1. Winter 2004 UCSC CMPE252B1 CMPE 257: Wireless and Mobile Networking SET 3p: Medium Access Control Protocols

2. Spring 2005CMPE257 UCSC2 MAC Protocol Topics n Time synchronization n Power saving

3. Spring 2005CMPE257 UCSC3 IEEE 802.11 Time Sync. Function n Bandwidth: p Up to 54 Mbps p Good for a few hundred nodes n Time Synchronization Function (TSF) p Not scalable p How to fix it? n Note: Only single-hop ad hoc networks are dealt with here ([HL02]).

4. Spring 2005CMPE257 UCSC4 IEEE 802.11 TSF n Time divided into beacon intervals, each containing a beacon generation window. n Each station: p Waits for a random number of slots; p transmits a beacon (if no one else has done so). n Beacon: several slots in length. window beacon interval

5. Spring 2005CMPE257 UCSC5 IEEE 802.11 TSF n Beacon contains a timestamp. On receiving a beacon, STA adopts beacon ’ s timing if T(beacon) > T(STA). n Clocks move only forward. faster adopts 12:0112:00 slower not adopts 12:01 12:02 12:01

6. Spring 2005CMPE257 UCSC6 Problems with 802.11 ’ s TSF n Faster clocks synchronize slower clocks. n Equal opportunity for nodes to generate beacons. 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:14 1:15 1:16 1:17 1:18 1:19 1:21 1:23 1:18 1:19 1:21 1:23 +3 +4 +5 +6 +7 +8 +3 +4 +5 +6 +7 +8 1:21 1:22 1:23 1:25 1:28 1:31 1:23 1:25 1:28 1:31

7. Spring 2005CMPE257 UCSC7 The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization

8. Spring 2005CMPE257 UCSC8 Two Types of Out-of-Sync Fastest-station out-of-sync – fastest station is out of sync with all others. k-global out-of-sync – k percent of links are out of sync. n Questions: How often? For how long?

9. Spring 2005CMPE257 UCSC9 Fastest-station out-of-sync (1) n Clock1 and Clock2: two fastest clocks n d = their difference in accuracy n T = length of beacon interval (0.1 sec.) n Clock drift: d*T per beacon interval. n In  /(d*T) intervals, fastest-station will be out of sync with all others. T

10. Spring 2005CMPE257 UCSC10 Fastest-station out-of-sync (2) n n = number of stations. w = size of beacon window. P ’ (n,w) = prob(fastest station wins beacon contention)

11. Spring 2005CMPE257 UCSC11 Prob(Fastest station sends a beacon)

12. Spring 2005CMPE257 UCSC12 Fastest-station out-of-sync (3) n H = # beacon intervals with F.S. out-of- sync. n L = # beacon intervals between async periods. n E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. LH

13. Spring 2005CMPE257 UCSC13 How often does fastest-node get out of sync with others?

14. Spring 2005CMPE257 UCSC14 Percentage of time fastest station out of sync with all others 802.11a 54 Mbps ∆ = 224  s d = 0.003%

15. Spring 2005CMPE257 UCSC15 How often does 25%-async occur?

16. Spring 2005CMPE257 UCSC16 Percentage of time with 25 percent of links out-of-sync 802.11a 54 Mbps ∆ = 224  s d = 0.01%

17. Spring 2005CMPE257 UCSC17 How to fix it? n Desired properties: simple, efficient, and compatible with current 802.11 TSF. n Causes of out-of-sync p Unidirectional clocks p Equal beacon opportunity p Single beacon per interval p Beacon contention (collision)

18. Spring 2005CMPE257 UCSC18 Improve fastest station ’ s chance n Let the fastest station contend for beacon generation more frequently than others.

19. Spring 2005CMPE257 UCSC19 Adaptive Clock Sync Protocol n Station x participates in beacon contention once every C(x) intervals. n Initially, C(x) =1. Always, 1 < C(x) < Cmax. n Dynamically adjust C(x): x faster C(x) +1 x slower C(x) -1

20. Spring 2005CMPE257 UCSC20 Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

21. Spring 2005CMPE257 UCSC21 What if the fastest node leaves the IBSS? n The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

22. Spring 2005CMPE257 UCSC22 What if a new fastest node enters the IBSS? n The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.

23. Spring 2005CMPE257 UCSC23 Performance n 802.11 Performance of TSFPerformance of TSF n ATSP Performance of ATSPPerformance of ATSP n TATSP Performance of Modified TSFPerformance of Modified TSF

24. Spring 2005CMPE257 UCSC24 Modified TSF n Divide stations into three groups: p Group 1: C(x) = Cmax1 = 1 p Group 2: C(x) = Cmax2 = a small number p Group 3: C(x) = Cmax3 = a large number

25. Spring 2005CMPE257 UCSC25 Performance of TSF

26. Spring 2005CMPE257 UCSC26 Performance of ATSP

27. Spring 2005CMPE257 UCSC27 Performance of Modified TSF

28. Spring 2005CMPE257 UCSC28 Summary n Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable. n Proposed: a simple remedy compatible with the current TFS. n Choice of Cmax?

29. Spring 2005CMPE257 UCSC29 What ’ s Next? n IBSS: single-hop n MANET: multi-hop transmission range

30. Spring 2005CMPE257 UCSC30 Comments n Need simulations with data traffic p Some data transmissions may go beyond the Target Beacon Transmission Time (TBTT) n More realistic analysis p Nodes may be still in defer state when in beacon window time: independent, uniform assumption doesn’t hold.

