1. Spring 2005UCSC CMPE2571 CMPE 257: Wireless Networking SET 3: Medium Access Control Protocols

2. Spring 2005UCSC CMPE2572 MAC Protocol Topics n IEEE 802.11 n Fairness n Directional antennas n Multiple channels n Modeling and performance analysis (very limited!) n Power efficiency and synchronization issues n Scheduled channel access

3. Spring 2005UCSC CMPE2573 IEEE 802.11 Standard n PHY/MAC standard for wireless LANs p First standardized in 1997 p Meet great success in starting from 1999 n Several working groups p IEEE 802.11a: high speed extension to the 5GHz band p 802.11b/g: high speed extension to the 2.4 GHz band p 802.11e: Quality of service (QoS) enhancement (still active) p 802.11i: Security enhancement p 802.11s: Mesh-networking support

4. Spring 2005UCSC CMPE2574 Requirements n Single MAC to support multiple PHYs p Handle ``hidden terminal’’ problem p Provisions time-bounded service n PHYs: p Direct sequence p Frequency hopping p Infrared (never implemented)

5. Spring 2005UCSC CMPE2575 Architecture n Infrastructure mode p Basic Service Set (BSS) p Access Point (AP) and stations (STA) take different roles p Distribution system (DS) interconnect multiple BSSs to form a single network (not specified in the standard). n Ad hoc mode p Independent Basic Service Set (IBSS) p Single-hop (the standard makes this assumption either explicitly or implicitly)

6. Spring 2005UCSC CMPE2576 Access Points n Stations select an AP and “associate” with it n Support roaming (not part of the standard) n Provide other functions p time synchronization (beaconing) p power management support p point coordination function (PCF) n Traffic typically (but not always) flows through AP p direct communication possible (in IEEE 802.11e)

7. Spring 2005UCSC CMPE2577 802.11 Protocol Entities MAC Sublayer PLCP Sublayer PMD Sublayer MAC Layer Management PHY Layer Management Station Management LLC MAC PHY

8. Spring 2005UCSC CMPE2578 802.11 Protocol Architecture n MAC Entity p basic access mechanism p fragmentation p encryption n MAC Layer Management Entity p synchronization p power management p roaming p MAC MIB n Physical Layer Convergence Protocol (PLCP) p PHY-specific, supports common PHY SAP p provides Clear Channel Assessment (CCA) signal (carrier sense)

9. Spring 2005UCSC CMPE2579 802.11 Protocol Architecture (cont.) n Physical Medium Dependent Sublayer (PMD) p modulation and encoding n PHY Layer Management p channel tuning p PHY MIB n Station Management p interacts with both MAC Management and PHY Management

10. Spring 2005UCSC CMPE25710 MAC in Detail n Channel access mechanism r Distributed Coordination Function (DCF) Carrier sense multiple access (CSMA) with immediate MAC-level ACK RTS/CTS exchange (optional) r Point Coordination Function (PCF) Polled access through AP and distributed access Contention-free period (CFP) and contention period (CP) Seldom implemented in practice n Synchronization and power management

11. Spring 2005UCSC CMPE25711 CSMA/CA Explained DIFS Contention Window Slot time Defer Access Backoff-Window Next Frame Select Slot and Decrement Backoff as long as medium is idle. SIFS PIFS DIFS Free access when medium is free longer than DIFS Busy Medium n Reduce collision probability where mostly needed. p Stations are waiting for medium to become free. p Select Random Backoff after a Defer, resolving contention to avoid collisions. n Efficient Backoff algorithm stable at high loads. p Exponential Backoff window increases for retransmissions. p Backoff timer elapses only when medium is idle. n Implement different fixed priority levels. p To allow immediate responses and PCF coexistence.

12. Spring 2005UCSC CMPE25712 CSMA/CA + ACK protocol Defer access based on Carrier Sense. –CCA from PHY and Virtual Carrier Sense state. Direct access when medium is sensed free longer then DIFS, otherwise defer and backoff. Receiver of directed frames to return an ACK immediately when CRC correct. –When no ACK received then retransmit frame after a random backoff (up to maximum limit). Ack Data Next MPDU Src Dest Other Contention Window Defer Access Backoff after Defer DIFS SIFS DIFS

13. Spring 2005UCSC CMPE25713 RTS/CTS Based Access Net Allocation Vector (NAV)Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector (NAV). Medium BusyDefer on either NAV or "CCA" indicating Medium Busy. Use of RTS / CTS is optional but must be implemented. RTS_ThresholdUse is controlled by a RTS_Threshold parameter per station. –To limit overhead for short frames. RTS CTS Ack Data NAV Next MPDU Src Dest Other CW Defer AccessBackoff after Defer NAV (RTS) (CTS) DIFS

14. Spring 2005UCSC CMPE25714 Frame Formats n MAC Header format differs per Type: p Control Frames (several fields are omitted) p Management Frames p Data Frames n Includes Sequence Control Field for filtering of duplicate caused by ACK mechanism. Frame Control Duration ID Addr 1Addr 2Addr 3Addr 4 Sequence Control CRC Frame Body 22666620-23124 802.11 MAC Header Bytes: Protocol Version TypeSubType To DS Retry Pwr Mgt More Data WEPRsvd Frame Control Field Bits: 22411111111 DS FromMore Frag

15. Spring 2005UCSC CMPE25715 Address Field Description n Addr 1 = All stations filter on this address. n Addr 2 = Transmitter Address (TA) p Identifies transmitter to address the ACK frame to. n Addr 3 = Dependent on To and From DS bits. (Wireless Distribution System) n Addr 4 = Only needed to identify the original source of WDS (Wireless Distribution System) frames.

