Scatternet Formation Scatternet Formation in Bluetooth in Bluetooth CSC 457 Bill Scherer November 8, 2001
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Scatternet Formation in Scatternet Formation in BluetoothBluetooth
CSC 457
Bill Scherer
November 8, 2001
OutlineOutline
IntroductionOverview of BluetoothScatternet Formation Protocols
What is Bluetooth?What is Bluetooth?
What is Bluetooth? Ad Hoc wireless networking Specification and protocol suiteInitiated by Ericsson in 1994
Why Should I Care About It?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: OverviewNext Up: Overview
IntroductionOverview of BluetoothScatternet Formation Protocols
Physical Layer: MediaPhysical Layer: Media
2.4 GHz Band (license-free)Slotted Bandwidth
– 79 hop frequencies (23 in Japan, France, Spain)– 1 MHz each– 625sec hop intervals (1600 hops/sec)
10/100 Meter rangeUp to 500 kbits/sec bandwidth
Frequency Hopping CDMAFrequency Hopping CDMA
Hop Pattern– Permutation of the available hop frequencies
Clock– Current offset within the hop pattern
Referred to as "Channels"
Organization of Bluetooth Organization of Bluetooth NetworksNetworks
Piconets– Master/Slave– Shared channel
Scatternets– Grouped Piconets– Bridges
Shared Slaves
SM
B
SS
S
MS
S
S
Next Up: Scatternet FormationNext Up: Scatternet Formation
IntroductionOverview of BluetoothScatternet Formation Protocols
SM
B
SS
S
MS
S
S
Scatternet FormationScatternet Formation
How do we go from (A) to (B)?
??
?
??
?
??
?
?
(A) (B)
SM
B
SS
S
MS
S
S
Establishing a ConnectionEstablishing 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 S1
M S2
M S3
M S4
Scatternet TopologiesScatternet Topologies
Roughly possible topologies for n nodes
6 topologies for 3 nodes:
n(n-1)
22
M S
M
S M
S
M M
S
S S
M
S M
M
M S
S
Good Topology PropertiesGood Topology Properties
Fully connected Masters belong to exactly one PiconetBridges connect only two Piconets
– Avoid overload on the bridge node
Minimal number of Piconets forming minimal diameter Scatternet– Reduce cost of routing
BTCP BTCP (Bluetooth Connection Protocol)(Bluetooth Connection Protocol)
Bluetooth Connection Protocol Based on Leader Election
– Identifying one node to be in charge
Two phase protocol1) Elect a leader
2) Assign roles
Leader ElectionLeader 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 modeElection ends when no more nodes found
Role AssignmentRole 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
BTCP Example: Leader ElectionBTCP Example: Leader Election
2
3
5
78
9
6
4
1
(1) (2) (3)
(4) (5) (6)
2
3
5
78
9
6
4
1
2
22
21 2
3
5
78
9
6
4
1
2
22
2
1
2
3
5
78
9
6
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1
3
2
4
2
3
5
78
9
6
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1
3
2
4
2
3
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78
9
6
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12
7
BTCP Example: RolesBTCP Example: Roles
2
3
5
78
9
6
4
1
2
3
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78
9
6
4
1 B
M
1
1
22
2
2
(1) (2) (3)
2
3
5
7M
9
6
S
1 B1
1
22
2
2
2
3
5
7M
9
6
M
1 B1
1
22
2
2
(4) (5) (6)
2
3
5
7M
9
B
M
11
1
22
2
2S
S
S
SM
S
B
M
S
Limitations of BTCPLimitations 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
LMSLMS
Law, Mehta, Siu from MITRandomized, distributedMultiple rounds, but no separate phasesEvery node starts out as a leaderAlso assumes all nodes can see each other
During a Round of LMSDuring 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
RetirementRetirement
Once two leaders connect, one must retireInvariants 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 1Case 1
One leader has no slaves– Join other Piconet and retire (if room)– Take a slave, other leader retires (otherwise)
S
M
SL
S
S
S
M
BL
S
S
retired
Case 2Case 2
The two leaders have < k - 1 slaves between them
S
M
S
S
M
SS
S
M
S
S
S
SS
retired
Case 3Case 3
At least k - 1 slaves between the leaders– fill up and retire one of them
retiredS
M
S
S B
S
M
S
S
*
S
M
S
S B
S
M
S
S
*
Cases 4, 5Cases 4, 5
Special cases to make the algorithm workRefer 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 TheoryA 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: ServicesTransport 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 AdvantagesFHCDMA 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 StatesConnection 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)
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