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
Organization of Bluetooth Networks

• Piconets
• Master/Slave
• Shared channel
• Scatternets
• Grouped Piconets
• Bridges
• Shared Slaves

9
Next Up Scatternet Formation

• Introduction
• Overview of Bluetooth
• Scatternet Formation Protocols

10
Scatternet Formation

• How do we go from (A) to (B)?

(A)
(B)
11
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
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
BTCP Example Leader Election
(1)
(2)
(3)
2
2
3
8
3
8
4
4
7
7
2
2
1
6
6
5
5
9
1
9
1
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
9
1
9
1
9
1
2
2
2
18
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
Case 1

• One leader has no slaves
• Join other Piconet and retire (if room)
• Take a slave, other leader retires (otherwise)

24
Case 2

• The two leaders have

25
Case 3

• At least k - 1 slaves between the leaders
• fill up and retire one of them

S
retired
S
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)