Scatternet Formation in Bluetooth

1. Scatternet Formation in Bluetooth CSC 457 Bill Scherer November 8, 2001

2. Outline • Introduction • Overview of Bluetooth • Scatternet Formation Protocols

3. What is Bluetooth? What is Bluetooth? • Ad Hoc wireless networking • Specification and protocol suite • Initiated by Ericsson in 1994

4. Why Should I Care About It? • Up and coming • In billions of devices by 2005 (Business Week, 18 September 2000) • Cool • Cordless desktop • Briefcase e-mail • Wire-free headphones • Cheap • As little as 29¢ incremental • 80K transistors

5. Next Up: Overview • Introduction • Overview of Bluetooth • Scatternet Formation Protocols

6. Physical Layer: Media • 2.4 GHz Band (license-free) • Slotted Bandwidth • 79 hop frequencies (23 in Japan, France, Spain) • 1 MHz each • 625sec hop intervals (1600 hops/sec) • 10/100 Meter range • Up to 500 kbits/sec bandwidth

7. Frequency Hopping CDMA • Hop Pattern • Permutation of the available hop frequencies • Clock • Current offset within the hop pattern • Referred to as "Channels"

8. S S M B S S S S S M Organization of Bluetooth Networks • Piconets • Master/Slave • Shared channel • Scatternets • Grouped Piconets • Bridges • Shared Slaves

9. S S M B S S S S S M Next Up: Scatternet Formation • Introduction • Overview of Bluetooth • Scatternet Formation Protocols

10. ? ? ? ? ? S ? ? S ? M ? B S ? S S S S M Scatternet Formation • How do we go from (A) to (B)? (A) (B)

11. M S M S M S M S 1 2 3 4 Establishing a Connection 0) Slave: must be in Page Scan mode 1) Master: enter Page mode 2) Slave: Slave response to page 3) Master: Master response to slave 4) Slave, Master are now connected

12. M S M M S M M S M n(n-1) S M S S M S 2 2 S M S Scatternet Topologies • Roughly possible topologies for n nodes • 6 topologies for 3 nodes:

13. Good Topology Properties • Fully connected • Masters belong to exactly one Piconet • Bridges connect only two Piconets • Avoid overload on the bridge node • Minimal number of Piconets forming minimal diameter Scatternet • Reduce cost of routing

14. BTCP (Bluetooth Connection Protocol) • Bluetooth Connection Protocol • Based on Leader Election • Identifying one node to be in charge • Two phase protocol • Elect a leader • Assign roles

15. Leader Election • All nodes start with VOTES = 1. • Look for other nodes (send/listen on special discovery channel) • When two nodes meet, higher VOTES wins, gets all votes and MAC addresses from loser. • Loser enters Page Scan mode • Election ends when no more nodes found

16. Role Assignment • Winner of election picks "sub-masters" and bridges for minimum possible Piconets • Winner forms temporary Piconet with sub-masters, gives them assignment, list of slaves • Sub-masters page in slaves

17. 2 3 8 4 7 2 1 6 2 5 1 9 2 2 BTCP Example: Leader Election (1) (2) (3) 2 2 3 8 3 8 4 4 7 7 2 2 1 6 6 5 5 1 9 1 9 2 2 (4) (5) (6) 3 4 3 4 7 3 8 3 8 3 8 4 4 4 7 7 7 2 2 2 6 6 6 5 5 5 1 9 1 9 1 9 2 2 2

18. M 3 8 3 8 3 M 4 4 S 2 2 7 7 7 2 2 2 6 6 1 6 1 2 2 5 5 5 B B 1 9 1 9 1 9 2 2 1 1 2 2 3 M 3 M S M M M M 2 2 7 7 S 2 2 S 6 1 B 1 B 2 2 5 5 S B 1 9 1 9 S S 2 2 1 1 2 2 BTCP Example: Roles (1) (2) (3) (4) (5) (6)

19. Limitations of BTCP • Assumes all nodes can see each other • Can get two isolated Scatternets otherwise • Time complexity: (n/k) for n nodes • Due to centralized nature • A group at MIT has achieved O(log n) • Assumes zero knowledge of network • Could reuse old topologies if semi-stable

20. LMS • Law, Mehta, Siu from MIT • Randomized, distributed • Multiple rounds, but no separate phases • Every node starts out as a leader • Also assumes all nodes can see each other

21. During a Round of LMS • Each leader flips a coin to see whether it goes into Scan or Seek mode • Scan mode: • Listen for another node (discovery channel) • If contacted, go into Page Scan mode • Seek Mode • Look for slave on discovery channel • Connect via Page

22. Retirement • Once two leaders connect, one must retire • Invariants for partial Scatternets: • Each leader either has no slave, or has at least one unshared slave • Each leader has fewer than k slaves in its Piconet • Five cases needed to preserve invariants

23. S S S S M retired M S S S B L L Case 1 • One leader has no slaves • Join other Piconet and retire (if room) • Take a slave, other leader retires (otherwise)

24. S S retired M M S M S S S S S S S S Case 2 • The two leaders have < k - 1 slaves between them

25. Case 3 • At least k - 1 slaves between the leaders • fill up and retire one of them S S retired M M M M B S S B S S S S * S S S * S

26. Cases 4, 5 • Special cases to make the algorithm work • Refer to paper if you want the full details • http://perth.mit.edu/~ching/pubs/ PerformanceOfScatternet.pdf • Important thing is that even in these cases, one of the leaders retires

27. A Bit of Theory • Time Complexity: BTCP • (n/k) for n nodes, k slaves per Piconet • Due to centralized nature • Time Complexity: LMS • O(log n) • ~1/2 the leaders retire each round

28. Transport Layer: Services • SCO (Synchronous Connection Oriented) • Fixed 64 kbit/sec symmetrical link • 2 slots at a time (one each direction) • ACL (Asynchronous Connectionless) • 432.6 kbit/sec symmetrical link • 721.0/57.6 kbit/sec asymmetrical link • 5 slots at a time • Choice: 1 ACL, 3 SCOs, or one of each

29. FHCDMA Advantages • Resistance to interference • Can still get through on other parts • Resistance to multipath effects • Reflection, like an echo • Multiple access for co-located devices • Multiple simultaneous hop patterns • Graceful bandwidth degradation

30. Connection States • Active • Sending/Receiving normally • Sniff • Typically slaves only • Low-power mode • Not listening on every receive slot • Hold (SCO communications only) • Park (not participating)