16. Spring 2005UCSC CMPE25716 Comments on CSMA/CA n IEEE 802.11 cannot avoid collisions of data packets (see [FAMA97]). n CSMA/CA works fine when hidden terminals are just a few. n For most single-hop wireless LANs, RTS/CTS is not useful (turned off by default in practice) n Spatial reuse is reduced in multi-hop networks

17. Spring 2005UCSC CMPE25717 Synchronization and Power Management n Synchronization p finding and staying with a WLAN p Synchronization functions r TSF Timer, Beacon Generation n Power Management p sleeping without missing any messages p Power Management functions r periodic sleep, frame buffering, Traffic Indication Map

18. Spring 2005UCSC CMPE25718 Synchronization in 802.11 n Timing Synchronization Function (TSF) n Used for Power Management p Beacons sent at well known intervals p All station timers in BSS are synchronized n Used for Point Coordination Timing p TSF Timer used to predict start of Contention Free burst n Used for Hop Timing for FH PHY p TSF Timer used to time Dwell Interval p All Stations are synchronized, so they hop at same time.

19. Spring 2005UCSC CMPE25719 n All stations maintain a local timer. n Timing Synchronization Function p keeps timers from all stations in synch p AP controls timing in infrastructure networks p distributed function for Independent BSS n Timing conveyed by periodic Beacon transmissions p Beacons contain Timestamp for the entire BSS p Timestamp from Beacons used to calibrate local clocks p not required to hear every Beacon to stay in synch p Beacons contain other management information r also used for Power Management, Roaming Synchronization Approach

20. Spring 2005UCSC CMPE25720 Infrastructure Beacon Generation APs send Beacons in infrastructure networks. Beacons scheduled at Beacon Interval. Transmission may be delayed by CSMA deferral. –subsequent transmissions at expected Beacon Interval –not relative to last Beacon transmission –next Beacon sent at Target Beacon Transmission Time Timestamp contains timer value at transmit time. Time Axis Beacon Interval XXXX "Actual time" stamp in Beacon Beacon Busy Medium

21. Spring 2005UCSC CMPE25721 Power Management n Mobile devices are battery powered. p Power Management is important for mobility. n Current LAN protocols assume stations are always ready to receive. p Idle receive state dominates LAN adapter power consumption over time. n How can we power off during idle periods, yet maintain an active session? n 802.11 Power Management Protocol: p allows transceiver to be off as much as possible p is transparent to existing protocols p is flexible to support different applications r possible to trade off throughput for battery life

22. Spring 2005UCSC CMPE25722 Power Management Approach n Allow idle stations to go to sleep p station’s power save mode stored in AP n APs buffer packets for sleeping stations. p AP announces which stations have frames buffered p Traffic Indication Map (TIM) sent with every Beacon n Power Saving stations wake up periodically p listen for Beacons n TSF assures AP and Power Save stations are synchronized p stations will wake up to hear a Beacon p TSF timer keeps running when stations are sleeping p synchronization allows extreme low power operation n Independent BSS also have Power Management p similar in concept, distributed approach

23. Spring 2005UCSC CMPE25723 Infrastructure Power Management n Broadcast frames are also buffered in AP. p all broadcasts/multicasts are buffered p broadcasts/multicasts are only sent after DTIM p DTIM interval is a multiple of TIM interval TIM TIM-Interval Time-axis Busy Medium AP activity TIM DTIM DTIM interval Broadcast

24. Spring 2005UCSC CMPE25724 Infrastructure Power Management n Broadcast frames are also buffered in AP. p all broadcasts/multicasts are buffered p broadcasts/multicasts are only sent after DTIM p DTIM interval is a multiple of TIM interval n Stations wake up prior to an expected (D)TIM. TIM TIM-Interval Time-axis Busy Medium AP activity TIM DTIM DTIM interval PS Station Broadcast

25. Spring 2005UCSC CMPE25725 Infrastructure Power Management n Broadcast frames are also buffered in AP. p all broadcasts/multicasts are buffered p broadcasts/multicasts are only sent after DTIM p DTIM interval is a multiple of TIM interval n Stations wake up prior to an expected (D)TIM. n If TIM indicates frame buffered p station sends PS-Poll and stays awake to receive data p else station sleeps again TIM TIM-Interval Time-axis Busy Medium Tx operation AP activity TIM DTIM DTIM interval PS Station Broadcast PS-Poll Broadcast

