1. Design and Analysis of Data Communication Problems in Bluetooth Wireless Personal-Area Networks Ting-Yu Lin ( 林亭佑 ) Department of Communication Engineering National Chiao-Tung University, Bun Lab.

2. 2 Agenda Bluetooth Standard Overview Talk Structure Piconet Issues Addressing (Parts I & II) Scatternet Problems Resolving (Parts III & IV) Conclusions Future Directions References

3. 3 Bluetooth Standard Overview  Master-driven short-range wireless radio technology  Operate at 2.4 GHz unlicensed ISM band  TDD/FHSS with nominal rate 1600 hops/sec and 23/79 frequencies available, each of 1 Mbps symbol rate  Transmission range: 10, up to 100 meters  Physical links: - SCO (Synchronous Connection-Oriented) for voice slot reservation at fixed intervals - ACL (Asynchronous ConnectionLess) for data polling access scheme

4. 4 Bluetooth Standard Overview  Bluetooth protocol stack  Addressing - 48-bit Bluetooth Device Address (BD_ADDR) - 3-bit Active Member Address (AM_ADDR) - 8-bit Parked Member Address (PM_ADDR)  Four operational modes - Active, Sniff, Hold, Park

5. 5 Bluetooth Network Topology  Piconet - Master can connect to at most 7 active/sniff/hold slaves simultaneously per piconet  Scatternet - Interconnecting multiple piconets to form a larger network Bridge

6. 6 Packets Exchange Scenario MASTER SLAVE 1 SLAVE 2 SLAVE 3 ACLSCO ACL

7. 7 Bluetooth Frequency Hopping

8. 8 Talk Structure

9. Part I Sniff Scheduling for Power Saving Slot pair

10. 10 Sniff Scheduling  Problem Statement - Open issue: how to determine the sniff-related parameters based on different traffic loads? - Goal: balancing tradeoff between power conservation and traffic need  Deficiencies of Previous Works - Each slave is considered independently of others - Most works are restricted to a naive exponential adjustment in either sniff interval or active window - The placement of active windows on the time axis when multiple sniffed slaves are involved is ignored

11. 11 Sniff Scheduling  Architecture of Our Sniff Scheduling Protocol

12. 12  Sniff timing for slave k (unit = slot pair) Basic Idea  Slot occupancy = (active window size) / (sniff interval)  Our Evaluator is performed periodically to check the slot utilization status and determine an appropriate slot occupancy (reduction/increase/no change) for slave k = 20/60 = 1/3 Evaluation Period

13. 13 Evaluator U k : the slot utilization of the sniff-attempt slots assigned to slave k. B k : the buffer backlog for slave k, indicating the number of packets currently queued in the local Baseband buffer. W k : a weighted value derived from U k and B k to indicate the utilization ratio of time slots assigned to slave k. B max is the maximum buffer space and 0 ≦ α ≦ 1 W k = β W k ’ + (1-β) W k W k ’ is the history value, 0 ≦ β ≦ 1 T k,N k,O k : the current sniff parameters (sniff interval, active window size, and offset, respectively) associated with slave k. S k : the desired slot occupancy of slave k derived by the following equation. This value is the expected ratio of the new N k to the new T k. where 0 < δ < 1

14. 14 Evaluator for slave k UkUk B k /B max Calculator X (factor α) Calculator Y (factor δ ) WkWk SkSk  Example Evaluation Period U k = 16 (used) / 80 (reserved) = 0.2 B k /B max = 7 (queued) / 50 (buffer size) = 0.14 W k = 0.5 x 0.2 + 0.5 x 0.14 = 0.17 α = 0.5 S k = 0.17 x 1/3 (slot occupancy) / 0.8 = 0.07 δ= 0.8 = 1/15

15. 15 Evaluator for slave k  Possible sniff timings to satisfy 1/15 slot occupancy

16. 16 Resource Pool (RP) 120 (max. sniff interval) 1-D infinite vector V 2-D finite matrix M 15 (min. sniff interval)

17.  Recall that the Evaluator for Slave1 concludes that its slot occupancy should be reduced from 1/3 to 1/15  Possible scheduling Scheduling Policy Slave1 must first give back the 1/3 slot occupancy Scheduler tries to find a sniff pattern satisfying 1/15 slot occupancy Or 2/30 Or 4/60 Or 8/120  Two scheduling policies are proposed - LSIF (Longest Sniff Interval First), which starts searching with the longest interval - SSIF (Shortest Sniff Interval First), which starts searching with the shortest interval