31. Spring 2005CMPE257 UCSC31 Power Saving Protocols n Various aspects of solution for saving power p Transmission power control p Power aware routing p Low-power mode n Power saving modes in IEEE 802.11 p Active mode p Power saving mode (PS) p Protocols under Infrastructure and ad hoc network are different

32. Spring 2005CMPE257 UCSC32 Power Saving at MAC Layer awake sleep Beacon window ATIM window Beacon interval

33. Spring 2005CMPE257 UCSC33 Challenges n MANET (Mobile ad hoc networks) p Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization p Clock synchronization r Radio interference r Variable packet delay (unpredictable mobility) r Lack of central control p Neighbor discovery r Because PS host will reduce its transmitting/receiving activity r Routing problem p Network partitioning and merging

34. Spring 2005CMPE257 UCSC34 Design guidelines n More beacon  A PS host should not inhibit its beacon in ATIM window even if it has heard other beacons  Inaccurate-neighbor problem prevention  Multiple beacon in a ATIM window n Overlapping Awake interval  No clock synchronization  Overlapping of Wake-up pattern of two PS host n Wake-up prediction  PS host’s wake-up pattern based on their time difference

35. Spring 2005CMPE257 UCSC35 Infrastructure and Ad Hoc Protocols n Access Point (AP) monitors each host n PS mode host wakes up periodically for incoming packet from AP. n Periodic beacon frames. n In each beacon frame, a Traffic Indication Map(TIM) will be delivered, which contains ID’s of those PS host with buffered unicast packet in the AP. n PS hosts wakes up periodically n ATIM window : short interval that PS hosts wake up n In the beginning of each ATIM window, each mobile host will contend to transmit a beacon frame. n Successful beacon synchronizes mobile host’s clock and prevents other hosts from sending their beacon

36. Spring 2005CMPE257 UCSC36 Dominating-awake-interval n PS host stay awake sufficiently long so as to ensure that neighboring host can know each other. n Dominating awake property p AW >= BI/2 + BW n Alternatively labeled odd and even sequence of beacon intervals

37. Spring 2005CMPE257 UCSC37 Periodically–fully-awake-interval n Two types of beacon interval p Low power intervals r Length of active window is reduced to minimum r Starts with an active window which contains a beacon window followed by a MTIM window AW = BW + MW, in the rest of the time, the host can go to the sleep mode. p Fully awake intervals r Length of active window is extended to the maximum r Arrives periodically, interleaved between low power intervals r AW = BI, rest of the time must remain awake n Suitable for slowly mobile environments

38. Spring 2005CMPE257 UCSC38 Periodically–fully-awake-interval Rest of active window

39. Spring 2005CMPE257 UCSC39 Properties n Each PS host’s beacon window overlaps with any neighbor’s fully-awake intervals in every T beacon intervals. n More power saving than previous protocol when T > 2. n Remark: Every node chooses the same T.

40. Spring 2005CMPE257 UCSC40 Quorum-based n Quorum p A set of identities one need to obtain before doing sth. p Two quorums have non-empty intersection to ensure atomicity. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 4 16

41. Spring 2005CMPE257 UCSC41 Quorum-based n PS host only needs to send beacon O(1/n) of all the beacon intervals n Quorum interval p Beacon + MTIM, AW = BI n Non quorum intervals p Starts with an MTIM window, after that, host may go to sleep mode, AW=MW p Amount of awaking time is less than 50%, provided n >=4 n Suitable for expensive transmission cost

42. Spring 2005CMPE257 UCSC42 Communication with PS hosts n Unicast p Predict PS host’s wakeup time and send MTIM packet during that time n Broadcast p Divide them into groups p Hosts within the same group have overlapping MTIM window p Need multiple transmissions

43. Spring 2005CMPE257 UCSC43 Summary n Three power saving protocol for asynchronous MANETs: p Dominating awake interval r Most power consumption, r Lowest neighbor discovery time. p Periodically-fully-awake interval r Balance both power consumption and neighbor discovery time. p Quorum based r The most power saving r Longest neighbor discovery time

44. Spring 2005CMPE257 UCSC44 Comments on Simulations n A custom-built simulator p Many details omitted n Carrier sensing and transmission range n Star-topology n Packet delivery delay (tradeoff?)

45. Spring 2005CMPE257 UCSC45 Future Work n More MANET scenarios n Adaptive beacon intervals?

46. Spring 2005CMPE257 UCSC46 References  [HL02] Lifei Huang and Ten-Hwang Lai, On the Scalability of IEEE 802.11 Ad Hoc Networks, in ACM MobiHoc 2002.  [THH02] Tseng et al., Power-Saving Protocols for IEEE 802.11-Based Multi- Hop Ad Hoc Networks, in IEEE INFOCOM 2002.

47. Spring 2005CMPE257 UCSC47 Acknowledgments n Parts of the presentation are adapted from the following sources: p Moses Pawar, USC, http://www.isi.edu/~weiye/teaching/cs558sm 04/slides/Mac_protocols.ppt http://www.isi.edu/~weiye/teaching/cs558sm 04/slides/Mac_protocols.ppt p Ten H. Lai, Ohio State University r http://www.cse.ohio-state.edu/~lai/788-Au03/2- scalibility.pdf http://www.cse.ohio-state.edu/~lai/788-Au03/2- scalibility.pdf r http://www.cse.ohio-state.edu/~lai/788-Au03/4- Power%20Saving.ppt http://www.cse.ohio-state.edu/~lai/788-Au03/4- Power%20Saving.ppt