26. Spring 2005UCSC CMPE25726 BSS (Basic Service Set) QBSS (Basic Service Set for QoS) ( Enhanced Station ) HCCAEDCAPCFDCF PC HC IEEE 802.11E

27. Spring 2005UCSC CMPE25727 IEEE 802.11E p QoS enhancements r EDCA - Enhanced Distributed Coordinated Access (a.k.a. Enhanced DCF) r HCCA - Hybrid Coordination Function (through HC, Hybrid Controller) Coordinated Access (a.k.a. Enhanced PCF) p TC – Traffic Categories p TXOP – Transmission Opportunity r Granted by EDCF-TXOP or HC- poll TXOP p AIFS – Arbitration Interframe Space p Draft only – subject to change!

28. Spring 2005UCSC CMPE25728 802.11e EDCA n Introduction of 4 Access Categories (AC) with 8 Traffic Classes (TC) n MSDU are delivered through multiple backoffs within one station using AC specific parameters. n Each AC independently starts a back off after detecting the channel being idle for AIFS n After waiting AIFS, each back off sets counter from number drawn from interval [1,CW+1] n newCW [AC] >= ((oldCW[TC] + 1 ) * PF ) - 1

29. Spring 2005UCSC CMPE25729 Access Category and Traffic Class n Prioritized Channel Access is realized with the QoS parameters per TC, which include : p AIFS[AC] p CWmin[AC] p PF[AC] AC_VO [0]AC_VI [1]AC_BE [2]AC_BK [3] AIFSN2237 CWmin3715 CWmax7151023

30. Spring 2005UCSC CMPE25730 EDCA Virtual Collision AC1AC2AC3AC4TC

31. Spring 2005UCSC CMPE25731 ACK BackOff[AC0] + Frame BackOff[AC1] + Frame BackOff[AC2] + Frame AIFS[AC0] AIFS[AC1] AIFS[AC2] BackOff[AC3] + Frame AIFS[AC3 ] Access Category based Back-offs

32. Spring 2005UCSC CMPE25732 HCCA ( Hybrid Coordination Function Controlled Channel Access ) n Extends the EDCA access rules. n CP : TxOP p After AIFS + Back off p QoS Poll ; After PIFS n CFP : TxOP p Starting and duration specified by HC using QoS Poll.

33. Spring 2005UCSC CMPE25733 HCCAEDCA HC PIFS DATAA AIFSSIFSAIFS PIFS DATA Hybrid Coordinator

34. Spring 2005UCSC CMPE25734 802.11e Operation in the CFP n Guaranteed channel access on successful registration n Each node will receive a TxOP by means of polls granted to them by the HC n TxOP based on negotiated Traffic specification (TSPEC) and observed node activity n TxOP is at least the size of one Maximum sized MSDU at the PHY rate. n Access Point advertises polling list

35. Spring 2005UCSC CMPE25735 Food for Thought n Use your favorite network simulator, try to experiment with IEEE 802.11 in multi-hop networks and see how well or bad it performs in such networks. Any interesting findings? n Simulate IEEE 802.11e in multi-hop networks and test its effectiveness for service differentiation.

36. Spring 2005UCSC CMPE25736 References n [IEEE99] IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std 802.11-1999. n [TK84] H. Takagi and L. Kleinrock, Optimal Transmission Range for Randomly Distributed Packet Radio Terminals, IEEE Trans. on Comm., vol. 32, no. 3, pp. 246-57, 1984. n [WV99] L. Wu and P. Varshney, Performance Analysis of CSMA and BTMA Protocols in Multihop Networks (I). Single Channel Case, Information Sciences, Elsevier Sciences Inc., vol. 120, pp. 159-77, 1999. n [WG02] Yu Wang and JJ, Performance of Collision Avoidance Protocols in Single-Channel Ad Hoc Networks, IEEE Intl. Conf. on Network Protocols (ICNP ’02), Paris, France, Nov. 2002.

37. Spring 2005UCSC CMPE25737 Acknowledgments n Parts of the presentation are adapted from the following sources: p Mustafa Ergen, UC Berkeley, http://www.eecs.berkeley.edu/~ergen/docs/IEEE- 802.11overview.ppt http://www.eecs.berkeley.edu/~ergen/docs/IEEE- 802.11overview.ppt p Greg Ennis, Symbol Technologies, http://grouper.ieee.org/groups/802/11/Tutorial/archit.pdf http://grouper.ieee.org/groups/802/11/Tutorial/archit.pdf p Phil Belanger, Aironet and Wim Diepstraten, Lucent Technologies, http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf p Abhishek Karnik, Dr. Ratan Guha, University Of Central Florida, http://www.cs.ucf.edu/courses/cda4527/WLAN/80211.ppt http://www.cs.ucf.edu/courses/cda4527/WLAN/80211.ppt