18. 18 Experimental Environment  2-state MMPP traffic model  5 slaves in the piconet  α = 0.7  δ = 0.5  Resource Pool (RP) size = 100 x 4 = 400 (slot pairs)

19. 19 AA: Always Active with Round Robin polling policy AS_VSI: Always Sniff Varying Sniff Interval AS_VAW: Always Sniff Varying Active Window With Buffer Size ≧ 30 Our LSIF/SSIF achieve (Compared to AA) 37 % reduction in power consumption 16 % improvement in throughput (a) Power Consumption (b) Throughput Buffer Size B max

20. 20 Summary of Contributions  Features of Proposed Solution - An adaptive sniff scheduling scheme is proposed to consider multiple slaves simultaneously. - Our scheduling is more accurate in determining the sniff- related parameters based on slaves’ traffic loads. - Our proposal includes the placement of active windows of sniffed slaves on the time axis.

21. Part II Link Polling Policy by Pattern Matching Observations 1.Few works consider the asymmetric up-/down-link traffics between master and slave. 2.The incorporation of packet type selection into polling policy remains unaddressed before this work.

22. 22 Pattern Matching Polling (PMP)  This work focuses on the Bluetooth ACL link.  Assuming error-free, only DH1/3/5 are considered.  Bandwidth Efficiency β is defined as the number of payload bytes per non-empty slot. β

23. 23 Motivation (A Naive Greedy Polling Example)  Bandwidth Efficiency β = (16*(20+2)) / (5+3) = 44 unit = bytes/slot

24. 24 β = (26*(20+2)) / (5+3+1+1) = 57.2 (23% improvement) Motivation (A Pattern Matching Polling Example)  Bandwidth Efficiency

25. 25  Two problems need further elaboration in PMP: Pattern Matching Polling (PMP) 1.How to determine a most bandwidth efficient polling pattern? 2. Given a most efficient pattern, how to schedule polling timings?

26. 26 Parameters - Consider a master-slave pair with λ M and λ S (bytes/slot) as their traffic loads. - Let λ H = max{ λ M, λ S }, λ L = min{ λ M, λ S }, and ratio ρ = λ H / λ L. - Denote by N H and N L the units with loads λ H and λ L, respectively. - Use numbers 1/3/5 to represent DH1/DH3/DH5 packets. - A polling pattern is a sequence of packet types that will be exchanged by a master-slave pair. Polling Patterns

27. 27

28. 28 Polling Patterns  A length-k pattern (k is a positive integer) consists of two k-tuples: (H 1, H 2, …, H k ) and (L 1, L 2, …, L k ), where H i, L i = 1, 3, or 5, each representing a packet type.  Intuitively, the sequence of packets (H 1, L 1, H 2, L 2, …, H k, L k ) will be exchanged by N H and N L, and the sequence will be repeated periodically, as long as the ratio ρ is unchanged and there is no bursty traffic.

29. 29 Impact of Pattern Length  As k grows, the number of offered traffic ratios ρ will increase exponentially.

30. 30  Bandwidth Efficiency  Given traffic loads λ H and λ L of a master-slave pair, we propose to select the polling pattern that gives the highest bandwidth efficiency β for use.

31. 31  Let j be a positive integer ≦ k (within one iteration of the pattern) Polling Timings Γ1Γ1 Γ2Γ2 ΓkΓk Reference point

32. 32  Based on λ H and λ L, the most efficient polling pattern (5, 3) (1, 1) is selected.  Γ 1 = 16, Γ 2 = 26, β = 57.2 (23% improvement)  Note that an overflow bit is also implemented in our PMP policy to prevent buffer overloading when bursty traffic occurs. Pattern Matching Polling Example Γ1Γ1 Γ2Γ2

33. 33 Experimental Environment  7 active slaves in a piconet  Buffer size for each slave = 2048 bytes  Three other polling strategies are implemented - NGP: Naive Greedy Polling (p.23) - ERR: Exhaustive Round Robin (ref. A. Capone et. al. ) - StickyAFP: Sticky Adaptive Flow-based Polling (ref. A. Das et. al. )

