BC

14. CSMA/CA

16. IEEE 802.11 MAC Layer

17. DCF and PCF  Distributed Coordination Function – fundamental access method for IEEE 802.11 – CSMA/CA – Used within both ad hoc and infrastructure  Point Coordination Function – an alternative access method – implemented on top of the DCF – a point coordinator is used to determine which station currently has the right to transmit

18. DCF and PCF  Different accesses to medium can be defined through the use of different values of IFS – PCF IFS < DCF IFS – PCF traffic has higher priority to access the medium to provide a contention-free accesses.  Coexistence of DCF and PCF in a superframe. Contention-free PeriodContention Period A Super Frame

19. DCF  CSMA/CA  Random backoff following a busy medium  all packets should be acknowledged immediately  Carrier sense – physical carrier sensing provided by the PHY – virtual carrier sensing provided by MAC

20. Virtual Carrier Sense  to resolve hidden terminal problem  via RTS (Request to Send) and CTS (Clear to Send) A BC RTS A BC CTS

21. Frame Exchange

22. Acknowledgement  Frames that should be acked – Data – Poll (for PCF)  Frames without ack – Ack – RTS, and CTS  An ACK should be returned immediately after a successfully received frame.  After receiving a frame, an ACK should be sent after SIFS (Short IFS) – SIFS < PIFS < DIFS

23. Priority  Priorities of frames are distinguished by the IFS incurred between two consecutive frames  IFSs – SIFS: ACK, RTS, CTS, Response to a poll in PCF – PIFS: PCF frames – DIFS: DCF frames

24. Random Backoff  Before transmitting asynchronous MPDUs, a STA shall use the CS function to determine the medium state.  If busy, the STA will: – defer a DIFS gap – then generate a random backoff period (within the contention window CW) for an additional deferral time to resolve contention Backoff time = CW* Random() * Slot time where CW = starts at CWmin, and doubles after each failure until reaching CWmax and remains there in all remaining retries (e.g., CWmin = 31, CWmax = 255) Random() = (0,1) Slot Time = Transmitter turn-on delay + medium propagation delay +medium busy detect response time

25. DCF Access Procedure  CSMA/CA  Can try when: – no PCF detected – or, Contention Period of a Superframe when using a PCF.  Basic Access – A STA with a pending frame may transmit when it detects a free medium for  DIFS time. – Otherwise, the Backoff procedure shall be followed. DIFS Busy Medium PIFS SIFS DIFS Contention Window Backoff-Window Next Frame Defer Access Slot time Immediate access when medium is free >= DIFS

26. Backoff Procedure  Backoff – The Backoff Timer should be frozen when medium is busy. – The timer should be resumed only when the medium is free for a period > DIFS. – Transmission shall commence whenever the Backoff Timer reaches 0. A B C D E DIFS CWindow Backoff Frame CWindow = Contention Window = Backoff = Remaining Backoff

27. DCF Transmissions Data DIFS Defer Access ACK DIFS Contention Window Backoff-Window Next Frame SIFS Backoff after Defer Sender Receiver Other Data DIFS Defer Access RTS CTSACK NAV(RTS) NAV(CTS) DIFS Contention Window Backoff-Window Next Frame SIFS T1 SIFS T3 Backoff after Defer Sender Receiver Other Without RTS/CTS With RTS/CTS

28. Recovery Procedure  RTS/CTS recovery – After an RTS is transmitted, if the CTS is not received within a predetermined CTS_Timeout (T1), then a new RTS shall be generated. – The CW is doubled in each failure. – Repeated until the RTS_Retransmit_Counter reaches an RTS_Retransmit_Limit.  DATA frame recovery – backoff mechanism shall be used when no ACK is received within a predetermined ACK_Window(T3) – This procedure shall be continued until the ACK_Retransmit_Counter reaches an ACK_Retransmit_Limit

29. IEEE 802.11 MAC

30. 30 IEEE 802.16 WMAN  Goal: Provide high-speed Internet access to home and business subscribers, without wires.  Base stations (BS) can handle thousands of subscriber stations (SS)  Access control prevents collisions.  Supports – Legacy voice systems – Voice over IP – TCP/IP – Applications with different QoS requirements.

31. 31 Time Division Duplexing (TDD)

32. 32 Media Acces Control (MAC)  Connection orienteded  Channel access: – UL-MAP Defines uplink channel access Defines uplink data burst profiles – DL-MAP Defines downlink data burst profiles – UL-MAP and DL-MAP are both transmitted in the beginning of each downlink subframe

33. 33 Bandwidth Request and Allocation  SSs may request bw in 3 ways: – Use the ” contention request opportunities ” interval upon being polled by the BS. – Send a standalone MAC message called ” BW request ” in an allready granted slot. – Piggyback a BW request message on a data packet.

34. 34 Bandwidth Request and Allocation BS grants/allocates bandwidth in one of two modes: – Grant Per Subscriber Station (GPSS) – Grant Per Connection (GPC) Decision based on requested bw and QoS requirements vs available resources. Grants are realized through the UL-MAP.

35. 35 4 types of Scheduling Service  Unsolicited Grant Service (UGS) – Real-time, periodic fixed size packets (e.g. T1 or VoIP) – Restrictions on bw requests (Poll-Me bit)  Real-Time Polling Service (rtPS) – Real-time, periodic variable sizes packets (e.g MPEG) – BS issues periodic unicast polls. – Cannot use contention requests, but piggybacking is ok.  Non-Real-Time Polling Service (nrtPS) – Variable sized packets with loose delay requirements (e.g. FTP) – BS issues unicast polls regularly (not necessarily periodic). – Can also use contention requests and piggybacking.  Best Effort Service – Never polled individually – Can use contention requests and piggybacking