Scatternet Formation in
Bluetooth
CSC 457
Bill Scherer
November 8, 2001
Outline

Introduction
 Overview of Bluetooth
 Scatternet Formation Protocols
What is Bluetooth?
What is Bluetooth?
 Ad Hoc wireless networking
 Specification and protocol suite
 Initiated by Ericsson in 1994
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
Next Up: Overview

Introduction
 Overview of Bluetooth
 Scatternet Formation Protocols
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
Frequency Hopping CDMA

Hop Pattern
– Permutation of the available hop frequencies

Clock
– Current offset within the hop pattern

Referred to as "Channels"
Organization of Bluetooth
Networks

Piconets
– Master/Slave
– Shared channel

Scatternets
– Grouped Piconets
– Bridges
 Shared Slaves
S
S
M
B
S
S
S
S
S
M
Next Up: Scatternet Formation

Introduction
 Overview of Bluetooth
 Scatternet Formation Protocols
S
S
M
B
S
S
S
S
S
M
Scatternet Formation

How do we go from (A) to (B)?
(A)
(B)
?
?
?
S
?
M
?
?
?
?
S
?
S
S
S
S
?
B
S
M
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
M
S
1
M
S
2
M
S
3
M
S
4
Scatternet Topologies
n(n-1)
2 2 possible

Roughly
topologies for n
nodes
 6 topologies for 3 nodes:
M
S
M
M
S
S
S
M
S
M
S
M
S
M
M
M
S
S
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
BTCP (Bluetooth Connection Protocol)


Bluetooth Connection Protocol
Based on Leader Election
–

Identifying one node to be in charge
Two phase protocol
1) Elect a leader
2) Assign roles
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
Role Assignment

Winner of election picks "sub-masters" and
bridges for minimum possible Piconets
 Winner forms temporary Piconet with submasters, gives them assignment, list of
slaves
 Sub-masters page in slaves
BTCP Example: Leader Election
(1)
3
2
1
(2)
8
4
6
5
2
3
7
9
2
1
2
1
1
7
6
5
2
9
2
4
4
8
3
7
6
9
2
2
2
2
1
1
2
4
1
8
7
6
5
9
2
2
(6)
3
4
4
5
2
3
(5)
3
5
8
4
(4)
3
(3)
8
3
7
6
9
2
2
1
7
4
5
8
7
6
9
2
BTCP Example: Roles
(1)
3
2
1
(2)
8
4
6
5
2
7
2
9
2
3
2
1
M
2
2
2
3
2
1
2
6
5
B
2
2
7
1
9
1
2
2
3
2
1
(5)
M
M
5
8
4
(4)
(3)
6
B
2
7
1
9
1
2
2
3
2
1
2
5
2
7
6
B
1
9
1
(6)
M
M
5
M
S
S
7
B
1
9
1
S
S
M
M
S
S
B
S
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
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
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
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
Case 1

One leader has no slaves
– Join other Piconet and retire (if room)
– Take a slave, other leader retires (otherwise)
S
S
S
S
M retired
M
S
S
S
L
B
L
Case 2

The two leaders have < k - 1 slaves between
them
S
S
M
M
S
S
S
S
M
S
S
S
S
retired
S
Case 3

At least k - 1 slaves between the leaders
– fill up and retire one of them
S
S
M
S
S
S
M
B
*
S
S
retired
M
B
S
S
S
*
M
S
S
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
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
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
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
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)