34. 34 K: max. allowable pattern length (a)Piconet Throughput (b)Average Delay When λ ≧ 65 (bytes/slot) Our PMP achieves (Compared to NGP) 17 % improvement in throughput 14 % reduction in average delay

35. 35 Summary of Contributions  Features of Proposed Solution - An efficient Pattern Matching Polling (PMP) policy is proposed to handle asymmetric up-/down-link traffics and exploit different Bluetooth packet types. - The ultimate goal is to reduce the unfilled, or even null, payloads in each busy slot. When multiple links (master-slave pairs) exist, bandwidth efficiency of each single link does determine the max. allowable throughput within a piconet (piconet capacity).

36. Part III BlueRing: A New Scatternet Topology for Bluetooth

37. 37 BlueRing Scatternet Structure Routing Protocol Topology Maintenance Mechanism

38. 38 Motivation  Deficiencies of Previous Works - Most star- or tree-shaped scatternet topologies suffer from a communication bottleneck at the root as the network enlarges. - How to route packets once the scatternet is formed remains unaddressed. - Topology maintenance (fault-tolerance) issues are not properly addressed.

39. 39 BlueRing Structure  Upstream/Downstream Piconet  Upstream/Downstream Master  Upstream/Downstream Bridge Master/Bridge interleaving

40. 40 BlueRing Routing Protocol  General baseband packet format  Payload header formats: (a) single-slot packets and (b) multi-slot packets

41. 41 BlueRing Routing Protocol  Payload formats in BlueRing: (a) single-hop unicast communication, (b) multi-hop unicast communication, and (c) scatternet broadcast communication  The fields in gray are what added by BlueRing

42. 42 BlueRing Recovery Protocol  We propose to use 2 DIACs (from 63 reserved DIACs), say DIAC1 and DIAC2, to facilitate BlueRing recovery/extension.  The general GIAC will be used to invite new hosts to join an existing BlueRing.  Bridge missing recovery: (a) DIAC1 discovering and (b) the reconnected BlueRing.

43. 43  Master missing recovery: (a) DIAC1 discovering and (b) the reconnected BlueRing. BlueRing Recovery Protocol

44. 44 BlueRing Extension Protocol  In BlueRing, each master should execute GIAC inquiry from time to time.  When the number of slaves belonging to a master exceeds a certain limit, say α ( α ≧ 4), we will split it into two piconets.  The master should send out a split_request token to obtain split permission from all other masters (concurrent splitting avoidance).

45. 45 (b)  Once the split request is approved by all piconets on the ring, the master detaches its upstream bridge and two non-bridge slaves. A BlueRing extension example with α= 4 (a) (c)

46. 46 Experimental Environment  Only DH1 packets are simulated  No mobility is modeled  Each ACL connection could be intra- or inter- piconet communication with data rate of 256K bps

47. 47 Simulated topologies with 21 hosts (a)Star-shaped structure with a piconet as the central gateway (b)BlueRing with 3 piconets, each containing 7 slaves (c)Single-piconet structure containing all 21 nodes in a single piconet (park mode is used)

48. 48 (a) Throughput (21 % increased, compared to Star-shaped) (b) Average packet delays (38 % reduced, compared to Star-shaped)

49. 49 Summary of Contributions  Features of Proposed Solution - Routing on BlueRing is stateless. - BlueRing architecture is simple and scalable. - Maintaining a BlueRing is an easy job.

50. Part IV Collision Analysis for a Multi-Piconet Environment

51. 51 Collision Analysis (Multi-Piconet Environment)  Problem Statement - Co-channel interference between Bluetooth piconets could have negative impact on the network throughput.

52. 52 Motivation  Limitations of Previous Works (ref. A. El-Hoiydi et. al.) - Only single-slot packets are considered. - It is assumed that each piconet is fully-loaded.  Thus, the results do not reflect general scenarios.

53. 53  Analysis Approach - Consider DH1/3/5 packets that lack for FEC error tolerance, and assume that they occupy the whole 1-slot, 3-slot, and 5-slot space, respectively. - For N coexisting piconets, we first analyze the packet success probability for two interfering piconets that are neither time- nor frequency-synchronized. Collision Analysis (Multi-Piconet Environment)  Goal - Derive the theoretical packet error probability and aggregate network throughput for a N-piconet environment.

54. 54 Collision Analysis  Assume that all piconets have homogeneous packet arrival rates: λ 1 for DH1, λ 3 for DH3, and λ 5 for DH5 (0 ≦ λ i ≦ 1 for i = 1, 3, and 5). - Case I: fully-loaded traffic (λ 1 + 3λ 3 + 5λ 5 = 1) - Case II: non fully-loaded traffic (λ 1 + 3λ 3 + 5λ 5 < 1)  Create a dummy 1-slot packet with arrival rate λ 0 to represent an empty slot, where

55. 55 Example with Fully-Loaded Traffic  Parameters - P 0 = 78/79 - P S (i): the success probability for an i-slot packet  Concept - P S (1) = 1/9 ． P 0 ． P 0 (B1) + 1/9 ． P 0 ． P 0 (B2) + 1/9 ． P 0 (B3) + 1/9 ． P 0 (B4) + 1/9 ． P 0 ． P 0 (B5) + 1/9 ． P 0 (B6) + 1/9 ． P 0 (B7) + 1/9 ． P 0 (B8) + 1/9 ． P 0 (B9) = 3/9 P 0 2 + 6/9 P 0 Slot delimiters 1-slot  3-slot packet  5-slot packet  B?B? Assuming λ 1 =λ 3 =λ 5, the possibility of each slot delimiter  A general probabilistic formula for P S (i) can be derived

56. 56 Collision Analysis  Slot delimiters  General formula Possibility of each slot delimiter

57. 57 Collision Analysis  can be solved recursively as follows:

58. 58 Collision Analysis  Examples  Network throughput of X (a piconet)  Aggregate network throughput of N piconets is N × T f(1) f(3)

59. 59 Experimental Environment  79 channels are available  DH1/3/5 packets are simulated  Equal arrival rates for DH1/3/5 packets (λ 1 = λ 3 = λ 5 )

60. 60 N: number of piconets The results suggest that when N > 42, network throughput starts to degrade as traffic load > 50% Aggregate Network Throughput against traffic loads for various network sizes (N)

61. 61 Summary of Contributions  Features of Proposed Analysis - All available packet types (1-slot, 3-slot, and 5-slot) are considered. - Each piconet is not necessarily fully-loaded.

62. 62 Conclusions  Sniff Scheduling (piconet) - Pioneer work to 1. Consider multiple slaves simultaneously when assigning sniff parameters, 2. Schedule different sniff patterns to slaves with different traffic loads, based on accurate calculation of traffic requirement.  Pattern Matching Polling (piconet) - Pioneer work to 1. Address the asymmetric up-/down-link traffics between master and slave, 2. Incorporate packet type selection into polling policy.

63. 63 Conclusions (Cont.)  BlueRing (scatternet) - Pioneer work to propose a scatternet structure with corresponding 1. Simple stateless routing protocol and 2. Structure maintenance mechanism to handle node leaving/joining.  Collision Analysis (multi-piconet environment) - Pioneer work to 1. Consider all packet types (1-slot, 3-slot, and 5-slot) and 2. Remove the assumption that each piconet is fully-loaded in the analysis model.

64. 64 Future Directions  Compaction/Re-organization strategies for the Resource Pool (sniff scheduling)  BlueRing performance analysis considering inter-piconet interference  A real implementation of the proposed BlueRing scatternet - Hardware support is uncertain (e.g., bridge functionality)

65. 65 References 1. Ting-Yu Lin and Yu-Chee Tseng, “An Adaptive Sniff Scheduling Scheme for Power Saving in Bluetooth,” IEEE Wireless Communications, Dec. 2002. 2. Ting-Yu Lin, Yu-Chee Tseng, and Yuan-Ting Lu, “An Efficient Link Polling Policy by Pattern Matching for Bluetooth Piconets,” The Computer Journal (SCI), 2003. 3. Ting-Yu Lin, Yu-Chee Tseng, and Keng-Ming Chang, “Formation, Routing, and Maintenance Protocols for the BlueRing Scatternet of Bluetooths,” Wireless Communications and Mobile Computing, 2003. 4. Ting-Yu Lin and Yu-Chee Tseng, “Collision Analysis for a Multi- Bluetooth Picocells Environment,” IEEE Communications Letters, 2004.